U.S. patent number 3,781,503 [Application Number 05/200,424] was granted by the patent office on 1973-12-25 for solid state induction cooking appliances and circuits.
This patent grant is currently assigned to General Electric Company. Invention is credited to John D. Harnden, Jr., William P. Kornrumpf.
United States Patent |
3,781,503 |
Harnden, Jr. , et
al. |
December 25, 1973 |
SOLID STATE INDUCTION COOKING APPLIANCES AND CIRCUITS
Abstract
An economical smooth-top cooking appliance for inductively
heating cooking utensils preferably comprises a flat air-core
induction heating coil driven at an ultrasonic frequency by a
simplified one-thyristor, one-transistor, or two-transistor
resonant inverter. The series and parallel resonant circuits in the
inverters are formed by a capacitor and the induction heating coil.
The one-thyristor series resonant circuit is desirable because
power control to adjust the cooking temperature is obtained by the
combined effect of increasing the frequency and amplitude of the
sinusoidal current pulses supplied to the coil. The coil is also
movable to change the gap spacing between coil and utensil to vary
the coupled power. Flux beneath the coil is used for warming
purposes.
Inventors: |
Harnden, Jr.; John D.
(Schenectady, NY), Kornrumpf; William P. (Schenectady,
NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
22741673 |
Appl.
No.: |
05/200,424 |
Filed: |
November 19, 1971 |
Current U.S.
Class: |
219/622; 363/124;
219/624; 219/675; 219/661 |
Current CPC
Class: |
H05B
6/062 (20130101); A47J 36/2483 (20130101); H02M
7/5236 (20130101); H02M 7/523 (20130101) |
Current International
Class: |
A47J
36/24 (20060101); H05B 6/06 (20060101); H02M
7/505 (20060101); H02M 7/523 (20060101); H05B
6/12 (20060101); H05b 005/04 () |
Field of
Search: |
;219/10.49,10.75,10.77,10.79 ;321/27,45,4,44 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Maphaim, "A Low Cost Ultrasonic Frequency Invention Using a Single
SCR," Application Note 200.49, General Electric Co., Syracuse,
N.Y..
|
Primary Examiner: Reynolds; Bruce A.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
Induction cooking appliances with other inverter power circuits and
multicylinder power circuits are respectively disclosed and claimed
more broadly in the following concurrently filed applications
assigned to the same assignee as this invention: U.S. Pat. Ser. No.
(200,526) by David L. Bowers, Donald S. Heidtmann, and John D.
Harnden, Jr.; and U.S. Pat. Ser. No. (200,528) by William P.
Kornrumpf and John D. Harnden, Jr.
Claims
What we claim as new and desire to secure by Letters Patent of the
United States is:
1. A solid state cooking appliance for inductively heating cooking
utensils comprising
a substantially non-metallic, plate-like support having a
substantially unbroken utensil supporting surface,
a nominally flat induction heating coil mounted adjacent said
support for generating an alternating magnetic field that extends
across a gap including said support and beyond said utensil
supporting surface, and
a static power conversion circuit including a unidirectional
voltage supply and a solid state inverter for generating an
ultrasonic frequency wave that drives said induction heating
coil,
said inverter comprising a capacitor and said induction heating
coil connected as the only resonating components of a single
resonant circuit that is operated by a single controlled solid
state switching device and control circuit therefor.
2. A cooking appliance according to claim 1 wherein said induction
heating coil is an air-core coil, and the appliance further
includes
a non-metallic warming tray mounted beneath said air-core coil for
supporting a utensil to be warmed.
3. A cooking appliance according to claim 1 wherein said induction
heating coil is adjustably mounted relative to said support to
change said gap and therefore the power coupled to the utensil.
4. A cooking appliance according to claim 1 wherein said induction
heating coil is a ferromagnetic-core coil and further includes a
ferrite plate mounted approximately parallel to said coil to serve
as a high permeability path for return magnetic flux.
5. A cooking appliance according to claim 1 wherein said induction
heating coil is a single layer coil made with braided ribbon
conductor.
6. A cooking appliance according to claim 1 wherein said induction
heating coil is a single-layer coil made with ribbon litz wire.
7. A cooking appliance according to claim 1 wherein said source of
unidirectional voltage supply is a source of variable input
unidirectional voltage to thereby modulate the power output of said
inverter and thus the power coupled to the utensil.
8. A cooking appliance according to claim 1 wherein said inverter
resonant circuit is a series resonant circuit in which said
capacitor and induction heating coil are effectively connected in
series circuit relationship with one another and said controlled
solid state switching device,
a diode connected in inverse-parallel with said controlled
switching device, and
reset inductor means operative during nonconducting intervals of
said controlled switching device and diode for resetting said
capacitor.
