U.S. patent number 3,775,577 [Application Number 05/245,924] was granted by the patent office on 1973-11-27 for induction cooking apparatus having pan safety control.
This patent grant is currently assigned to Environment/One Corporation. Invention is credited to Philip H. Peters, Jr..
United States Patent |
3,775,577 |
Peters, Jr. |
November 27, 1973 |
INDUCTION COOKING APPARATUS HAVING PAN SAFETY CONTROL
Abstract
An improved induction cooking unit and power supply therefore
having a pan safety control for automatically sensing improper
operation of the induction cooking unit due to the use of an
unsuitable pan fabricated from a high conductivity material such as
aluminum or copper. Lower power versions of the unit (115v.) employ
a chopper inverter circuit utilizing a single SCR and feedback
diode pair as the power controlling element of the circuit and
higher power versions (230v.) use two such SCR/diode pairs
connected in series for converting direct current potential to a
high frequency pulsating excitation potential that is supplied to
the induction heating coil of the unit. The pan safety control
feature senses whether the conduction interval of the SCR/diode
pair falls within preset limits which normally would occur with a
pan of suitable conductivity characteristics being heated by the
coil. Under conditions where a high conductivity pan such as
aluminum or copper is employed, the conduction interval of the
single SCR and diode pair will be shortened under conditions where
the circuit has been running and the high conductivity pan is
brought into proximity with the inductive heating coil. Under these
conditions, the pan safety control circuit senses the shortened
conduction interval of the SCR and feedback diode pair and shuts
down the chopper inverter. A second situation that can occur is
where the high conductivity pan is physically disposed over the
inductive heating coil at the time of initial turn-on of the
induction cooking unit. Under these conditions the pan safety
control senses a prolonged conduction interval falling outside a
preset limit and similarly shuts down the chopper inverter.
Inventors: |
Peters, Jr.; Philip H.
(Greenwich, NY) |
Assignee: |
Environment/One Corporation
(Schenectady, NY)
|
Family
ID: |
22928656 |
Appl.
No.: |
05/245,924 |
Filed: |
April 20, 1972 |
Current U.S.
Class: |
219/626; 219/665;
363/56.01 |
Current CPC
Class: |
H02M
7/5157 (20130101); H02H 7/122 (20130101); H02M
5/45 (20130101); H05B 6/06 (20130101) |
Current International
Class: |
H02M
7/515 (20060101); H02M 5/00 (20060101); H02H
7/122 (20060101); H05B 6/06 (20060101); H02M
7/505 (20060101); H02M 5/45 (20060101); H05B
6/00 (20060101); H05b 005/04 () |
Field of
Search: |
;219/10.49,10.75,10.77,10.79 ;321/4,11,18,24,45 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Staubly; R. F.
Assistant Examiner: Reynolds; B. A.
Claims
What is claimed is:
1. An improved induction cooking unit power supply system including
in combination inverter circuit means comprised by a controlled
conductivity control device for converting unidirectional potential
to a high frequency pulsating excitation potential, the frequency
and period of which is determined by the conduction interval of the
controlled conductivity control device, said inverter circuit means
including as a component thereof an induction heating coil that is
excited by the high frequency pulsating excitation potential, and
pan safety control circuit means synchronously operable with said
inverter circuit means for assuring safe operation of said inverter
circuit means, said pan safety control circuit means comprising
sampling means responsive to conduction through the controlled
conductivity control device for deriving an indication of the
conduction intervals of the control conductivity control device,
conduction interval limit setting means responsive to conduction
through the controlled conductivity control device for defining
safe limits to the respective conduction intervals for the control
conductivity control device, comparison means responsive to said
sampling means and said conduction interval limit setting means for
comparing the actual conduction intervals to the preset safe
limits, and safety means responsive to the output from the
comparison means and controlling operation of said inverter circuit
means in response to the actual conduction of the controlled
conductivity control device failing to conform to the preset safe
limits.
2. An improved induction cooking unit power supply system according
to claim 1 wherein the inverter circuit means comprises a
chopper-inverter circuit means utilizing a silicon controlled
rectifier having a feedback diode connected in inverse parallel
circuit relationship therewith across a pair of power supply
terminals and having a gating circuit for controlling the turn-on
of the silicon controlled rectifier coupled to the gating
electrode, said self-excited gating circuit including a built-in
low voltage gating circuit power source that energizes the gating
circuit and this is connected across and derives its power from the
power supply terminals connected across the silicon controlled
rectifier device, and wherein said safety means comprises a fast
responding, gate control conduction device connected across the
gating circuit power source in parallel circuit relationship
therewith for shunting the gating circuit power source in response
to an output signal from the comparison means and thereby shut down
the chopper-inverter.
3. An improved induction cooking unit power supply system according
to claim 2 further including full wave rectifier means connected to
and supply the chopper-inverter circuit means and adapted for
excitation from a conventional residential source of alternating
current electric energy with said pair of power supply terminals
connecting the output from said full wave rectifier means across
the load terminals of the silicon controlled rectifier and parallel
inverse connected feedback diode, a filter inductor connected in at
least one of said power supply terminals intermediate the output of
the full wave rectifier means and the chopper-inverter circuit
means, a filter capacitor connected across the output of the full
wave rectifier means intermediate the full wave rectifier and the
filter inductor, and said chopper-inverter circuit means further
comprising at least a commutating inductor and series connected
commutating capacitor with the series connected commutating
inductor and commutating capacitor being connected across the power
supply terminals in parallel circuit relationship across the
silicon controlled rectifier and parallel inverse connected
feedback diode.
4. An improved induction cooking unit power supply system according
to claim 3 further including a radio frequency suppression inductor
and series connected capacitor connected in parallel circuit
relationship across the commutating capacitor, the radio frequency
suppression inductor also comprising the induction heating
coil.
5. An improved induction cooking unit power supply system according
to claim 4 further comprising zero point switch means connected in
at least one of said power supply terminals intermediate the output
from the full wave rectifier and the filter inductor and filter
capacitor for controlling energization of the filter capacitor and
the chopper-inverter circuit means, and zero point switching
control means connected to and controlling operation of said zero
point switch means for assuring soft starting of the
chopper-inverter circuit means upon initially being placed in
operation and for further reducing radio frequency interference
effects of the circuit.
6. An improved induction cooking unit power supply system according
to claim 5 further including power control means for changing the
level of output power derived from the chopper-inverter circuit
means by varying the commutating frequency of the chopper-inverter
circuit means and hence the conduction intervals of the silicon
controlled rectifier, and means for auto-matically changing the
limit setting of said conduction interval limit setting means
simultaneously with each change in the conduction interval of the
silicon controlled rectifier by said power control means.
7. An improved induction cooking unit power supply system according
to claim 6 further including over-temperature sensing and control
means for sensing an over-temperature condition in a pan or other
object being heated by the induction cooking unit, and inhibit
means for inhibiting further operation of said zero point switching
control means in response to the output of an over-temperature
condition alarm signal from said over-temperature sensing and
control means, said inhibit means also being responsive to the
output from said comparison means in said pan safety control
circuit means for shutting off said zero point switch means in
response to an alarm output signal from said comparison means, and
common resetting switching means manually controlled by an operator
of the induction cooking unit for resetting both said pan safety
control and said over-temperature sensing and control means after
actuation thereof.
8. An improved induction cooking unit power supply system according
to claim 7 further including pan temperature control means for
directly sensing the temperature of a pan or other metal base
cooking vessel being heated by the induction cooking unit and
having its output coupled to and controlling operation of said
inhibit means for controlling operation of the zero point switching
means in response to the sensed pan temperature.
9. An improved induction cooking unit power supply system according
to claim 5 further including inhibit means for inhibiting further
operation of said zero point switching control means in response to
the output from said comparison means in said pan safety control
circuit means, said inhibit means serving to shut-off said
chopper-inverter circuit means in response to an alarm output
signal from said comparison means, and resetting switching means
manually controlled by an operator of the induction cooking unit
for resetting said pan safety control circuit means after actuation
thereof.
10. An improved induction cooking unit power supply system
according to claim 1 further including power control means for
changing the level of output power derived from the
chopper-inverter circuit by varying the commutating frequency of
the chopper-inverter circuit means and hence the conduction
intervals of the silicon controlled rectifier, and means for
automatically changing the limit setting of said conduction
interval limit setting means simultaneously with each change in the
conduction interval of the silicon controlled rectifier by said
power control means.
11. An improved induction cooking unit power supply system
according to claim 1 wherein said sampling means comprises first
and second differentiator circuit means responsive to the
conduction of the controlled conductivity control device for
developing first and second timing signal pulses representative of
the start and the termination of the conduction interval,
respectively, said conduction interval limit setting means
comprises monostable multi-vibrator circuit means responsive to the
first timing signal pulse for generating after a preset delay
period a pedestal gating signal pulse of preset time duration
definitive of the preset safe period of condution limits, said
comparison means comprises coincidence circuit means responsive to
the pedestal gating signal pulse and to said second timing signal
pulse for determining whether said second timing signal pulse
occurs within the safe limits defined by the delayed pedestal
gating signal pulse, and said safety means is connected to and
controlled by the output from said coincidence circuit means.
12. An improved induction cooking unit power supply system
according to claim 11 wherein the inverter circuit means comprises
a chopper-inverter circuit means utilizing a silicon controlled
rectifier having a feedback diode connected in inverse parallel
circuit relationship therewith across a pair of power supply
terminals and having a gating electrode, a self-excited gating
circuit for controlling the turn-on of the silicon controlled
rectifier coupled to the gating electrode, said self-excited gating
circuit including a built-in low voltage gating circuit power
source that energizes the gating circuit and that is connected
across and derives its power from the power supply terminals
connected across the silicon controlled rectifier device, and
wherein said safety means comprises a fast responding gate control
conduction device connected across the gating circuit power source
in parallel circuit relationship therewith for shunting the gating
circuit power source in response to an output signal from the
coincidence circuit means and thereby shut down the
chopper-inverter.
13. An improved induction cooking unit power supply system
according to claim 12 further including full wave rectifier means
connected to and supplying the chopper-inverter circuit means and
adapted for excitation from a conventional residential source of
alternating current electric energy with said pair of power supply
terminals connecting the output from said full wave rectifier means
across the load terminals of the silicon controlled rectifier and
parallel inverse connected feedback diode, a filter inductor
connected in at least one of said power supply terminals
intermediate the output of the full wave rectifier means and the
chopper-inverter circuit means, a filter capacitor connected across
the output of the full wave rectifier means intermediate the full
wave rectifier means and the filter inductor, and wherein said
chopper-inverter circuit means further comprises at least a
commutating inductor and series connected commutating capacitor
with the series connected commutating inductor and commutating
capacitor being connected across the power supply terminals in
parallel circuit relationship with the silicon controlled rectifier
and parallel inverse connected feedback diode and, a radio
frequency suppression inductor and series connected capacitor
connected in parallel circuit relationship across the commutating
capacitor with the radio frequency suppression inductor also
comprising the induction heating coil.
14. An improved induction cooking unit power supply system
according to claim 13 further comprising zero point switch means
connected in at least one of said power supply terminals
intermediate the output from the full wave rectifier and the filter
inductor and filter capacitor for controlling energization of the
filter capacitor and the chopper-inverter circuit means, and zero
point switching control means connected to and controlling
operation of said zero point switch means for assuring soft
starting of the chopper-inverter circuit means upon initially being
placed in operation and for further reducing radio frequency
interference effects of the circuit.
15. An improved induction cooking unit power supply system
according to claim 14 further including power control means for
changing the level of output power derived from the
chopper-inverter circuit means by varying the commutating frequency
of the chopper-inverter circuit means and hence the conduction
intervals of the silicon controlled rectifier, and means for
automatically changing the limit setting of said conduction
interval limit setting means simultaneously with each change in the
conduction interval of the silicon controlled rectifier by said
power control means.
16. An improved induction cooking unit power supply system
according to claim 15 further including over-temperature sensing
and control means for sensing an over-temperature condition in a
pan or other object being heated by the induction cooking unit, and
inhibit means for inhibiting further operation of said zero point
switching control in response to the output of an over-temperature
condition alarm signal from said over-temperature sensing and
control means, said inhibit means also being responsive to the
output from said comparison means in said pan safety control
circuit means for shutting-off said zero point switch means in
response to an alarm output signal from said comparison means, and
common resetting switching means manually controlled by an operator
of the induction cooking unit from resetting both said pan safety
control and said over-temperature sensing and control means after
actuation thereof.
17. An improved induction cooking unit power supply system
including in combination inverter circuit means connected across a
pair of power supply terminals for converting a supply potential
applied across the power supply terminals to a high frequency
pulsating excitation potential the frequency and period of which is
determined by the conduction intervals of a controlled conductivity
control device that comprises a part of the inverter circuit means,
said inverter circuit means including as a component thereof an
induction heating coil that is excited by the high frequency
pulsating excitation potential, and pan safety control circuit
means synchronously operable with the inverter circuit means for
assuring safe operation of the inverter circuit means, said pan
safety control circuit means comprising a low voltage pan safety
circuit power source that is connected across and derives its power
from the pair of power supply terminals, monostable multi-vibrator
circuit means energized by said low voltage pan safety circuit
power source and having a presetable period of operation for
deriving a pedestal limit setting signal pulse of preset pulse
duration after a preset period of delay upon the monostable
multi-vibrator circuit means being triggered from its stable to its
unstable state of operation, first sampling means for deriving a
first timing signal pulse representative of the start of each
conduction interval of the controlled conductivity control device
and for supplying the same to the monostable multi-vibrator circuit
means to control operation thereof, second sampling means for
deriving a second timing s1gnal pulse representative of the end of
each conduction interval of the controlled conductivity control
device, coincidence circuit means energized by said low voltage pan
safety circuit power source, means for supplying said pedestal
limit setting signal pulse and said second timing signal pulse to
said coincidence circuit means for deriving an output control
signal from said coincidence circuit means upon the second timing
signal pulse failing to conform to the preset safe limits defined
by the pedestal limit setting signal pulse, and safety circuit
means responsive to the output from said coincidence circuit means
for shutting down said inverter circuit means in response to an
output control signal from the coincidence circuit means.
18. An improved induction cooking unit power supply according to
claim 17 wherein said monostable multi-vibrator circuit means
comprising a first NPN junction transistor connected in series
circuit relationship with first voltage dividing resistor means
across the low voltage pan safety circuit power source, a first PNP
feedback transistor connected in series circuit relationship with a
second set of voltage dividing resistors across the low voltage pan
safety circuit power source and having the base thereof connected
to an intermediate point on the first mentioned voltage dividing
resistor means, a first programmable unijunction transistor having
its anode gate connected to an intermediate point on said second
set of voltage dividing resistors, a series connected first
variable resistor and capacitor circuit connected in parallel
circuit relationship with the second set of voltage dividing
resistors and having the junction of the variable resistor and
capacitor connected to the gate of the programmable unijunction
transistor, said last mentioned series connected resistor and
capacitor circuit being variable to control the turn-on time of
said first programmable unijuntion transistor, and a load resistor
connecting the cathode of the first programmable unijunction
transistor to the negative terminal of the low voltage pan safety
circuit power source, a second NPN junction transistor connected in
series circuit relationship with a third set of voltage dividing
resistors across the low voltage pan safety circuit power source,
the load resistor of the first programmable unijunction transistor
being connected through a current limiting resistor to the base of
the second NPN junction transistor, a second PNP feedback
transistor connected in series circuit relationship with a fourth
set of voltage dividing resistors between the base of the second
NPN junction transistor and the positive terminal of the low
voltage pan safety circuit power source, the base of the second PNP
feedback transistor being connected to an intermediate point on the
third set of voltage dividing resistors, and a second programmable
unijunction transistor device having its anode connected through a
second variable resistor to the collector of the second PNP
feedback transistor and to a timing capacitor connected in series
circuit relationship with the second variable resistor and the
collector-emitter of the second PNP feedback transistor across the
low voltage pan safety circuit power source, said second
programmable unijunction transistor having its anode gate connected
to an intermediate point on the fourth set of voltage dividing
resistors and having its cathode connected directly to the negative
terminal of the low voltage pan safety circuit power source, the
delayed pedestal timing signal pulse output from the monostable
multi-vibrator circuit means being derived from the collector of
the second PNP feedback transistor after a delay period determined
by the setting of the first variable resistor and with the duration
of the delayed pedestal timing signal pulse being determined by the
setting of the second variable resistor, the trigger input signal
to the monostable multi-vibrator circuit means being coupled to the
base of the first NPN junction transistor through a differentiator
network supplied through a voltage dividing resistor with the
potential appearing across the controlled conductivity control
device.
19. An improved induction cooking unit power supply according to
claim 18 wherein said coincidence circuit means comprises a set of
two PNP transistors having their emitter-collector circuits
connected in series circuit relationship with a load resistor
across the low voltage pan safety circuit power source, the base of
one of the PNP transistors being connected through a current
limiting resistor to the output from the collector of the second
PNP feedback transistor in the monostable multi-vibrator circuit
means and the base of the remaining PNP transistor being connected
through an isolating diode to the output of a differentiator
network supplied through a voltage dividing resistor with the
potential appearing across the controlled conductivity control
device, the output from the coincidence circuit being derived from
the load resistor connected in series circuit relationship with the
two PNP transistors, and being applied to the gating electrode of a
latching silicon controlled rectifier that comprises the safety
circuit means and that is connected in parallel circuit
relationship with the self-excited low voltage gating circuit power
supply for the main power silicon controlled rectifier and feedback
diode pair through an isolating diode.
20. An improved induction cooking unit power supply system
according to claim 19 wherein the inverter circuit means comprises
a chopper-inverter and the controlled conductivity control device
comprises a silicon controlled rectifier having a feedback diode
connected in inverse parallel circuit relationship therewith across
the pair of power supply terminals, said silicon controlled
rectifier having a gating electrode, a self-excited gating circuit
coupled to the gating electrode for controlling the turn-on of the
silicon controlled rectifier, said self-excited gating circuit
including a built-in low voltage gating circuit power source that
energizes the gating circuit and that is connected across and
derives its power from the power supply terminals connected across
the silicon controlled rectifier device, and wherein said safety
means comprised by the latching silicon controlled rectifier is
connected across the low voltage gating circuit power source in
parallel circuit relationship therewith for shunting the low
voltage gating circuit power source in parallel circuit
relationship therewith for shunting the low voltage gating circuit
power source in response to an output signal from the coincidence
circuit and thereby shut down the chopper-inverter.
21. An improved induction cooking unit power supply system
according to claim 20 further including full wave rectifier means
connected to and supplying the chopper-inverter circuit means and
adapted for excitation from a conventional residential source of
alternating current electric energy with said pair of power supply
terminals connecting the output from said full wave rectifier
across the silicon controlled rectifier and parallel inverse
connected feedback diode, a filter inductor connected in at least
one of said power supply terminals intermediate the output of the
full wave rectifier means and the chopper-inverter circuit means, a
filter capacitor connected across the output of the full wave
rectifier means intermediate the full wave rectifier and the filter
inductor, and wherein said chopper-inverter circuit means further
comprises at least a commutating inductor and series connected
commutating capacitor with the series connected commutating
inductor and commutating capacitor being connected across the power
supply terminals in parallel circuit relationship across the
silicon controlled rectifer and parallel inverse connected feedback
diode, and a radio frequency suppression inductor and series
connected capacitor connected in parallel circuit relationship
across the commutating capacitor with the radio frequency
suppression inductor also comprising the induction heating
coil.
22. An improved induction cooking unit power supply system
according to claim 21 further including zero voltage switch means
connected in at least one of said power supply terminals
intermediate the output from the full wave rectifier means and the
filter inductor and filter capacitor for controlling energization
of the filter capacitor and the chopper-inverter circuit means, and
zero point switching control means connected to and controlling
operation of said zero point switching means for assuring soft
starting of the chopper-inverter circuit means upon intially being
placed in operation and for further reducing radio frequency
interference effects of the circuit.
23. An improved induction cooking unit power supply system
according to claim 22 further including power control means for
changing the power level output derived from the circuit by varying
the frequency of operation of the chopper-inverter circuit means
through varying the conduction intervals of the silicon controlled
rectifier, means for automatically changing the limit setting of
said conduction interval limit setting means simultaneously with
each change in the conduction interval of the silicon control
rectifier by said power control means, over-temperature sensing and
control means for sensing an over-temperature condition in a pan or
other object being heated by the induction cooking unit, means for
inhibiting further operation of said zero point switching control
in response to the output of an over-temperature condition alarm
signal from said over-temperature sensing means, and common
resetting switching means manually controlled by an operator of the
induction cooking unit for resetting both said pan safety control
and said over-temperature sensing and control means after actuation
thereof.
24. A pan safety control circuit for an induction cooking unit
inverter circuit having a controlled conductivity control device
for assuring safe operation of the inverter circuit, said pan
safety control comprising sampling means responsive to conduction
through the controlled conductivity control device for deriving an
indication of the conduction intervals of the control conductivity
control device, conduction interval limit setting means responsive
to conduction through the controlled conductivity control device
for defining safety limits to the respective conduction intervals
for the control conductivity control device, comparison means
responsive to said sampling means and said conduction interval
limit setting means for comparing the actual conduction intervals
to the preset safe limits, and safety means responsive to the
output from the comparison means and controlling operation of the
inverter circuit for shutting down operation of the inverter
circuit in response to the actual conduction of the controlled
conductivity control device failing to conform to the preset safe
limits.
25. A pan safety control circuit for an induction cooking unit
inverter circuit according to claim 24 wherein the sampling means
comprises first and second differentiator circuit responsive to the
conduction of the controlled conductivity control device for
developing first and second timing signal pulses representative of
the start and the termination of the condution interval,
respectively, said conduction interval limit setting means
comprises monostable multi-vibrator circuit means responsive to the
first timing signal pulse for generating after a preset delay
period a timing pedestal gating signal pulse of preset time
duration definitive of the preset safe period of conduction limits,
said comparison means comprises coincidence circuit means
responsive to the pedestal gating signal pulse and to said second
timing signal pulse for determining whether said second timing
signal pulse occurs within the safe limits defined by the delayed
pedestal gating signal pulse, and said safety means is connected to
and controlled by the output from said coincidence circuit
means.
26. A pan safety control circuit for assuring safe operation of an
induction cooking unit power inverter circuit having a silicon
controlled rectifier supplied from a pair of power supply
terminals, said pan safety control comprising a low voltage pan
safety circuit power source that is connected across and derives
its power from the pair of power supply terminals, monostable
multi-vibrator circuit means energized by said low voltage pan
safety circuit power source and having a presetable period of
operation for deriving a pedestal limit setting signal pulse of
preset pulse duration after a preset period of delay upon the
monostable multi-vibrator circuit means being triggered from its
stable to its unstable state of operation, first sampling means for
deriving a first timing signal pulse representative of the start of
each conduction interval of the silicon controlled rectifier and
for supplying the same to the monostable multi-vibrator circuit
means to control operation thereof, second sampling means for
deriving a second timing signal pulse representative of the end of
each conduction interval of the silicon controlled rectifier and
feedback diode pair, coincidence circit means energized by said low
voltage pan safety circuit power source, means for supplying said
pedestal limit setting signal pulse and said second timing signal
pulse to said coincidence circuit means for deriving an output
control signal from said coincidence circuit means upon the second
timing signal pulse failing to conform to the preset safe limits
defined by the pedestal limit setting signal pulse, and safety
circuit means responsive to the output said coincidence circuit
means for shutting down said inverter circuit in response to an
output control signal from the coincidence circuit means.
27. A pan safety control circuit for an induction cooking unit
power inverter according to claim 26, wherein said monostable
multi-vibrator circuit means comprising a first NPN junction
transistor connected in series circuit relationship with first
voltage dividing resistor means across the low voltage pan safety
control circuit power source, a first PNP feedback transistor
connected in series circuit relationship with a second set of
voltage dividing resistors across the low voltage pan safety
circuit power source and having the base thereof connected to an
intermediate point on the first mentioned voltage dividing resistor
means, a first programmable unijunction transistor having its anode
gate connected to an intermediate point on said second set of
voltage dividing resistors, a series connected first variable
resistor and capacitor circuit connected in parallel circuit
relationship with the second set of voltage dividing resistors and
having the junction of the variable resistor and capacitor
connected to the gate of the programmable unijunction transistor,
said variable resistor and capacitor circuit being variable to
control the turn-on time of said first programmable unijunction
transistor, and a load resistor connecting the cathode of the first
programmable unijunction transistor to the negative terminal of the
low voltage pan safety control circuit power source, a second NPN
junction transistor connected in series circuit relationship with a
third set of voltage dividing resistors across the low voltage pan
safety control circuit power source, the load resistor of the first
programmable unijunction transistor being connected through a
current limiting resistor to the base of the second NPN junction
transistor, a second PNP feedback transistor connected in series
circuit relationship with a fourth set of voltage dividing
resistors between the base of the second NPN junction transistor
and the positive terminal of the low voltage pan safety control
circuit power source, the base of the second PNP feedback
transistor being connected to an intermediate point on the third
set of voltage dividing resistors, and a second programmable
unijunction transistor device having its anode connected through a
second variable resistor to the collector of the second PNP
feedback transistor and to a timing capacitor connected in series
circuit relationship with the second variable resistor and the
collector-emitter circuit of the second PNP feedback transistor
across the low voltage pan safety control circuit power source,
said second programmable unijunction transistor having its anode
gate connected to an intermediate point on the fourth set of
voltage dividing resistors and having its cathode connected
directly to the negative terminal of the low voltage pan safety
control circuit power source, the delayed pedestal timing signal
pulse output from the monostable multi-vibrator circuit means being
derived from the collector of the second PNP feedback transistor
after a delay period determined by the setting of the first
variable resistor and with the duration of the delayed pedestal
timing signal pulse being determined by the setting of the second
variable resistor, the trigger input signal to the monostable
multi-vibrator circuit means bieng coupled to the base of the first
NPN junction transistor through a differentiator network supplied
through a voltage dividing resistor with the potential appearing
across the silicon controlled rectifier and feedback diode
pair.
28. A pan safety control circuit for an induction cooking unit
power inverter according to claim 27 wherein said coincidence
circuit means comprises a set of two PNP transistors having their
emitter-collector circuits connected in series circuit relationship
with a load resistor across the low voltage pan safety control
circuit power source, the base of one of the PNP transistors being
connected through a current limiting resistor to the output from
the collector of the second PNP feedback transistor in the
monostable multi-vibrator circuit means and the base of the
remaining PNP transistor being connected through an isolating diode
to the output of a differentiator network supplied through a
voltage dividing resistor with the potential appearing across the
silicon controlled rectifier and feedback diode pair, the output
from the coincidence circuit being derived from the load resistor
connected in series circuit relationship with the two PNP
transistors, and applied to the gating electrode of a latching
silicon control rectifier that comprises the safety means and is
connected in parallel circuit relationship with the self-excited
low voltage gating circuit power supply for the main power silicon
controlled rectifier and feedback diode pair through an isolating
diode.