9. A solid state cooking appliance for inductively heating a
cooking utensil comprising
a substantially non-metallic, plate-like support having a
substantially unbroken utensil supporting surface,
a nominally flat air-core induction heating coil mounted adjacent
said support for generating an alternating magnetic field that
extends across a gap including said support and beyond said utensil
supporting surface, and
a static power conversion circuit including full wave rectifying
means and a solid state inverter for generating an ultrasonic
output frequency wave that drives said induction heating coil,
said inverter comprising a capacitor and said induction heating
coil connected as the only resonating components of a single
resonant circuit that is operated by a single controlled solid
state switching device and control circuit therefor.
10. A cooking appliance according to claim 9 wherein said inverter
resonant circuit is a series resonant circuit in which said
capacitor and induction heating coil are effectively connected in
series circuit relationship with one another and said controlled
solid state switching device, said controlled switching device
being a thyristor,
a diode connected directly in inverse-parallel with said thyristor
to form a thyristor-diode combination, and
reset inductive means for resetting said capacitor during
nonconducting intervals of said thyristor and diode.
11. A cooking appliance according to claim 10 wherein said series
connected capacitor, induction heating coil and thyristor-diode
combination are effectively coupled between the output terminals of
said rectifying means, and
said reset inductive means is connected across said capacitor.
12. A cooking appliance according to claim 10 wherein said reset
inductive means and thyristor-diode combination are effectively
coupled in series circuit relationship between the output terminals
of said rectifying means, and
said series connected capacitor and induction heating coil are
connected across said thyristor-diode combination.
13. A cooking appliance according to claim 10 wherein said series
connected capacitor, induction heating coil, and thyristor-diode
combination are effectively coupled between the output terminals of
said rectifying means, and
said reset inductive means is connected across said series
connected capacitor and induction heating coil.
14. A cooking appliance according to claim 10 wherein said control
circuit for said thyristor controlled switching device operates at
a variable repetition rate to change both the amplitude and
frequency of said output frequency wave and therefore the power
coupled to the utensil.
15. A cooking appliance according to claim 10 wherein said
rectifying means comprises a phase controlled rectifier which
supplies a variable input voltage to thereby change the amplitude
of said output frequency wave and thus the power coupled to the
utensil.
16. A cooking appliance according to claim 9 wherein said induction
heating coil is adjustably mounted relative to said support to
change said gap and therefore the power coupled to the utensil.
17. A solid state induction cooking appliance comprising
a substantially non-metallic, plate-like support having a planar,
substantially unbroken utensil supporting surface,
a nominally flat air-core induction heating coil mounted beneath
said support for generating an alternating magnetic field that
extends across a gap including said support and is coupled with
metallic portions of a cooking utensil placed thereon, and
a static power conversion circuit including unidirectional voltage
supply and a solid state inverter for generating an ultrasonic
frequency output wave that drives said induction heating coil,
said inverter comprising a capacitor and said coupled induction
heating coil and utensil connected as a resonant circuit that is
operated by a single controlled solid state switching device and
control circuit.
18. A cooking appliance according to claim 17 further including a
non-metallic warming tray movably mounted beneath said induction
heating coil for supporting a utensil to be warmed.
19. A cooking appliance according to claim 17 wherein said
induction heating coil is adjustably mounted relative to said
support to change said gap and therefore the power coupled to said
utensil.
20. A solid state cooking appliance for inductively heating a
cooking utensil comprising
a substantially non-metallic, plate-like support having a
substantially unbroken utensil supporting surface,
a nominally flat air-core induction heating coil mounted adjustably
beneath said support for generating an alternating magnetic field
that extends across a gap including said support and beyond said
utensil supporting surface,
a static power conversion circuit including a unidirectional
voltage supply and a solid state inverter for generating an
ultrasonic frequency wave that drives said induction heating
coil,
means for modulating the power output of said inverter to thereby
control the power coupled to the utensil, and
means for moving said induction heating coil relative to said
support to independently control the power coupled to the utensil
by changing the gap between said coil and support.