29. An improved induction cooking unit power supply comprising
inverter circuit means including gate controlled power thyristor
means coupled to and supplying an induction heating coil load with
periodic energization currents, rectifier means for supplying a
rectified undulating semi-filtered unidirectional high voltage
excitation potential for said inverter circuit means and gating
circuit means for developing gating signal pulses for application
to the control gate of the gate controlled power thyristor means
comprising a part of the inverter circuit means, selectively
operable zero point switching means connected intermediate the
rectifier means and the inverter circuit means for selectively
supplying energizing potential from the rectifier means to the
inverter circuit means, zero point switching control means for
selectively turning-on said zero point switching means at or near
the beginning of each cycle of the rectified undulating
semi-filtered unidirectional excitation potential, and pan safety
control means responsive to the conductivity characteristics of the
metal base cookware being inductively heated for controlling
operation of said zero point switching control means for assuring
safe operation of said inverter circuit means.
30. An improved inductive cooking unit power supply according to
claim 29 further including start-up delay inhibit control means for
inhibiting operation of said zero point switching control means,
and control means for applying an on-off control signal to the
inhibit control means for controlling operation of said inhibit
control means.
31. An improved inductive cooking unit power supply according to
claim 30 wherein said zero point switching means comprises a
silicon control rectifier connected in one of the power supply
leads from the rectifier means to the inverter circuit means, said
zero point switching conrol means comprises voltage divider means
connected across the output of the rectifier means intermediate the
rectifier means and the zero point switching means and a low
voltage pilot switching silicon control rectifier having a control
gate connected to the voltage divider and havng its output
connected to supply gating current to the zero point switching
silicon control rectifier, and said start-up delay inhibit control
means comprises an inhibit latching silicon control rectifier
having its gate connected to its anode and connected across at
least a portion of said voltage divider means for clamping off the
supply of gating potential to the pilot switching silicon control
rectifier, an inhibit control transistor for selectively shunting
the control gate of the inhibit latching silicon control rectifier,
a delay timing resistor capacitor charging circuit connected to
control the inhibit control transistor, inhibiting transistor means
responsive to on-off control signals connected to control the
charging of the capacitor in the resistor-capacitor charging
circuit, and discharge transistor means connected across the delay
timing capacitor for discharging the delay timing cpacitor upon the
circuit being turned-off following a period of operation.
32. An improved induction cooking unit power supply according to
claim 31 wherein the gate controlled power thyristor means
comprises at least two series connected SCR/feedback diode pairs
connected to and conrolling excitation of the inverter circuit
means and supplied by said rectifier means with the gate electrodes
of the SCRs being supplied in parallel with the output from the
gating signals output circuit means whereby the power supply is
capable of operation at higher power levels.
33. An improved inductive cooking unit power supply according to
claim 31 further including capacitor switching means operable
during non-conducting intervals of the inverter circuit means to
switch different values of commutating capacitance into effective
circuit relationship in the inverter circuit means to thereby
control the power generated by the circuit, and inhibit switching
ganged to operate in conjunction with said capacitor switching
means for applying inhibit control potentials to the inhibiting
transistor means for maintaining the zero point switch silicon
control rectifier turned-off while the capacitor switching means is
being switched to change the power output of the inductive cooking
unit.
34. An improved induction cooking unit power supply according to
claim 29 wherein said gating circuit means includes means for
deriving a low voltage unidirectional self-excitation potential
from the high voltage rectifier unfiltered unidirectional
excitation potential supplied from said rectifier means, timing
circuit charging means supplied by said low voltage excitation
potential, voltage responsive switching means connected to and
controlled by said timing circuit charging means, gate controlled
semiconductor switching means connected intermediate said low
voltage excitation potential and said timing circuit charging means
for controlling application of said low voltage excitation
potential to said timing circuit charging means, the control gate
of said semiconductor switching means being coupled to and
controlled by the voltage across the gate controlled power
thyristor means for controlling supply of the low voltage
excitation potential to the timing circuit charging means, said
voltage responsive switching means being rendered conductive upon
the voltage of said timing circuit charging means attaining a
preset value, a second gate controlled semiconductor switching
means having a relatively flat current vs voltage conducting
characteristic and having its load terminals connected through
suitable voltage dividing means across the output from the
rectifier means, the control gate of said second constant current
semiconductor switching means being connected to and supplied by
the voltage responsive switching means, and gating signal output
circuit means connected in series circuit relationship with the
constant current high voltage switching means and coupled to the
control gate of the gate controlled power thyristor means.
35. An improved inductive cooking unit power supply according to
claim 29 wherein the inverter circuit means is a chopper-inverter
comprised by a filter inductor having an inductance L.sub.2, at
least two series connected bidirectional conducting gate control
semiconductor thyristor switching devices connected in series
circuit relationship with the filter inductor across the output
from said rectifier means with the filter inductor interposed
between the thyristor switching devices and the rectifier means, a
commutating inductor having an inductance L.sub.1 and commutating
capacitor having a capacitance C.sub.1 connected in series circuit
relationship across the thyristor switching devices and tuned to
series resonance at a desired commutating frequency that provides a
combined thyristor conduction and commutating period t.sub.1 during
each cycle of operation, means for deriving output power coupled to
at least one of the commutating components, the gating circuit
means being coupled to the control gate of the thyristor switching
device for rendering the device conductive at a controlled
frequency of operation that provides an operating period T for the
chopper-inverter including a quiescent charging period t.sub.2 in
each cycle of operation where T = t.sub.1 + t.sub.2 such that the
vlue w.sub.2 t.sub.2 equals substantially .pi./2 radians or greater
where w.sub.2 = 1/.sqroot.L.sub.2 C.sub.1 whereby the reapplied
forward voltage across the semiconductor thyristor switching
devices following each conduction interval is maintained
substantially independent of load.
36. An improved inductive cooking unit power supply according to
claim 35 further including a smoothing inductor having an
inductance L.sub.3 and a smoothing capacitor having a capacitance
C.sub.3 that are connected in series circuit relationship across at
least one of the commutating components and have impedance values
such that the combined impedance of the commutating capacitor, the
smoothing inductor and the smoothign capacitor is capacitive in
nature and series resonates with the commutating inductor to
establish the commutating period t.sub.1, and wherein the smoothing
inductor and capacitor shape the output current flowing through the
smoothing inductor to substantially a sinusoidal waveshape having
little or no radio frequency interference emission effects, and the
smoothing inductor is an inductive heating coil.
37. An improved inductive cooking unit power supply according to
claim 35 wherein said safety control means for assuring safe
operation of the inverter circuit means comprises sampling means
responsive to conduction through the controlled conductivity
silicon control rectifier and feedback diode pair control device
for deriving an indication of the conduction intervals of the
silicon control rectifier and feedback diode pair, conduction
interval limit setting means responsive to conduction through
silicon control rectifier and feedback diode pair for defining
safety limits to the respective conduction intervals therefore,
comparison means responsive to said sampling means and said
conduction interval limit setting means for comparing the actual
conduction intervals to the preset safe limits, and safety means
responsive to the output from the comparison means and controlling
operation of the inverter circuit means for shutting down operation
of the inverter circuit means in response to the actual conduction
of the silicon control rectifier and feedback diode pair failing to
conform to the preset safe limits.
Description
BACKGROUND OF INVENTION
1. Field of Invention
This invention relates to a new and improved induction heating and
cooking apparatus of the type using an inductive heating coil
excited with high frequency electric currents for inductively
heating pans or other similar cooking vessels placed in proximity
to the coil.
More particularly, the invention relates to such an inductive
cooking apparatus and power supply therefore having an improved pan
safety control for sensing whether a pan or other cooking vessel
placed over the induction heating coil of the apparatus is composed
of a suitable material which allows the apparatus to operate
properly and safely, and if not, then shuts down the apparatus
thereby notifying the operator that the pan is not suitable for use
with an inductive cooking apparatus, and should be removed.
2. Prior Art Situation
In U.S. application Ser. No. 131,648--filed Apr. 6, 1971, now U.S.
Pat. No. 3,710,062, entitled "Metal Base Cookware Induction Heating
Apparatus Having Improved Power Supply and Gating Control Circuit
Using Infra-Red Temperature Sensor and Improved Heating Coil
Arrangement"--Philip H. Peters, Jr.--inventor-- assigned to the
Environment/One Corporation-applicant has described a novel metal
base cookware induction heating apparatus having an improved power
supply comprised by a chopper inverter circuit using a shunt fed
single SCR feedback/diode pair and series connected inductor and
capacitor commutating components for converting direct current
potential to a high frequency pulsating excitation potential that
is supplied to an induction heating coil that comprises a component
of the chopper inverter circuit. In this chopper inverter circuit,
the gating pulse that is applied to the control gate of the chopper
SCR/feedback diode pair is developed by a self-excited t.sub.2
timer circuit operating off of the supply terminal buses supplying
the SCR chopper inverter. The SCR chopper inverter is designed to
operate at some preset t.sub.1 commutation time determined by the
value of the commutating components of the circuit, and the trigger
gating pulse developed by the t.sub.2 timer gating circuit, is
developed after a fixed period t.sub.2 following each conduction
interval of the SCR-feedback diode pair. Thus, the repetition time
or rate T of the chopper inverter circuit is given by the
expression T = t.sub.1 + t.sub.2 and varies directly with
variations either in t.sub.1 or t.sub.2 . For a given SCR there is
a set of concurrent ratings at a given SCR device temperature which
determine the time of the commutation period below which the SCR
will fail to turn-off. For example, there are a number of SCR
devices on the market which at normal room temperature require that
the current through the device be below a nominal holding current
value for at least a period of 15 - 30 microseconds in order for
the device to turn-off and retain its current blocking capability.
If this minimum turn-off period is not provided for either in the
design of a circuit or in its subsequent operation, the SCR will
fail to turn-off (fail to commutate), and give rise to the
development of quite large currents flowing through the device in
excess of the device rating which can possibly destroy the
device.
It has been recognized by the inventor that when a copper or
aluminum or other pan or cooking vessel fabricated of high
conductivity metal, is placed over the inductive heating coil of an
induction heating apparatus such as that described in the above
mentioned pending U. S. application, the high electrical
conductivity of the pan or other cooking vessel greatly reduces the
inductive reactance of the inductive heating coil. This is in
contrast to stainless steel, titanium, iron and other lossy metal
materials which effect only a slight change in the inductance of
the inductive heating coil and serve to introduce primarily a
resistance in series with the coil. If the pan load imposed on the
inductive heating coil reduces the commutation period of the
overall chopper-inverter system including the inductive heating
coil below that which assures the SCR an adequate minimum turn-off
time, the SCR will remain on and the input current to the
chopper-inverter will rise to a very high value at the peak of the
line voltage. In the case of the above-noted circuit is limited
only in magnitude by the series choke coil L.sub.2 employed in the
circuit. In such event the main breaker, fuse or other protective
element included in the line supplying the chopper-inverter, will
trip and disconnect the chopper-inverter from the input supply
line. However, in any such failure, large transients of current are
developed as the stored energy in the inductive circuit is
dissipated, and it is possible to destroy the SCR in the process.
In order to overcome this problem, the present invention was
devised.
SUMMARY OF INVENTION
It is therefore a primary object of this invention to provide a new
and improved inductive cooking apparatus and power supply therefore
having a pan safety control which senses whether a particular pan
or other cooking vessel placed over the inductive heating coil of
the apparatus is fabricated from a high conductivity material such
as aluminum or copper, and if so, operates to shut down the
inductive cooking apparatus until the pan is removed by the
operator.
Another object of the invention is to provide such an inductive
cooking apparatus having a power supply and pan safety control
which may be reset readily by an operator of the unit, but which
also helps to school the operator to recognize that a particular
pan which he or she is using, is made of a high conductivity metal
such as copper or aluminum that should not be used with the
inductive cooking apparatus.
Still another object of the invention is to provide both low power
(115v.) and high power (230v.) models of an inductive cooking
apparatus, power supply and pan safety control having the above
capabilities which are fast responding and work under either of the
following conditions:
Case I -- The chopper-inverter power supply of the inductive
cooking apparatus is turned-on and running and a high conductivity
pan of aluminum, copper, etc. is brought into proximity with the
inductive heating coil by the operator; or
Case II -- The high conductivity pan is disposed over the inductive
heating coil upon initial turn-on of the inductive cooking
apparatus chopper-inverter.
A further object of the invention is the provision of an inductive
cooking apparatus having the above characteristics, and which also
include an over-temperature sensor and cutoff control that utilizes
a common reset connection with the pan safety control whereby
operation of the common reset connection readily can reset the
apparatus to condition it for renewed operation irrespective of
whether it was shut down because of the prior use of an improper
pan or over-temperature;
A still further object of the invention is to provide an induction
cooking apparatus and improved power supply therefore having a pan
safety control which is relatively simple to construct, operate and
service and is relatively inexpensive so that it can be employed as
a kitchen range in home use.
BRIEF DESCRIPTION OF DRAWINGS
These and other objects, features and many of the attendant
advantages of this invention will be appreciated more readily as
the same becomes better understood by reference to the following
detailed description, when considered in connection with the
accompanying drawings, wherein like parts in each of the several
figures are identified by the same reference character, and
wherein:
FIG. 1 of the drawings is a functional block diagram of a new and
improved induction heating and cooking apparatus having a pan
safety control constructed in accordance with the invention;
FIGS. 2A and 2B comprise a detailed schematic circuit diagram of a
lower power model (115v.) of the new and improved induction heating
cooking apparatus shown functionally in FIG. 1;
FIGS. 3(a) through 3(o) illustrate a series of voltage versus time
characteristic wave shapes produced at different points in the
circuit of FIG. 2 during the operation thereof; and
FIGS. 4A and 4B comprise a detailed schematic circuit diagram of a
higher power model (230v.) of the apparatus and which includes an
improved gating circuit for the chopper-inverter.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 is functional block diagram of a new and improved induction
cooking unit chopper-inverter power supply circuit including a pan
safety control constructed in accordance with the invention. The
circuit of FIG. 1 is intended to be energized from a conventional
commercial or residential 115 volt, 15-20amp, 60 cycles per second,
alternating power supply connected to the power supply terminals 11
and 12 through a conventional fuse plug or a circuit breaker such
as 13 preferably having a fast response time to assure protection
for the overall system. The alternating current power supply
terminals 11 and 12 are connected to a full wave rectifier 14 of
conventional construction having its output connected across a pair
of relatively high voltage (115 volts) direct current power supply
terminal means 15 and 16. It should be noted at this point that
while the circuit being described is intended primarily for use at
the lower voltage rating of 115 volts, 15-20amps, it may readily be
adapted for use at higher voltages such as 220-230 volts,
20-30amps, by appropriate modification to include high voltage
rated power switching devices as will be described hereinafter in
connection with FIG. 4. The output from the full wave rectifier 14
is unfiltered, and hence the potential appearing across the direct
current power supply terminals 15 and 16, while unidirectional, is
in the form of a series of sinusoidal-shaped, half wave rectified
high voltage pulses that drop substantially to zero value
intermediate each half wave pulse and have a frequency on the order
of 120 cycles per second or approximately double the frequency of
the alternating current supply connected to the input supply
terminals 11 and 12.
The full wave rectifier 14 supplies the full wave rectified
excitation potential for a chopper inverter circuit comprised by a
filter inductor L.sub.2, a filter capacitor C.sub.2, a
bidirectional conducting, gate controlled conductivity control,
semiconductor thyristor switching device formed by a power rated
silicon control rectifier 17 and reversely poled, parallel
connected feedback diode 18. The silicon control rectifier 17
(hereinafter referred to as a SCR) and feedback diode 18 are
connected across, and serve to excite at a relatively high
excitation frequency (of the order of 20-30 kilocycles per second)
chopper-inverter circuit commutating components 19 to the described
more fully hereinafter in connection with FIG. 2 of the drawings. A
conventional resistor-capacitor snubbing network for reducing dv/dt
effects on the SCR 17 (not shown in FIG. 1), also may be connected
across the SCR 17 and feedback diode 18. Energization of the
chopper-inverter circuit takes place through the filter capacitor
C.sub.2, and filter inductor L.sub.2 which preferably is connected
in the direct current power supply terminal 16, but alternately
could be connected in the power supply terminal 15. However, the
preferred location of the inductor L.sub.2 is as shown in FIGS. 1
and 2 whereby the SCR 17 anode and diode 18 may be connected to the
positive terminal 15 in order to minimize capacitive voltage
coupling effects to the heat sinks provided for these devices.
Energization of the high frequency chopper-inverter power supply
circuit takes place only during intervals while a self-starting,
zero point energization switching means in the form of a zero point
switching SCR 21, is conductive. The zero point switch comprised by
SCR 21 is rendered conductive by a zero point sensing turn-on
control 42 that in turn is controlled by a start-up delay inhibit
circuit 23 which in turn is under the control of a heating rate
control 25, and over-temperature control 35, and if provided, a pan
temperature control 24 or other similar control. The heating rate
control 25 operates to adjust the power level at which power is
produced by the chopper-inverter power supply circuit that in turn
supplies the high frequency pulses of electrical energy for
exciting the inductive heating coil that comprises a component of
the chopper-inverter power supply commutating circuit, and that
serves to inductively heat metal based cookware or other objects
disposed over the heating coil. The heating rate control 25
operates to establish the power level or rate at which heat will be
produced by the inductive heating coil in the metal based cookware.
The pan temperature control 24 (if used) directly senses the
temperature of the metal based cookware being inductively heated by
the inductive heating coil, and thereafter develops an on-off
control signal which controls operation of the start-up delay
inhibit circuit 23. Low voltage direct current excitation power to
operate the pan temperature control 24 and over-temperature control
35 is obtained directly from the output of full wave rectifier 14
as will be explained more fully hereinafter in connection with FIG.
2.
The soft start, zero-point switching SCR 21 does not, itself,
directly control gating-on of the chopper-inverter SCR 17, but only
serves to enable operation of the chopper-inverter by controlling
application of the high voltage, unidirectional excitation
potential from full wave rectifier 14 across the chopper-inverter
power supply terminals 15 and 16A. Thus, the voltage appearing
across power supply terminals 15 and 16 may or may not be present
across power supply terminals 15 and 16A dependent upon whether
zero point switch SCR 21 is conducting or not. The purpose of the
soft start, zero point switching control 22 is to assure that
energization potential is supplied across SCR 17 only at or near
the beginning of the rectified, substantially unfiltered,
sinusoidally-shaped, half wave rectified high voltage pulses
appearing at the output of full wave rectifier 14. In this manner,
surge charging of the filter capacitor C.sub.2 and the commutating
components of the chopper-inverter circuit with initial high
voltages, is avoided.
One of the undesirable consequences avoided by the use of the zero
point switching SCR 21, is possible misfiring of the
chopper-inverter SCR 17 due to the lack of a sufficient gating-on
signal at a desired turn-on point. This feature also assures that
the rate at which the applied voltage increases, is always limited
to a value which assures that the energy stored in the commutating
circuit components 19 will always be sufficient to commutate the
SCR 17 and diode 18 under all conditions of loading. This latter
condition requires that the commutating current flowing in the
commutating components be several fold greater than the current
I.sub.2 flowing through the inductor L.sub.2 at the moment of
commutation. A too rapid application of a supply voltage (such as
might occur if the circuit were initially turned-on at a point
corresponding to the peak output voltage from the full wave
rectifier 14), could result in a charging current I.sub.2 through
the conductor L.sub.2 which would be larger than the available
commutating current from the commutating components 19, and would
result in SCR 17 failing to turn-off. By appropriate design of the
start-up delay inhibit 23 and the zero point switching control 22,
proper delays are provided so as to assure proper gating-on and
commutation-off of the chopper-inverter switching SCR 17 at all
times including initial start-up of the circuit and under varying
load conditions.
Gating-on of the chopper-inverter switch SCR 17 is achieved through
the medium of a gating pulse transformer 31 whose secondary winding
is connected to the control gate of the chopper-inverter switching
SCR 17, and whose primary winding is supplied from a gating pulse
amplifier and DC supply circuit 32. The gating pulse amplifier 32
in turn is supplied from and controlled by a ramp delay and trigger
pulse generator circuit 33 for generating turn-on trigger pulses
that are amplified by the gating pulse amplifier 32 and supplied to
the control gate of SCR 17 through gating pulse transformer 31.
These trigger pulses have a repetition rate (which in one
embodiment of the invention can be varied within a predetermined
range determined by the setting of a variable timing resistor 34
for controlling output power from the circuit) and which determines
the operating frequency of the chopper inverter circuit. The ramp
delay and trigger pulse generator 33 is supplied by a conductor 91
that is connected to the right of the L.sub.2 filter inductor so
that the energizing potential supplied to the ramp delay and
trigger pulse generator 33 is the same potential that is
established across the chopper-inverter SCR/diode pair 17,18, and
is established only after the zero point switching SCR 21 has been
rendered conductive to supply enabling potential across the
chopper-inverter circuit. The built-in ramp charging delay of
circuit 33, and the nature of the gating-on pulses supplied from
amplifier 32 to the control gate of SCR 17, then assures the
production of a gating-on pulse which is of sufficient magnitude to
guarantee turn-on of the SCR 17 under all con-ceivable operating
conditions, and irrespective of loading on the induction heating
coil or the magnitude of the supply voltage at any moment.
As described more fully in the above-referenced copending U. S.
application Ser. No. 131,648, now U.S. Pat. No. 3,710,062, the
heating coil supplied by the chopper-inverter circuit is in the
form of a pancake-shaped, spiral inductive heating coil employed to
heat a metal base pan or other cookware physically positioned in
inductively coupled relation to the coil. The magnetic lines of
flux produced by the pancake-shaped inductive heating coil are
tightly coupled to and generate heat within the metal-based pan due
to the build-up and collapse of the magnetic lines of flux
inductively coupled to the metal-base pan at the relatively high
chopping rate or repetition frequency on the order of 20-30
kilocycles per second. More heat is produced per unit current at
higher frequencies than at lower frequencies in an inductively
heated load such as metal-base cookware due to the higher surface
resistivity of the metal at the higher frequency. The
pancake-shaped spiral configuration of the induction heating coil
provides very close magnetic coupling between the coil and the
metal based cookware placed in close proximity to the plane of the
coil. However, it has been determined that by proper design of the
coil, the radial magnetic field of such coil becomes
self-cancelling at relatively short distances away from the coil so
that the electro-magnetic radiation levels are kept low to thereby
minimize electro-magnetic interference (EMI) and radio frequency
interference (RFI) effects. If desired, such inductive heating
coils may be connected in series, in parallel, or in
series-parallel, to provide power output for multiple loads.
Upon initial start-up of the induction cooking unit shown in FIG.
1, the circuit breaker 13 is closed to supply alternating current
and excitation potential to the full wave rectifier 14. The
relatively high voltage, full wave rectified, unidirectional
potential appearing at the output of rectifier 14 is then supplied
across terminals 15 and 16 to the gating pulse amplifier and DC
supply 32, the start-up delay inhibit control circuit 23 and zero
point switching control circuit 22. However, at initial start-up,
the zero point switching SCR 21 will be maintained off due to the
delay-inhibit circuit 23 until power is called for. Consequently,
no voltage is developed across filter capacitor C.sub.2 until such
time that zero point switching SCR 21 is made conducting. At this
point, if the heating rate control has not been set previously,
heating rate control 25 is adjusted to provide a desired heating
rate and thereby control the amount of output power produced in
each output pulse of the chopper-inverter, and supplied through the
inductive heating coil. This heating rate control serves in much
the same manner as the flame control adjustment provided on certain
gas ranges to allow adjustment of the size of the heating flame,
and is in addition to any temperature control such as that shown at
24, that may be provided. If it is used, the desired temperature
setting may be made by appropriate adjustment to the pan
temperature control 24. Thereafter, the pan temperature control 24
will sense the temperature of the metal base pan or other cookware
being heated by the inductive heating coil, and will operate the
chopper-inverter in an on-off control manner through start-up delay
inhibit circuit 23, zero point switching control 22 and zero point
switching SCR 21, to maintain the temperature of the pan at or near
the set point determined by the setting of the pan temperature
control 24.
Assume that the initial start-up adjustments described above have
been made, and that the start-up delay inhibit circuit 23 has
enabled the zero point switching control 22 so as to allow it to
turn-on the zero point switching SCR 21 at or near the beginning of
a half wave of the rectified, unfiltered, high voltage output
excitation potential appearing across the supply terminals 15 and
16. Upon zero point switching SCR 21 being turned-on, the inverter
circuit commutating capacitor (to be described in connection with
FIG. 2) will begin to charge through filter inductor L.sub.2, and
voltage developed across inductor L.sub.2 (initially zero prior to
turn-on of SCR 21 as measured from terminal bus 15), will go
negative with respect to terminal bus 15 at the point of connection
of conductor 91. Thus, it will be appreciated that upon turn-on of
the zero point switching SCR 21, the right side terminal bus 16A
will go negative with respect to the positive terminal bus 15
thereby enabling the chopper-inverter circuit. The rate of rise of
voltage across the chopper-inverter circuit components, and
particularly the SCR 17 and diode 18 pair will be determined
primarily by the value of the filter capacitor C.sub.2, and the
dv/dt snubbing network comprising a series connected resistor and
capacitor normally connected across the chopper-inverter SCR 17 for
limiting this rate of rise of reapplied voltage to some predesigned
value consistent with the rating of the SCR.
The development of the voltage across the chopper-inverter in turn
enable ramp delay and trigger pulse generator 33 through conductor
91 so as to initiate operation of this circuit, and produce a
trigger pulse of precise timing but low voltage amplitude, at some
point in time subsequent to turn-on of zero point switching SCR 21,
as determined by the setting of the timing resistor 34. This low
voltage trigger pulse is then amplified by the gating pulse
amplifier 32 and supplied through gating pulse transformer 31 to
the control gate of the chopper-inverter switching SCR 17 to cause
it to turn-on. Thereafter, the SCR 17 and diode pair 18
automatically will be turned-on and commutated off at a relatively
high commutating frequency higher than the overall chopper-inverter
operating frequency, and as determined by the value of the
commutating components of the circuit in a manner well known in th
art. For so long as power is called for by the temperature sensor
circuit 24, the chopper-inverter will produce output pulses of
power at a level determined by the heating rate control 25 setting
and having an operating frequency of the order of 20-30 kilocycles
per second as determined by the repetition rate of the trigger
pulses generated by the ramp delay of trigger pulse generator 33.
This repetition rate in turn is determined by the setting of the
adjustable resistor 34 and other parameters of the chopper-inverter
circuit as discussed above. Upon reaching the set point
temperature, the pan temperature control 24 (if provided) will
cause the start-up delay inhibit circuit 23 to inhibit further
operation of the zero point switching control 22 so that zero point
switching SCR 21 is maintained off until such time that additional
heat is called for due to a drop in temperature of the pan or other
metal based cookware being inductively heated by the unit.