21. A solid state cooking appliance for inductively heating a
cooking utensil comprising
an induction heating coil mounted adjacent a substantially
non-metallic unbroken utensil support and generating an alternating
magnetic field,
a static power conversion circuit having a pair of unidirectional
voltage terminals and including a solid state inverter for
converting the unidirectional voltage to an ultrasonic output
frequency wave that drives said induction heating coil,
said inverter being a one-thyristor series resonant inverter
comprising reset inductive means connected in series circuit
relationship with said thyristor and an inverse-parallel connected
diode between said unidirectional voltage terminals, and only a
commutating capacitor and said induction heating coil connected in
series circuit relationship directly across said thyristor and
inverse-parallel diode, and
a single control circuit for turning on said thyristor at a
variable repetition rate to adjust both the amplitude and frequency
of said output frequency wave and therefore the power coupled to
the cooking utensil.
22. A cooking appliance according to claim 21 wherein said reset
inductive means is a reset inductor that operates to recharge said
commutating capacitor during nonconducting intervals of said
thyristor and inverse-parallel diode.
23. A cooking appliance according to claim 21 wherein said static
power conversion circuit further includes unidirectional voltage
power supply connected to said pair of unidirectional voltage
terminals that converts an alternating supply voltage to a variable
unidirectional voltage to further control the power coupled to the
cooking utensil.
Description
BACKGROUND OF THE INVENTION
This invention relates to cooking appliances based on induction
heating and to solid state circuits therefor, and more particularly
to improved, economical power circuits for induction cooking
appliances operating in the ultrasonic frequency range.
The use of induction heating to heat a food containing cooking
utensil is a theoretically efficient process since heat is
generated only in the metallic utensil where it is wanted. The
ordinary gas range and electric range by comparison have greater
losses due to poor coupling of heat to the utensil and heating the
surrounding atmosphere. Although known in principle for a number of
years, prior food cooking appliances based on induction heating
have been unsatisfactory. Equipment using available line
frequencies of 50 Hz or 60 Hz was inadequate and cumbersome, and
eddy current cookers employing a circular arrangement of
alternately poled permanent magnets rotated by an electric motor
were bulky, expensive, and for other reasons not suitable for wide
usage.
Induction cooking appliances employing solid state power conversion
circuits to drive the induction heating coil at the higher
frequencies that are desirable have made possible a significant
reduction in cost and size of the equipment. By operating at
ultrasonic frequencies of 18 kHz and above, the appliance is
inaudible to most humans, and solid state power conditioning at
these frequencies is presently reliable. The above-identified
application of Bowers et al discloses a power converter including a
rectifier and a two-thyristor series sine wave inverter that drives
a pancake ferromagnetic-core induction heating coil. To be truly
competitive for commercial introduction, however, improved and more
economical appliances and circuitry are needed.
SUMMARY OF THE INVENTION
The solid state induction cooking appliance to which the invention
applies comprises a substantially nonmetallic, plate-like support
having a substantially unbroken utensil supporting surface. A flat
or nominally flat induction heating coil is mounted adjacent the
support and generates an alternating magnetic field that couples
with metallic portions of a utensil placed on the support. A static
power conversion circuit typically comprising a rectifier and an
inverter generates an ultrasonic output frequency wave for driving
the induction heating coil. To obtain an economical appliance, such
as a cooktop unit or a portable counter-top warming or cooking
appliance, the improvement is made of using a relatively simple
inverter comprising a capacitor and the induction heating coil
connected as a series or parallel resonant circuit that is
controlled by a single solid state switching device and control
circuit therefor. While it is desirable to use an inexpensive
air-coil core, the simplified inverters are usable with the ferrite
ferromagnetic-core coils also.
In the preferred embodiments, the inverter is a series resonant
sine wave inverter in which a thyristor (SCR) and an
inverse-parallel connected diode are connected in series with the
capacitor and coil, and reset inductor means is provided to reset
the capacitor during nonconducting intervals of the thyristor-diode
combination. Power control to adjust the cooking temperature or
utensil heating level is obtained by the combined effect of both
increasing the frequency and amplitude of the sinusoidal current
pulses supplied to the coil. Other embodiments are one-thyristor
and one-transistor parallel resonant circuits comprising a
capacitor and the coil connected in parallel. The inverter power
output and thus the power coupled to the utensil is adjusted by
respectively changing the amplitude or frequency of the sinusoidal
current pulses.