For a more detailed description of the construction and operation
of the chopper-inverter circuit, reference is made to the
above-identified copending U. S. application Serial No. 131,648,
now U.S. Pat. No. 3,710,062. Briefly, however, as will be described
more fully hereinafter, in connection with FIG. 2B, the
chopper-inverter is comprised by the filter inductor L.sub.2 which
has an inductance L.sub.2 which must be properly related to the
value of the commutating components. The SCR 17/diode 18 pair
comprise a bidirectional conducting gate control semiconductor
thyristor switching device that is connected in series circuit
relationship with the filter inductor across power supply terminals
15, 16A and hence filter capacitor C.sub.2 with the filter inductor
L.sub.2 being interposed between SCR 17 and diode 18 and the source
of direct current excitation potential comprised by capacitor
C.sub.2. A commutating inductor having inductance L.sub.1 and a
commutating capacitor having a capacitance C.sub.1 is connected in
series circuit relationship across the SCR 17/diode 18 pair, and
are tuned to series resonance at a desired commutating frequency
that provides a combined thyristor conduction and commutating
period t.sub.1 during each cycle of operation of the
chopper-inverter. Output power is derived from at least one of the
commutating components in a manner to be described hereinafter.
The gating circuit coupled to the control gate of the SCR 17
renders SCR 17 conductive at a controlled frequency of operation
that provides an operating period T for the chopper-inverter
circuit including a quiescent charging period t.sub.2 in each cycle
of operation where T = t.sub.1 + t.sub.2 such that the value
w.sub.2 t.sub.2 equals substantially .pi./2 radians or greater at
the operating frequency and where w.sub.2 equals /.sqroot.L.sub.2
C.sub.1. By designing the chopper-inverter in this manner, the
reapplied forward voltage appearing across the SCR/17 diode pair
following each conduction interfal will be maintained substantially
independent of load. A preferred embodiment of the chopper-inverter
also includes a smoothing inductor L.sub.3 and a smoothing
capacitor C.sub.3 connected in series circuit relationship across
one of the commutating components, either L.sub.1 or C.sub.1, in a
manner such that the combined impedance of the commutating
component, the smoothing inductor and the smoothing capacitor is
series resonant with the remaining commutating component at a
frequency which establishes the commutating period t.sub.1. In a
preferred arrangement, the smoothing inductor L.sub.3 comprises the
planar, inductive heating coil and in combination with the
smoothing capacitor C.sub.3 shapes the output current flowing
through the smoothing inductor (and hence inductive heating coil)
to substantially a sinusoidal wave shape producing little or no
spurious emission at radio frequencies above the operating
frequency of the inverter.
It has been determined, that where the pan or other metal based
cookware used in conjunction with the inductive heating coil, is
fabricated from stainless steel, iron, titanium, or other lossy
metal material (by lossy metal material is meant a metal material
having low electrical conductivity), then the effect of the pan is
to introduce a resistance in series with the coil with only a
minimal or slight change in inductance of the inductive heating
coil. However, this is not true of high conductivity pans
fabricated from materials having high electrical conductivity such
as aluminum, copper, alloys thereof, etc. Where such high
conductivity pans or cookware are placed over the inductive heating
coil, they result in greatly reducing the effective inductive
reactance of the coil. The value of inductance of the inductive
heating coil is reduced due to a reduction in the net flux produced
by the current flowing through the coil so that in effect, the high
conductivity pan or other utensil acts as a short circuited
secondary coil of a transformer having a primary coil with an
inductance equal to that normally of the inductive heating coil in
an unloaded condition. Measurements have shown that where utensils
fabricated from a para- or ferromagnetic material (lossy material)
are used, perhaps a one percent reduction in load coil inductance
occurs. In contrast where high conductivity pans are employed
having a magnetic permeability of unity, they very markedly reduce
the inductance of inductor L.sub.3 and reduce the effective
capacitance present at the terminals of capacitor C.sub.1 with the
result that the commutation period t.sub.1 is reduced. A similar
reduction in t.sub.1 is caused by such a high conductivity load
placed near inductor L.sub.1.
In the chopper-inverter circuit which excites the inductive heating
coil, the trigger pulse supplied to the SCR 17 is developed after a
fixed period t.sub.2 by a t.sub.2 trigger pulse generator 33, and
is initiated immediately following the t.sub.1 commutation time. As
set forth above the period T of the chopper-inverter circuit where
T = t.sub.1 + t.sub.2 varies directly as t.sub.1. Hence, a decrease
in the t.sub.1 commutation time caused by the use of a high
conductivity load results in a reduction of the operating period T
of the chopper-inverter and an increase in operating frequency.
For any given SCR device there is a set of concurrent ratings at a
given device temperature which determines the minimum time of the
commutation period t.sub.1 below which the SCR will fail to
turn-off. If the load (in the form of a high conductivity pan)
reduces the commutation period t.sub.1 below this minimum turn-off
time, the SCR will remain on and the input current to the
chopper-inverter will rise to a very high value at the peak of the
line voltage, and is limited in magnitude only by the series choke
coil L.sub.2. In such event, the best that can happen is that the
main circuit breaker 13 will trip and disconnect the
chopper-inverter from the AC input supply line. However, the
allowable turn-off time of a given SCR device is least when the
supply voltage is greatest, and commutation failure most often
times occurs at the peak of the input alternating current supply
wave rather than near a zero point. Consequently, large transients
of current can be developed as the stored energy in the inductive
circuits is dissipated, and it is possible to destroy the SCR in
the process.
The chopper-inverter can be protected from commutation failure of
the SCR under conditions where the high conductivity pan of
aluminum or copper is used in connection with the inductive cooking
unit by means of a pan safety control circuit 30 shown in FIG. 1.
The pan safety control circuit 30 will be described more fully
hereinafter in connection with FIG. 2 of the drawngs. Briefly,
however, it should be noted that rather than measure or sense the
actual reactance and resistance of the chopper-inverter circuit,
and the changes produced in these parameters by reason of the use
of a high conductivity pan or other cookware, the pan safety
control 30 monitors the relative t.sub.1 commutation time and the
t.sub.2 charging time of the chopper-inverter which inherently are
governed by the load impedance reflected into the chopper-inverter
circuit by the nature of the pan or other cookware placed over the
inductive heating coil. The pan safety control 30 protects the
chopper-inverter circuit from commutation failure when an aluminum,
copper or other high conductivity pan is placed over the inductive
heating coil, by monitoring the t.sub.1 commutation time of the
commutating circuit, and turning off the trigger pulses to the
chopper SCR 17 when this t.sub.1 commutation time reaches a pre-set
minimum value. The circuit provides effective protection for both
the mode of operation where the charging period t.sub.2 is
constant, and the case where the charging period t.sub.2 is varied.
In either instance, the use of high conductivity pans or other
cookware with the inductive cooking unit chopper-inverter, will
produce changes in either the ratio (t.sub.1)/t.sub.2) or the
difference (t.sub.1 - t.sub.2) of the respective t.sub.1 and
t.sub.2 operating times.
FIG. 2 is a detailed, schematic circuit diagram of a new and
improved induction cooking unit power supply system including a pan
safety control 30 constructed in accordance with the invention, and
illustrated in functional block diagram form in FIG. 1. From FIG.
2, it will be seen that the negative potential appearing at
terminal 16A on the inverter end of filter inductor L.sub.2, is fed
through conductor 91 and voltage dropping resistor 93 across a
zener diode 92. The stabilized voltage appearing across zener diode
92 is applied across a charging capacitor 94 through the variable
timing resistor 34. This voltage is applied to a silicon unilateral
switch (SUS) or a programmable unijunction transistor (PUT) 95
which is a threshold device that breaks down and conducts upon the
voltage across the device reaching a fixed threshold voltage value.
Upon this threshold voltage value being obtained by capacitor 94, a
trigger pulse is produced across a small load resistor connected in
series with the SUS 95 that is coupled through an RC network to the
base of a PNP gating transistor 97. The time interval required for
the voltage across zener diode 92 to charge capacitor 94 to the
threshold breakover value of SUS 95 is the quiescent t.sub.2
charging time of the chopper-inverter circuit.
The discharge of capacitor 94 through SUS 95 and RC network 96,
produces a sharp, negative-going pulse of voltage on the base of a
PNP power amplifier transistor 97 which turns the transistor on for
the duration of the pulse, and produces a much larger, amplified
gating-on pulse across the primary winding 98 of a pulse
transformer T.sub.1. This gating-on pulse is transformed to the
secondary winding 99 of the pulse transformer T.sub.1 and causes
the large power rated chopper SCR 17 to be gated on at a repetition
rate determined by the repetition rate of the trigger pulses
generated by SUS 95. The circuit is completed by a large value
discharge resistor 108 that is connected across the filter
capacitor C.sub.2 and assures rapid and complete discharge of
capacitors C.sub.1, C.sub.2, C.sub.3, etc., upon the zero point
switching SCR 21 being turned-off. It is important that these
capacitors be discharged to obtain conduction of the zero point
switching SCR 21 as near to the zero point of the supply
alternating current as possible upon a turn-on gating signal being
applied to zero point switching SCR 21. It is also desirable that a
conventional dv/dt snubber circuit comprised by a series connected
resistor 109 and capacitor 111 be connected across the large power
rated chopper SCR 17 and feedback diode 18 for limiting the rate of
reapplied forward voltage across the SCR 17 following turn-off of
the feedback diode 18 during each commutation interval in a manner
well known to those skilled in the art.
For conveneience, the filter capacitor C.sub.3 is comprised by two
smaller capacitors C.sub.3a and C.sub.3b connected in series
circuit relationship with the inductive heating coil L.sub.3 with
the series circuit thus comprised being connected across the
C.sub.1 commutating capacitor C.sub.1a - C.sub.5a, in the manner
shown in FIG. 2. Energizing potential for supplying appropriate
biasing to the power gating transistor 97 is obtained by a diode
rectifier 101 and voltage dropping resistor 103 connected in series
circuit relationship with a zener diode 105 across the high voltage
direct current power supply terminal 15 and 16. The pulse
transformer T.sub.1 also has a third or auxiliary winding 106 that
is connected in series circuit relationship with a pulse sustaining
network comprised by a capacitor 107 and current limiting resistor
and that is connected to the base of PNP gating transistor 97 for
prolonging its conduction over a predetermined pulse duration
period to assure adequate gating power being supplied through the
pulse transformer T.sub.1 to gate on the power chopper SCR 17.
Alternatively, a low voltage, gate signal developing pilot SCR
could be employed in place of the third auxiliary winding
connection for the same purpose. A convenient and useful device for
indicating when the chopper-inverter circuit is operating, is
provided by a neon lamp 100 connected in series with a dropping
resistor across the choke inductor L.sub.2. The neon lamp 100
turns-on only while the high frequency potential is developed
across the inductor L.sub.2, and since the DC resistance of
inductor L.sub.2 is very low, the lamp does not respond to the DC
current passing through the inductor. The brightness of the lamp
100 will remain uniform because the high frequency component of the
current I.sub.L flowing through the choke inductor L.sub.2 does not
change greatly for changes in loading ranging from no load to full
load conditions.
The important characteristic to note in connection with the
above-described t.sub.2 timer gating circuit, is that the gating
pulse energy supplied to the control gate of the power chopper SCR
17 is derived from the DC storage capacitor 102 connected across
zener diode 105, which in turn is charged directly from across the
high voltate power supply terminals 15 and 16. The start-up delay
inhibit circuit 23 assures that the capacitor 102 comes full
charged before zero point switching SCR 21 is turned-on, and the
supply voltage to the chopper-inverter appears across filter
capacitor C.sub.2. As a result, pulses of ample and equal magnitude
are provided at the gate of chopper SCR 17 at all times including
those portions of the supply, full wave rectified potential near
the zero point (identified as the valley of the ripple). Hence, the
gating circuit arrangement provides gating pulses of sufficient
strength and of minimum charging delay t.sub.2 to assure turn-on of
the chopper SCR 17, and to assure an inverter chopping rate which
remains essentially constant and independent of the value of the
rectifier full wave potential appearing between power supply
terminals 15 and 16. In this circuit, the SUS 95 provides pulses of
equal magnitude and with constant delay down to a voltage as low as
12 volts across the power supply terminals 15 and 16. It is a
function of the filter capacitor C.sub.2 to prevent the value of
the supply voltage from falling below 12 volts in the valley of the
ripple, even under conditions of greatest chopper-inverter loading.
By reason of the independent power supply comprised by diode
rectifier 101, dividing resistor 103, zener diode 105 and capacitor
102, the gating pulse amplifier 97 will assure provision of gating
pulses of sufficient amplitude or strength to gate-on the high
power chopper SCR 17 due to the built-in delay inherent in the
operation of the zero point switching SCR 21.
FIG. 3(a) of the drawings illustrates the wave form of the load
current flowing through inductor L.sub.1 of the chopper-inverter
circuit over two cycles of operation. The positive sinusoid
represents the interval of conduction of the SCR 17 and the
negative sinusoid the interval of conduction of the feedback diode
while the t.sub.2 charging time is indicated as a ramp voltage
gradually increasing from some negative value towards zero just
prior to the commencement of a new t.sub.1 commutation interval
represented by the combined conduction periods of the SCR 17 and
feedback diode 18 pair.
FIG. 3(b) illustrates the asymmetric square wave voltage appearing
across the zener diode 92 which supplies the t.sub.2 timer SUS 95
and charging capacitor 94 of trigger pulse generator 33. From a
consideration of FIG. 3(b), it will be appreciated that the voltage
across zener diode 92 is zero referred to the positive side 15 of
the full wave rectified potential appearing across power supply
terminals 15 and 16. The intervals of conduction S and D of the SCR
17 and feedback diode 18, respectively, also are shown. During the
t.sub.2 off charging period, the voltage across zener diode 92 is
negative and at the regulating level (minus 12 volts) of the zener
diode. From a further consideration of the voltage wave form shown
in FIG. 3(b), it will be seen that the direction of a voltage
change at time t = 0 and at time t = t.sub.1 are positive-going and
negative-going, respectively, where t = t.sub.0 is the point of
turn-on of SCR 17 and t = t.sub.1 corresponds to the point of
turn-off of the feedback diode 18 (and hence the termination of the
commutation period). Accordingly, if the wave form shown in FIG.
3(b) is differentiated and rectified appropriately, a positive t =
0 pulse and a negative t = t.sub.1 pulse can be derived at separate
terminals with respect to the positive main terminal bus 15, and
would appear in time and wave shape as shown in FIG. 3(c) of the
drawings.
Under conditions where a pan of suitable ferromagnetic material
such as stainless steel, iron, etc., is employed in conjunction
with the inductive heating coil, the t.sub.1 period remains
essentially unchanged, and the t = t.sub.1 timing pulse shown in
FIG. 3(c) will occur within some preset or peedictable range of
values. Suppose now that the t = t.sub.0 pulse shown in FIG. 3(c)
is used in a pan safety circuit to develop a timing pedestal gating
signal pulse of voltage of fixed amplitude and stable time duration
t = t.sub.1 .sub.' - t.sub.1 .sub." as shown in FIGS. 3(e), 3(j)
and 3(n) of the drawings. By appropriately delaying the generation
of this timing pedestal gating signal pulse t.sub.1 .sub.' -
t.sub.1 .sub." so that it occurs only during the preset or
predictable period when the t.sub.1 timing signal pulse would occur
under conditions where a pan of proper ferromagnetic material is
being used with the induct1ve heating coil, it will be seen that
the timing pedestal gating signal pulse in effect then can be used
to define a preset safe limit on the conduction interval of the
chopper SCR/diode pair. If then, an aluminum, copper or other
highly conductive pan load is employed with the induction cooking
coil, the t = t.sub.1 timing signal pulse will occur early,
depending on the area of the surface of the utensil and its
distance from the inductive heating coil as well as its electrical
conductance at the operating frequency, assuming that the circuit
is on and running and the pan is brought into proximity with the
inductive heating coil (hereinafter referred to a Case I
condition). Under Case I condition, the t = t.sub.1 timing signal
pulse will occur in advance of the timing pedestal gating signal
pulse t.sub.1 .sub.' - t.sub.1 .sub." as shown in FIGS. 3(j) - 3(k)
of the drawings Under such circumstances, it will be seen that
anticoincidence occurs, and that the application of the two signals
to a coincidence circuit would result in the production of an
anticoincidence output signal pulse shown in FIG. 3(k) for
application to a safety means controlling operation of the
chopper-inverter. Conversely, under operating conditions
(hereinafter referred to as Case II condition) where the aluminum,
copper or other highly conductive pan load first is disposed over
the inductive heating coil prior to initially placing the circuit
in operation, the t = t.sub.1 timing signal pulse will occur late
with respect to the timing pedestal gating signal pulse t.sub.1
.sub.' - t.sub.1 .sub." as shown in the FIGS. 3(m) - 3(o) of the
drawings. Here again it will be appreciated that an anticoincidence
situation will occur resulting in the production of an
anticoincidence output signal that can be used to operate an alarm
or safety means for shutting down operation of the chopper-inverter
circu1t.
The pan safety control circuit shown at 30 in FIG. 1, and in
greater detail in FIG. 2 of the drawings, is designed to provide
the above-described control features and is comprised by (1)
sampling means responsive to conduction through the
chopper-inverter SCR/diode pair for deriving the t.sub.o and
T.sub.1 timing signal pulses indicative of the conduction intervals
of the SCR/diode pair, (2) conduction interval limit setting means
responsive to the t.sub.0 timing signal pulse for defining safe
limits to the respective conduction intervals for the
chopper-inverter SCR/diode pair (3) comparison means in the form of
a coincidence circuit responsive to (1) and (2) for comparing the
actual conduction intervals of the chopper-inverter SCR/diode pair
to the preset safe limits, and (4) safety means responsive to the
output from (3) for controlling operation of the chopper-inverter
in response to an alarm output signal from (3).
As best shown in FIG. 2A the pan safety control circuit 30 is
supplied with direct current excitation voltage through a voltage
dividing resistor 201 from a source between points A and C
connected across filter capacitor C.sub.2. A zener diode 202 clamps
the pan safety supply voltage to a constant level of about -16
volts. This level of supply voltage for the pan safety control
(hereinafter referred to PSC), is arranged to be a few volts more
than the regulating level of about 12 volts of the zener diode 92
used in the t.sub.2 timer trigger pulse signal generating circuit
33. An isolating diode 203 is connected so as being reverse biased
between the negative terminals of the two zener diodes 202 and 92.
Diode 203 also is connected to the cathode of a latching SCR 204
having its load terminals connected between the power supply
terminals 15 and 16(B), and operating through the isolating diode
203 to shunt zener diode 92. By this arrangement, if the latching
SCR 204 is caused to conduct, it will latch into conduction and
short circuit both the zener diode 202 and the t.sub.2 - timer
zener diode 92 via the inter-connecting isolating diode 203. As a
consequence, trigger pulses for supply to the gating electrode of
the chopper SCR 17 can no longer be generated by the t.sub.2
trigger circuit 33, and the chopper-inverter will cease to operate.
The latching SCR 204 will remain latched in its conducting
condition despite turn-off of the chopper-inverter since the DC
supplied to the filter capacitor C.sub.2 remains on. To restart the
chopper-inverter, the direct current voltage to the filter
capacitor C.sub.2 must be removed. This is easily done with the
zero point switching SCR 21 as will be described hereinafter. Thus,
it will be appreciated that the latching SCR 204 serves as a safety
means for turning off the chopper-inverter upon being rendered
conductive by an output alarm signal from the pan safety control
circuit 30 in a manner to be described more fully hereinafter.
However, during normal operation of the inductive cooking unit, the
t.sub.2 - timer trigger circuit 33 is free to operate independently
of the pan safety control to produce trigger pulses for the main
chopper SCR 17 because of the isolation provided by isolating diode
203.
The sampling means for the pan safety control 30 is comprised in
part by a first coupling capacitor 205 which in conjunction with a
resistor 206A and first transistor 206 operates to differentiate
the positive going potential appearing across points A-D at the
start of the t.sub.1 conduction interval of the SCR/diode pair to
thereby produce a first t.sub.0 timing signal pulse representative
of the start of conduction of the main chopper SCR/diode pair. This
first t.sub.0 timing signal pulse is applied to the base of a first
NPN junction transistor 206 connected in series circuit
relationship with a first set of voltage dividing resistors 207 and
208 across terminals 15-16B of the low voltage pan safety control
circuit power source 202. A first feedback PNP junction transistor
209 has its base connected to the junction of voltage dividing
resistors 207 and 208 and has its emitter-collector connected in
series circuit relationship with a second set of voltage dividing
resistors comprised by the resistors 211, 212 and 206. A first
programmable unijunction transistor 213 (hereinafter referred to as
PUT 213) has its anode gate connected to an intermediate point on
the second set of voltage dividing resistors 211, 212 and 206 and
has its cathode connected through a load resistor 214 to the
negtive terminal 16(B) of the low voltage pan safety circuit power
supply. A series connected set of first variable resistors 215,
215(a) through 215(e) are connected in series circuit relationship
with a timing capacitor 216 with the series circuit thus comprised
being connected in parallel circuit relationship with the second
set of voltage dividing resistors 211, 212 and 206. The anode of
PUT 213 is connected to the junction of capacitor 216 with the
first set of variable resistors 215(a) through 215(e). The first
set of variable resistors 215(a) through 215(e) preferably
comprises a plurality of independently adjustable resistors which
may selectively be connected in an out of series circuit
relationship with the timing capacitor 216 through the medium of a
delay period setting switch 217 that comprises a means for setting
the delay period of a delayed pedestal timing signal pulse produced
by the monostable multivibrator circuit comprised in part by the
PUT 2B. If desired, this switch may be ganged for selective
operation in conjunction with the power level selector switch
S.sub.1 and S.sub.2 of the chopper-inverter as would be obvious to
one skilled in the art. Alternatively, resistor 215 could be preset
to provide a pedestal timing signal pulse of adequate pulse
duration for all levels of power. The PUT (programmable unijunction
transistor) is a three terminal, planar passivated PNPN
semiconductor switching device manufactured and sold by the General
Electric Company. For a detailed description of the PUT, reference
is made to application notes 60.20 issued November 1967, by the
General Electric Company-Semiconductor Products Department with
respect to the D13T1 and D13T2 programmable unijunction transistor
devices.
Briefly, the operation of the PUT 213 is as follows: Upon the
voltage across timing capacitor 216 being built up to a value
exceeding that of the reference value established by the voltage
dividing resistors 211, 212 and 206, PUT 213 becomes forward biased
and breaks down and conducts. This threshold conduction point can
be varied by adjustment of the selector switch 217 to connect in
different ones of the variable value resistors 215(a) - 215(e).
Upon PUT 213 being rendered conductive, a positive going voltage is
produced across the load resistor 214 which is supplied to the base
of a second NPN junction transistor 221 that has its
emitter-collector connected in series circuit relationship with a
third set of voltage dividing resistors 222 and 223. A second PNP
feedback transistor 224 has its base connected to the junction of
resistors 222 and 223 and has its emitter-collector connected in
series circuit relationship with a fourth set of voltage dividing
resistors 225 and 226 which in turn are connected in series with a
current limiting resistor 227 and the load resistor 214 for PUT 213
with the junction of resistors 226 and 227 being connected to the
base of the second NPN junction transistor 221. A second variable
resistor 228 is connected in series circuit relationship with a
second timing capacitor 229 between the collector of the second PNP
feedback transistor 224 and the negative terminal 16(B) of the low
voltage direct current pan safety control power source comprised by
zener diode 202. The junction of variable resistor 228 and timing
capacitor 229 is connected to the anode of a second PUT 231 whose
anode gate is connected to the juncture of the fourth set of
voltage dividing resistors 225 and 226 and whose cathode is
connected directly to the negative terminal 16(B).
In operation, the monostable multivibrator circuit comprised by the
above-described elements functions in the following manner. The
positive going voltage pulse supplied across conductor 91 and
appearing at point D is differentiated by capacitor 205 and
resistor 206A and applied to the base of the first NPN junction
transistor 206 causing transistor 206 to turn-on. Turn-on of
transistor 206 results in driving the base of the first PNP
feedback transistor 209 negatively toward the potential of negative
terminal 16(B) and causes transistor 209 to turn-on. Transistors
209 and 206 are regeneratively interconnected so that turn-on of
209 serves to maintain 206 in its conducting condition. Turn-on of
transistor 209 also starts charging the timing capacitor 216 toward
the potential of the collector of the PNP transistor 209 through a
selected one of the variable resistors 215(a) through 215(e). After
some preset delay period determined by the setting of selector
switch 217, PUT 213 turns-on and produces a positive going voltage
that is supplied through limiting resistor 227 to the base of the
second NPN junction transistor 221. Turn on of PUT 213 also
operates to remove the feedback to the base of transistor 206 so
that it is allowed to turn-off. This in turn results in turning off
the first PNP feedback transistor 209 and results in removing the
positive voltage for the anode gate of PUT 213 thereby allowing PUT
213 to turn-off after timing capacitor 216 has become sufficiently
discharged through load resistor 214. The delay period established
prior to conduction of PUT 213 is depicted by the wave form of the
voltage at point E applied to the base of PNP transistor 209 and is
illustrated in FIG. 3(d) of the drawings.
The positive going, delayed trigger pulse developed across load
resistor 214 upon PUT 213 being rendered conductive, causes the
second NPN junction transistor 221 to turn-on and results in
turning on the second feedback PNP transistor 224. Transistors 221
and 224 are connected regeneratively through the second set of
voltage dividing resistors 225 and 226 so that upon transistor 224
turning-on, a feedback potential is applied to the base of
transistor 221 to hold this transistor turned-on. Turn-on of the
PNP transistor 224 will cause the second timing capacitor 229 to
start charging towards the potential of the collector of the PNP
transistor 224 and after a preset period determined by the setting
of the second variable resistor 228, will cause the second PUT 231
to break down and conduct. Conduction of the second PUT 231 results
in removing feedback potential to the base of transistor 221 and
causes this transistor together with the second PNP transistor 224
to turn-off. As a consequence of this operation, the voltage at
point P on the emitter of the second feedback PNP transistor 224
will appear as shown in FIG. 3(e) of the drawings and will have a
duration t.sub.1 .sub.' - t.sub.1 .sub." which is determined by the
setting of the second variable resistor 228, and constitutes the
desired delayed pedestal limit setting signal pulse that is
supplied through a current limiting resistor 232 to the base of the
PNP junction transistor 233 comprising a part of the comparison
circuit means of the PSC.
Referring again to FIGS. 3(b) and 3(c) of the drawings, it will be
seen that at the end of the t.sub.1 commutation interval when
conduction through the SCR/diode pair terminates, the voltage at
point D drops sharply to the -12 volt regulating level of zener
diode 92 as shown in FIG. 3(b) of the drawings. This drop in
voltage is sampled by the sampling circuit means comprised by a
second differentiating circuit including capacitor 235 and resistor
236. The differentiated, negative going, second timing signal pulse
produced by differentiating network 235 and 236 is coupled acorss
an isolating diode 237 and resistor 238 to the base of a PNP
junction transistor 244 also comprising a part of the comparison
circuit means of the PSC.