Other low cost power conversion circuits disclosed are a
multi-cylinder inverter based on the one-thyristor series resonant
inverter and, two-transistor inverters using parallel and series
resonant circuits. Output power control in any of these power
conversion circuits is also achieved by using a phase controlled
rectifier to vary the d-c supply to the inverter. Another technique
is to mount the coil adjustably relative to the utensil support to
change the gap spacing between coil and utensil. These power
control techniques can be used in any selected combination to
obtain the range of cooking temperatures and settings desired. An
additional feature of an appliance with a flat air-core induction
heating coil is a non-metallic warming tray beneath the coil for
supporting covered utensils heated by magnetic flux emanating from
the bottom of the coil.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a solid state converter for supplying
ultrasonic frequency power to a flat spiral induction heating coil,
shown in plan view, in an induction cooking appliance constructed
according to the invention;
FIG. 2 is a diagrammatic cross-sectional view illustrating the
relation of the induction heating coil, support plate, cooking
utensil, and optional warming tray in a cooking appliance;
FIG. 3 shows several modifications of FIG. 2 including a coil made
with braided ribbon, an improved ferromagnetic core for the coil,
and adjustment of the cooking temperature by changing the coil
position;
FIG. 4 is a perspective view of the upper portion only of an
electric range with a food warming facility;
FIG. 5 is a detailed schematic circuit diagram of one embodiment of
a static power conversion circuit comprising an economical series
resonant inverter using only one thyristor and the induction
heating coil in a dual function as the commutating inductance;
FIG. 6 is the equivalent circuit diagram of the induction heating
coil and cooking utensil load;
FIGS. 7a and 7b are waveform diagrams of the induction coil current
and commutating capacitor voltage for the circuit of FIG. 5,
showing in each diagram the waveforms at two different inverter
output frequencies;
FIG. 8 is a modification of the FIG. 5 converter employing a phase
controlled rectifier and a modified gating and power circuit;
FIG. 9 shows another modification of the inverter of FIG. 5;
FIG. 10 is a schematic circuit diagram of a low cost multi-cylinder
power converter (less the rectifier) based on the inverter
configuration of FIG. 5;
FIG. 11 is an induction coil current waveform diagram useful in
explaining the operation of FIG. 10;
FIGS. 12 and 13 are schematic circuit diagrams of parallel resonant
inverters respectively using a single thyristor and a
transistor;
FIG. 14a shows a two-transistor inverter circuit;
FIG. 14b is a plan view in schematic form of the series aiding,
equal-turn induction heating coil used in the FIG. 14a circuit;
and
FIG. 15 is a detailed circuit diagram of another two-transistor
resonant inverter.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The induction cooking appliance shown in FIGS. 1-4 will be
described with regard to an induction surface unit in an electric
range, but essentially the same mechanical structure and circuitry
in a lower power version is also suitable for a portable
counter-top food cooking or warming appliance. The static power
conversion circuit indicated generally at 12 is preferably
energized by a single phase commercially available 60 Hz, 120 volt
or 240 volt source of alternating current potential, however in
appropriate cases this equipment can be designed for use with other
low frequency, low voltage or d-c sources. Static power converter
12 comprises generally a rectifier 13 and a solid state inverter 14
for converting the unidirectional rectifier output to an ultrasonic
frequency wave for driving an induction heating coil 15. Induction
heating coil 15 is a single layer, annular, flat spiral, air-core
coil wound with solid flat strip conductors or braided ribbon with
a rectangular cross section. To generate sufficient magnetic flux
to heat the utensil to the desired cooking temperature, coil 15 is
tightly wound with the short cross sectional dimension facing
upwards and adjacent turns separated by a flat insulating strip 20.
While coil 15 is usually perfectly flat, it is within the scope of
the invention to use a nominally flat coil such as one that is
slightly dished to improve the magnetic field distribution.
In the cooking appliance (FIG. 2), induction heating coil 15 is
appropriately mounted in a horizontal position immediately below a
non-metallic support 16 typically made of a thin sheet of glas,
ceramic, or plastic. Support plate 16 is commonly referred to as
the cooking surface and supports the metallic cooking utensil 17 to
be heated. If required, support 16 can have some metallic content
for electrostatic shielding or decorative purposes, but this is
necessarily limited to a small amount to permit nearly full power
to be coupled to the utensil. Cooking utensil 17 is more
particularly an ordinary cooking pot or pan, a frying pan, or some
other available metallic utensil used in food preparation. The
utensil can be made of a magnetic material such as magnetic
stainless steel, enameled steel, or cast iron; a non-magnetic
material such as aluminum; or a laminated product such as
copper-clad stainless steel or triple-clad (stainless-cast
iron-stainless steel). Special cooking utensils are not required,
although the best and most efficient results are obtained by
optimizing the size, shape, and material of the cooking utensil.