The comparison circuit means of pan safety control circuit 30 is
comprised by a coincidence circuit formed by the two PNP junction
transistors 244 and 233 which are connected in series so that the
collector of transistor 244 connects to the emitter of transistor
233. This pair of transistors is then connected in series circuit
relationship with a diode 242 and load resistor 243 across the low
voltage pan safety direct current power supply terminals 15 and
16B. The load resistor 243 in turn is directly connected to the
control gate of the latching safety SCR 204. With this circuit
arrangement, by appropriate adjustment of the first variable
resistor comprised by selector switch 217 and the adjustable
resistors 215(a) - 215(e), and by further adjustment of resistor
228 to the width of the pedestal limit setting pulse appearing at
point P and supplied to the base of the PNP transistor 233 in the
coincidence circuit, it will be seen from FIGS. 3(e) and 3(f) of
the drawings that the limit setting pedestal pulse applied to
resistor 233 can be made to be coincident with t.sub.1 timing pulse
applied to the base of coincidence transistor 244 during normal
operation of the chopper-inverter where a proper ferromagnetic pan
load is imposed on the conductive heating coil being excited by the
chopper-inverter. Since the pedestal limit setting signal pulse has
a positive polarity as shown in FIG. 3(e), coincidence transistor
233 will be maintained off at the point in time when the negative
going t.sub.1 timing pulse is applied to the base of the
coincidence transistor 244. Hence, no output will appear across the
load resistor 243 as shown in FIG. 3(g) of the drawings and as a
consequence the latching safety SCR 204 will be maintained in its
off condition thereby allowing trigger pulses to be generated by
the SUS 95 in the normal manner described above.
In contrast to the above-described normal operating condition, if
now an aluminum, copper or other non-ferrous, highly conductive pan
load is placed over the inductive heating coil, the t = t.sub.1
timing signal pulse will occur at a much earlier point in time as
indicated by FIGS. 3(h) and 3(i) of the drawings. This is due to
the markedly shortened commutation interval t.sub.1 caused by the
detuning effects of the highly conductive pan as explained earlier.
Consequently, from a comparison of FIGS. 3(i) to 3(j) it will be
seen that the t.sub.1 timing pulse indicative of the termination of
conduction of the chopper-inverter SCR/diode pair, occurs at a
point in time ahead of the limit setting pedestal timing pulse
t.sub.1 .sub.' - t.sub.1 .sub." shown in FIG. 3(j). In the pan
safety circuit, the base of PNP transistor 233 of the coincidence
circuit normally is maintained at a negative potential relative to
its emitter. Hence, PNP transistor 233 will be in normally
conducting condition at the point in time when the t.sub.1 timing
signal pulse is produced at the end of the conduction interval of
the chopper-inverter SCR/diode pair, at which point transistor 244
in the coincidence circuit is rendered conductive. With both
transistors 244 and 233 conductive, a positive going signal pulse
similar to that illustrated in FIG. 3(k) of the drawings will be
produced and applied to the gate of the latching safety SCR 204
causing it to turn-on.
Upon SCR 204 being turned-on, isolating diode 203 becomes forward
biased and operates through the conducting SCR 204 to shunt the
zener diode 92 thereby terminating further supply of excitation
potential to the SUS 95 in trigger generator circuit 33. The
latching SCR 204 will remain conductive and hence inhibit further
operation of the trigger generator circuit 33 for so long as direct
current power remains on the circuit. Accordingly, in order to
place (reset) the circuit back in operation, it is necessary for
the operator to return the power level control back to zero or at
least from one power level setting to another so as to turn-off the
zero point switching SCR 21 momentarily. This exercise has been
built into the circuit in order to educate the operator that he
will need to remove the highly conductive pan from the conductive
heating coil before the circuit can be rendered operative in a
normal and safe manner.
The above-described operation is true for Case I where the
chopper-inverter is operating in a normal manner under no load
conditions, and thereafter a highly conductive aluminum or copper
pan is placed over the inductive heating coil. However, for Case II
where thb conductive pan is placed over the inductive heating coil
in advance of placing the inverter circuit in operation, different
conditions prevail. Under these last stated conditions of Case II,
it has been determined that due to the detuning effect of the
highly conductive pan on the inductive heating coil, the normal
conduction interval of the chopper-inverter SCR/diode pair is
greatly extended in the manner predicted in FIG. 3(1) of the
drawings due to the fact that the highly conductive pan causes a
surge current to be oscillated back and forth between the C.sub.3,
L.sub.3 and C.sub.1, L.sub.1 components prior to being returned or
reflected back across the SCR/diode pair with sufficient energy to
reserve bias the SCR/diode pair and cause turn-off of these
devices. Under these operating conditions, the t.sub.1 timing
signal pulse will be produced at a point in time after t the
t.sub.1 .sub.' - t.sub.1 .sub." limit setting pedestal signal pulse
as shown in FIG. 3(m) and 3(n) of the drawings. Here again,
transistor 233 in the coincidence circuit will be in its normally
conducting condition at the point in time when the t.sub.1 timing
signal pulse occurs so that an output gating signal is produced
across the load resistor 243 and applied to the latching safety SCR
204 in the manner described above. Thus, it will be appreciated
that it does not matter whether the circuit is being operated under
either Case I or Case II conditions, the pan safety control circuit
30 is operative to turn-off and inhibit further production of
trigger signal pulse and the trigger signal generator 33.
In addition to the pan safety control 30, the new and improved
induction cooking power supply system shown in FIG. 2 also includes
an improved zero point switching control circuit 22 and an improved
start-up and delay inhibit circuit 23 for controlling operation of
the zero point switching SCR 21. As shown in FIG. 2, the zero point
switching SCR 21 has its gate electrode connected to the juncture
of a gating resistor 301 connected in series circuit relationship
with a low voltage, pilot turn-on SCR 62 and a voltage dividing
resistor 62A with the series circuit thus comprised being connected
across the power supply terminals 15 and 16 to the left of the zero
point switching SCR 21. Consequently, the direct current potential
is maintained across this pilot switching SCR 62 and resistor 301
being stabilized by a zener diode 302 connected across these two
elements. With this arrangement, upon a positive polarity gating
potential being applied to the control gate of the low voltage
pilot switching SCR 62, SCR 62 turns-on and applies a positive
polarity gating current to the control gate of the zero point
switching SCR 21 to cause this larger, power rated SCR to
turn-on.
The gating electrode of the switching SCR 62 is connected to the
juncture of a set of voltage dividing resistors 64, 65 and 66 which
in turn are connected in series circuit relationship across thh
power supply terminals 15 and 16 to the left or rectifier side of
the zero point switching SCR 21. In the absence of any further
control, a positive polarity gating-on potential will be applied
from resistor 65 and 66 to the gate of the low voltage pilot
switching SCR 62 during each half cycle of the full wave rectified
potential appearing across terminals 15 and 16 with SCR 21 in its
nonconducting, current blocking condition. Note that with SCR 21 in
its current blocking condition, the bleeder resistor 108 around
filter capacitor C.sub.2 will have bled off any charge on this
capacitor so that the potential appearing across power supply
terminal buses 15 and 16 is in effect the potential appearing at
the output of a full wave rectifier 14. However, the voltage
dividing resistors 65 and 66 have a low voltage, latching inhibit
SCR 72 connected in parallel therewith between the junction of
resistors 64 and 65 and the negative terminal bus 16. A bleeding
resistor 71 is connected between the juncture of voltage dividing
resistors 64 and 65 and the control gate of the latching SCR 72.
The delays and threshold potentials designed into the circuit are
such that the latching inhibit SCR 72 will be gated-on normally in
advance of turn-on of the SCR 62. Hence, turn-on of the latching
SCR 72 will inhibit thereafter through the remaining half cycle of
the supply alternating current wave any turn-on of the low voltage
pilot switching SCR 62 and hence, the zero point switching SCR 21.
The inhibiting action will be reproduced for each sinusoidal half
cycle of the full wave rectified potential appearing acros the
power supply terminal buses 15 and 16.
In order to prevent turn-on of the inhibit latching SCR 72 (and
thereby allow the low voltage pilot switching SCR 62 to turn-on
with consequent turn-on of the zero point switching SCR 21), an NPN
junction turn-on control transistor 73 is provided with its
collector connected to the control gate of the inhibiting SCR 72
and its emitter connected directly to the negative terminal bus 16.
With this arrangement, upon the control transistor 73 being
turn-on, it will clamp the potential of the control gate of the
inhibiting SCR 72 to the potential of the negative terminal bus 16
thereby preventing turn-on of the inhibiting SCR 72. With the
inhibiting SCR 72 thus maintained off, the low voltage pilot
switching SCR 62 will be allowed to turn-on thereby turning-on the
zero point switching SCR 21.
To control turn-on and turn-off of the control transistor 73, the
start-up delay and inhibit circuit 23 further includes a delay
timing capacitor 82 that is connected through a limiting resistor
311 to the base of control transistor 73 and is connected through a
limiting resistor 74 to the juncture of a set of voltage dividing
resistors 75, 76 and 324. With this arrangement, the delay timing
capacitor 82 will be charged from the potential appearing across
the voltage dividing resistors 75, 76 and 324 at some predetermined
charging rate depending upon the resistance-capacitance time
constant of the circuit. Transistor 73 is an NPN transistor and
normally is nonconductive until such time that the bias applied to
its base by the delay timing capacitor 82 becomes sufficiently
positive to turn it on. Thus, there is a built-in delay time
required to charge capacitor 82 to a level sufficient to turn-on
transistor 73. Upon reaching this point, transistor 73 normally
will turn-on. However, should transistor 73 turn-on at some
intermediate point in a half cycle of the supply full wave
rectified potential, inhibiting SCR 72 will have already turned-on
and be latched-in a conducting condition until the end of the half
cycle. In the next succeeding half cycle, because of the conducting
condition of the transistor 73, SCR 72 will be inhibited from
conducting, thereby allowing the low voltage pilot switching SCR 62
and zero point switching SCR 21 to be turned-on at or near a zero
point of the supply full wave rectified potential. Thus, soft
starting at or near the zero point of the supply full wave
rectified potential is assured thereby preventing surge charging of
the filter capacitor C.sub.2 and the commutating capacitor elements
of the chopper-inverter power circuit along with the undesirable
consequences that can result from such surge charging.
In order to control the turn-on and turn-off of the control
transistor 73 in accordance with the output of some control
function such as the pan temperature control 24, for example, a set
of parallel connected, NPN junction inhibiting transistors 81 and
85 are provided with their emitters connected together and their
collectors connected together, are this parallel combination
connected in series circuit relationship through a limiting
resistor 80 between the junctures of the resistors 74 and 311 and
the negative power supply terminal bus 16. With this arrangement,
upon either one or both of the inhibiting transistors 81 or 85
being turned-on, the potential of the base of the control
transistor 73 will be lowered towards the potential of the negative
power supply terminal bus 16 so as to prevent its turn-on. In this
condition, the inhibiting SCR 72 will be latched-on at the
commencement of each half cycle of the supply full wave rectified
potential thereby inhibiting further gating signal to the low
voltage pilot switching SCR 62 and the zero point switching SCR 21
and this will result in turning off the inductive cooking unit
power supply. For a more detailed description of the pan
temperature control circuit 24, reference is made to the
above-identified copending U. S. application Ser. No. 131,648, now
U.S. Pat. No. 3,710,062. Briefly, however, it can be stated that
the pan temperature control senses whether the temperature of the
pan 51 being inductively heated has exceeded a preset desired pan
temperature, and produces a positive polarity control signal pulse
that is supplied to the base of the inhibiting transistor 85 to
cause it to turn-on. Upon transistor 85 being turned-on, it will
discharge the delay timing capacitor 82 sufficiently to cause the
control transistor 73 to turn-off. This condition will then be
maintained for so long as the pan temperature control 24 calls for
removal of power. Thereafter, upon the pan 51 temperature dropping
below the desired temperature value, the pan temperature control 24
will remove the positive bias applied to the base of the inhibiting
transistor 85 thereby causing transistor 85 to turn-off. Upon this
occurrence, charge on the delay timing capacitor 82 will again
build up to the point that it turns-on the control transistor 73
thereby preventing inhibiting SCR 72 from being turned-on, and
allowing the pilot switch SCR 62 and zero point switching SCR 21 to
turn-on at or near the zero point of the next succeeding half cycle
of the full wave rectified potential appearing across power supply
terminal buses 15 and 16.
In addition to the above-described control action, it should be
noted that the base of the inhibiting transistor 85 is connected
through a limiting resistor 312 back to the juncture of a set of
voltage dividing resistors 313 and 314. This juncture is also
connected back through a conductor 315 to a set of contact points 1
through 5 of power level selector switch S.sub.2 whose movable
contact is connected through a conductor 316 back to the negative
power supply terminal 16. By this arrangement, whenever the power
level selector switch S.sub.2 is closed on one of its contacts 1-5
the mid-tap point of the voltage dividing resistor 313 and 314 will
be clamped to the potential of the negative power supply terminal
16. However, in between each of the contact points this clamping
potential is removed so that as the power level selector switch
S.sub.2 is moved from one contact point to the next in order to
switch in different values of commutating capacitance C.sub.1A -
C.sub.1E through the medium of a progressively shorting switch, a
positive turn-on potential will be applied through resistor 312 to
the base of inhibiting transistor 85 so as to maintain zero point
switching SCR 21 turned-off and in its current blocking condition
during the switching action affecting the commutating capacitors
C.sub.1A - C.sub.1E in a manner described more fully in the
above-referenced copending U. S. application Ser. No. 131,648, now
U.S. Pat. No. 3,710,062. In this manner operation of the
chopper-inverter circuit concurrently with the switching of the
commutating capacitor elements C.sub.1A - C.sub.1E is avoided
thereby avoiding dangerous arcing or sparking which otherwise might
occur if such switching took place while the chopper-inverter was
in operation.
In order to assure fast response to the turn-off and then rapid
turn-on of the power line switch or circuit breaker 13, a discharge
transistor 321 is incorporated which has its emitter-collector
connected in parallel circuit relationship across the delay timing
capacitor 82. The base of the discharge transistor 321 is connected
through a resistor 322 to the negative power supply terminal 16 and
through a conductor 323 to the juncture of voltage dividing
resistors 324 and 75. The discharge transistor 321 is a PNP
junction transistor so that upon initial turn-off of input power
through breaker 13 the transistor 321 is biased into conduction and
quickly discharges any residual potential on capacitor 82. When the
main power is turn-on via breaker 13, the discharge transistor 321
is prevented from conducting by biasing its base of a potential
above that which appears across resistor 76 by an amount determined
by voltage dividers 75, 76 and 324 and supplied across conductor
323. Thus, discharge transistor 321 upon turn-on of the circuit,
conducts for a negligible length of time and charge builds up on
delay capacitor 82 and the circuit operates to control turn-on of
the control transistor 73 as described previously.
A second on-off control over the operation of the inductive cooking
unit power supply is provided through the medium of the inhibiting
transitor 81 whose base is connected back through a conductor 421
to the alarm output signal of the over temperature sensor 35. For a
more detailed description of the construction and operation of the
over temperature sensor, reference is made to the above-identified
copending U. S. application Ser. No. 131,648, now U.S. Pat. No.
3,710,062. Briefly, however, it can be said that the
over-temperature sensor circuit 35 is comprised by a thermistor
shown at 411 which is positioned to sense the temperature in the
vicinity of the inductive heating coil L.sub.3, and upon an over
temperature condition being sensed, serves to develop an alarm
output signal that operates the inhibiting transistor 81 and shuts
down the inductive cooking unit chopper-inverter power supply by
turning off the zero point switching SCR 21.
The over-temperature sensor circuit 35 is supplied through a
voltage dropping resistor 408 connected in series circuit
relationship with a filter capacitor 410 across the power supply
terminals 15 and 16. A zener diode 409 is connected across the
filter capacitor 410 for regulating the DC excitation potential for
the over temperature circuit. This DC excitation potential is
developed across the low voltage power supply terminals 15B and 16,
and is supplied across the thermistor temperature sensor 411
through an adjustable resistor 412 for variably controlling the
sensitivity or threshold operating level of the over temperature
circuit. The juncture of the variable resistor 412 and thermistor
411 is connected to the anode gate of a PUT with the capacitor 413
being connected across the variable resistor 412 for stabilization
purposes. The anode of the PUT is connected to the juncture point
of a pair of voltage dividing resistors 416 and 417 connected
across the low voltage over temperature circuit power supply
terminals 15B and 16. The cathode of the PUT 414 is connected
through a cathode load resistor 415 to the negative terminal 416 of
the low voltage over temperature circuit power source. Alarm output
signals from the over temperature circuit are derived across the
load resistor 415 and supplied over conductor 421 to the base of
inhibiting transistor 81.
With the above-described arrangement, when an over-temperature
condition occurs in the inductive heating assembly, the thermistor
411 which has a negative temperature coefficient will drive the
anode gate of PUT 414 sufficiently positive with respect to its
cathode to cause PUT 414 to break down and conduct. Upon this
occurrence, a positive going enabling potential will be applied to
the base of the inhibiting transistor 81 across conductor 421 from
load resistor 415 and cause inhibiting transistor 81 to shut down
the inductive cooking unit power supply by turning off zero point
switching SCR 21 in the above-described manner. In order to reset
the over temperature circuit 35 after it has caused a shut down of
the inductive cooking unit power supply in the above-described
manner, the zero (0) fixed contact of selector switch S.sub.2 is
connected back through a conductor 408 to the juncture of the
voltage divider resistors 416 and 417. Upon the movable contact of
the power level selector switch S.sub.2 being moved to the zero
fixed contact, it will be seen that the juncture of the voltage
dividing resistors 416 and 417 will be clamped to the potential of
the negative power supply terminal bus 16 thereby causing PUT 414
to be turned-off. Thereafter, power level selector switch S.sub.2
may be moved to any other of its fixed contacts where it is
designed that the inductive cooking unit power supply circuit
operate, and the over-temperature control circuit 35 will ramain in
the off condition provided that the over temperature condition
which originally caused the circuit to operate, has been corrected
or allowed to cool. It should be noted at this point that the same
power level selector switch S.sub.2 upon being switched to the zero
fixed contact point will cause turn-on of the inhibiting transistor
85 due to the positive polarity turn-on potential supplied across
resistor 312 from the voltage dividing resistors 313 and 314. As
stated earlier turn-on of the inhibiting transistor 85 will cause
turn-off of the zero point switching SCR 21 which in turn will
remove power from the low voltage pan safety control power
terminals 15 and 16B thereby causing this circuit to be
de-energized and turning-off the latching SCR 204 in the event that
SCR has been rendered conductive by an alarm output from the pan
safety control circuit 30. Hence, actuation of the single common
power level selector switch S.sub.2 to its zero fixed contact
serves to reset to their normal (off) conditions both the pan
safety control circuit 30 and the over-temperature control circuit
35 in the event either one or both of the circuits has been
actuated in the above-described manner.
FIGS. 4A and 4B of the drawings comprise a detailed, schematic
circuit diagram of a second embodiment of an induction cooking unit
power supply system constructed in accordance with the invention.
The embodiment of the invention shown in FIGS. 4A and 4B is
designed for operation at higher power levels than the embodiment
shown in FIGS. 2A and 2B. For example, while the FIGS. 2A and 2B
circuit is designed primarily for operation with 115 volt,
15-20amp. alternating current power source, the circuit shown in
FIGS. 4A and 4B is designed for operation with a 230 volt, 20-30
amp. alternating current power source and, of course, the
parameters of the various circuit components are proportioned for
operation at this different, higher level. One of the primary
distinguishing features of the circuit shown in FIGS. 4A and 4B is
the use of two (or more where required) series connected SCRs 17A
and 17B which are connected in series circuit relationship across
the power supply terminals 15 and 16A. Each of SCRs 17A and 17B has
an inverse, parallel connected feedback diode 18A and 18B,
respectively, connected across it along with its own respective
snubber circuit comprised by serially connected capacitors 11A and
resistor 109A and capacitor 111B and resistor 109B, respectively.
The cathode of SCR 17A and the anode of SCR 17B are connected by a
common conductor 112 to the juncture of the anode of feedback diode
18A and the cathode of feedback diode 18B and the juncture of the
snubbing resistors 109A and 109B. The gating electrodes of SCRs 17A
and 17B are connected to respective gating resistors 107A and 107B
which, in turn, are connected in the cathodes of respective, low
voltage, pilot gating SCRs 105A and 105B whose anodes are connected
through the respective voltage dividing resistors 110A and 110B to
the power supply terminal 15 and common connecting conductor 112,
respectively. The control gates of the SCRs 105A and 105B are
connected to respective, secondary windings 99A and 99B of the
gating pulse transformer T.sub.1 whose primary winding 98 is
excited upon the NPN gating transistor 97 being rendered
conductive. Upon this occurrence, positive polarity gating pulses
will be induced in the secondary windings 99A and 99B of the pulse
transformer T.sub.1 which cause turn-on of the low voltage, pilot
gating SCRs 105A and 105B. The secondary windings 99A and 99B may
be shunted by reverse connected diodes (not shown) which limit any
flyback in voltage generated by the transformer T.sub.1. The series
connected voltage dividing resistors 110A, 107A, 110B and 107B
assure simultaneous turn-on of the pilot gating SCRs 105A and 105B
so that appropriate polarity gating pulses of sufficient strength
are applied simultaneously to the control gates of the power rated
chopping SCRs 17A and 17B causing these devices to turn-on
simultaneously. Thereafter, the chopper-inverter operates in
precisely the same manner as explained earlier in connection with
the lower voltage form of the power supply circuit.
Another important feature of construction of the circuit shown in
FIG. 4A is the design of the trigger generator circuit 33 for
supplying trigger pulses to the base of the gating transistor 97.
The FIG. 4A trigger generator circuit 33 is again comprised by a
silicon unilateral switch 95 which is a voltage sensitive switch
for sensing the build up in voltage across a timing capacitor 94
that is supplied through a variable resistor 34 and series
connected fixed resistor 405 upon a triggering PNP junction
transistor 401 being turned on. The PNP transistor 401 has its
emitter-collector connected in series circuit relationship with the
fixed resistor 405, variable resistor 34 and timing capacitor 94
across the stable, low voltage (-20 volts) direct current power
source comprised by capacitor 102 and regulating zener diode 105
supplied through voltage dividing resistor 103 and diode rectifier
101 with the series connected capacitor 102, resistor 103 and diode
rectifier 101 being connected across the power supply terminals 15
and 16 that are supplied directly with the full wave rectified
output of full wave rectifier 14. By this arrangement, it will be
seen that upon turn-on of PNP transistor 401, the pre-established
low voltage, direct current excitation potential existing across
capacitor 102 will be applied directly across the
resistance-capacitance timing circuit comprised essentially by the
resistors 405, 95 and capacitor 94. In this manner full operating
voltage for assuring proper operation of the timed triggering of
SUS 95 is provided even in the valley of the ripple of the full
wave rectified potential appearing across power supply terminals 15
and 16 thereby assuring proper production of triggering pulses even
at very low power supply potentials.
The improved trigger pulse generator 33 is completed by a set of
series connected voltage dividing resistors 402 and 404 which are
connected in series circuit relationship with the voltage dividing
resistor 93 between the power supply terminal 16A (right hand side
of filter conductor L.sub.2) and power supply terminal 15. A set of
series connected diode rectifiers 403 are connected across the
voltage dividing resistor 402 for limiting or clamping the
potential supplied to the base of the triggering PNP transistor 401
to a maximum of two diode drops. At the end of the t.sub.1
commutation interval following termination of conduction through
the SCR and diode pairs 17A, 18A and 17B and 18B, respectively, the
terminal voltage of power supply terminal 16A will swing negatively
during the timing period t.sub.2. Upon this occurrence, an enabling
potential will be applied to the base of the trigger PNP transistor
401 causing this transistor to turn-on. Turn-on of transistor 401
will result in applying the full 20 volt DC potential across filter
capacitor 102 to the RC timing network comprised by resistors 405,
34 and capacitor 94. Upon the voltage of capacitor 94 building up
to the threshold level of SUS 95, SUS 95 breaks over and conducts
to produce a gating signal pulse that is applied across
resistance-capacitance coupling network comprised in part by the
capacitor 96 to the base of the NPN gating transistor 97. As a
consequence, NPN transistor 97 turns-on and produces a gating
signal pulse in the primary winding 98 that in turn produces
turn-on gating pulses in the respective secondary windings 99A and
99B resulting in turn-on of the pilot SCRs 105A and 105B in the
previously described manner. A bleeder resistor 406 is connected
across a timing capacitor 94 to assure complete dissipation of any
charge on the timing capacitor during the succeeding t.sub.1
commutation intervals while trigger transistor 401 is turned-off.
In this manner precise control over the timing of turn-on of the
SUS 95 and hence repetition rate of the resulting trigger pulses,
is maintained.
Another distinguishing feature of the FIG. 4A and 4B circuit worthy
of mention is the inclusion of suitable radio frequency filters
such as the inductive .pi. filter 411 and capacitive .pi. filter
412 connected to the power supply conductors 11 and 12 at their
point of connection to the full wave bridge rectifier 14. By the
provision of these radio frequency filter arrangements, which may
be entirely conventional in construction and operation, further
suppression of undesired radio interference effects is achieved by
preventing or eliminating any radio frequency currents that
otherwise might leak out over the power supply conductors 11 and
12. Additionally, a further radio frequency .pi. filter arrangement
413 may be connected at the terminals which connect to the
inductive heating coil L.sub.3. Harmonic components of the current
supplied to coil L.sub.3 are suppressed and by passed capacitively
to the chassis ground for the equipment. By the provision of the
inductive .pi. filter 413 connected in this manner maximum
suppression of radio frequency emissions from the unit can be
achieved.
From the foregoing description it will be appreciated that the
invention makes available a new and improved inductive cooking
apparatus power supply and pan safety control therefore which
senses whether a particular pan or other cooking vessel placed over
the inductive heating coil of the apparatus, is fabricated from a
suitable ferromagnetic material, and, if not, automatically
shuts-down the apparatus until the pan is removed. The pan safety
control can be reset readily by an operator of the inductive
cooking unit, but by reason of its design, it helps to school the
operator (who may be an inexperienced housewife) so that she
recognizes that particular pans fabricated from highly conductive
material such as aluminum and copper cannot be used safely with the
inductive cooking unit. Further, the pan safety control is
operative to protect the inductive cooking unit power supply
whether the highly conductive pan of aluminum, copper, etc. is
placed over the inductive cooking coil after the unit is operating,
or whether it is placed there before the inductive cooking unit is
turned on. Additionally, the improved inductive cooking unit power
supply includes an over-temperature cutoff control and a common
reset switching mechanism which will serve to reset not only the
over-temperature control circuit, but also will reset the pan
safety control in the event that either one or both of these
controls have been actuated. An improved gating circuit and zero
point switching inhibit and control circuit constructions also are
described which assure proper gating and operation of the zero
point switch employed in the inductive cooking unit power supply
along with a higher power (230v.) version of the apparatus and all
of these features are included in an overall power supply circuit
which is relatively simple to construct, operate and service, and
is of relatively low cost. Having described two embodiments of a
new and improved induction cooking unit power supply system and pan
safety control circuit therefore constructed in accordance with the
invention, it is believed obvious that other modifications and
variations of the invention will be suggested to those skilled in
the art in the light of the above teachings. It is therefore to be
understood that changes may be made in the particular embodiments
of the invention described which are within the full intended scope
of the invention as defined by the appended claims.
* * * * *