Because of their relatively thin skin depth at ultrasonic
frequencies which results in a high equivalent impedance and also
because of the high magnetic losses, magnetic steel utensils couple
well to coil 15 and are heated most efficiently. Some copper-clad
utensils and thick aluminum utensils do not couple well to the coil
due to their low equivalent impedance, while the laminated and cast
iron utensils are inbetween. Any of these may be used, however, in
an induction cooking appliance when the coil 15, static power
conversion circuit 12, and the gap between the coil and utensil are
properly designed. Ordinarily a gap of at least one-eighth inch is
required between the top of coil 15 and the bottom of utensil 17 to
allow space for non-metallic support 16, and the gap is no greater
than about one-half inch at full power rating in order to couple
sufficient power into the utensil bottom to produce adequate
heating for cooking purposes. It is also important to impedance
match the utensil to the power circuit for best overall
efficiency.
Operation of solid state inverter 14 to impress an ultrasonic
frequency wave on induction heating coil 15 results in the
generation of an alternating magnetic field. The magnetic flux, in
particular flux emanating from the top of coil 15, is coupled
across the air gap through nonmetallic support 16 to utensil 17. An
ultrasonic frequency above 18 kHz or so is normally considered to
be the upper range of human hearing and is selected to make the
cooking appliance inaudible to most people. The induction heating
coil 15 shown in FIG. 2 is constructed with solid copper flat strip
with a rectangular cross section. Conveniently, the coil is wound
using alternating flat strips of copper conductor and a suitable
insulating material. To reduce the high frequency losses due to the
skin effect, it is preferable to use a coil 15' made with braided
ribbon as is illustrated schematically in FIG. 3. A still further
reduction in losses is obtained by using ribbon litz wire. Because
of the usual physical constraints, only a limited number of
ampere-turns can be included in a practical induction heating coil.
Since the power coupled to the utensil is theoretically
proportional to the square of the ampere-turns, the reduction of
losses is of consequence in increasing the efficiency.
Some of the other options that are possible as to the mechanical
and electrical arrangement of induction heating coils 15 and 15'
are also illustrated in FIG. 3. One of these is to use a
ferromagnetic-core coil rather than air-core coil. To this end, a
ferromagnetic core 18 for coil 15' is mounted horizontally beneath
the coil spaced from it by a predetermined air gap. The core serves
as a high permeability path for the return magnetic flux, one such
path being illustrated at 19, and is preferably a thin plate of
ferrite dispersed for instance in an extruded rubber matrix. The
ferrite plate runs cool and has superior high frequency performance
as compared to a laminated steel plate, and for a given induction
heating coil the power output requirements of static power
conversion circuit 12 are lower as compared to the air-core coil
situation. Usually, coils 15 and 15' are mounted stationary beneath
utensil support 16 with a fixed air gap spacing by means of an
appropriate mechanical support structure not here shown. Increasing
or decreasing the gap between the coil and utensil 17, however,
changes the amount of power coupled to the utensil and therefore
the temperature to which the utensil is heated. Other ways of
adjusting the cooking temperature by control of the power output of
static power conversion circuit 12 will be discussed later. Rather
than moving utensil support 16 relative to coil 15' (FIG. 3), a
preferred arrangement in view of the fact that coil 15 is
relatively light is to change the gap spacing by mounting coil 15'
for vertical movement. As illustrated here schematically, coil
support 21 underlies coil 15' and has a vertical extension that
extends above the surface of support 16 so as to be available to
the person doing the cooking to make a manual adjustment. Another
suitable arrangement employs a wire wound on the shaft of a control
knob on the range control panel.
FIG. 4 shows an induction cooking appliance in the form of an
electric range with four induction surface unit positions indicated
in dotted lines on the surface of utensil support 16. An important
feature of the invention is that support 16 is made of a
non-metallic material such as plastic or glass, and preferably has
a smooth and unbroken utensil supporting surface. This is not to
preclude the inclusion of sensors or indicators in support 16. At
ultrasonic frequencies there are insignificant reaction forces
which at lower frequencies would cause utensil 17 to move
horizontally when placed on the support plate approximately
centered with respect to one of the induction surface unit
positions. The transfer of energy to the utensil to heat it is
relatively efficient since heat is generated only in the utensil
and none is lost because of mismatch in size between the coil and
utensil. Although induction heating coil 15 may require some
cooling, the surface of support 16 is relatively cool since the
highest temperatures involved are about 450.degree. F, the
approximate maximum temperature to which the bottom of utensil 17
is heated to cook food as for example in frying operations. Because
of the cool cooking surface, spilled foods do not burn and char and
both support 16 and utensil 17 are easy to clean. The cool, smooth
support 16 also makes it possible to use this surface before
cooking, or even immediately after cooking, for other functions
related to food preparation such as opening cans, trimming and
cutting vegetables, transferring cooked food from the cooking
utensil to a serving dish, etc. Another advantage of induction
cooking is that induction heating results in a low thermal mass
system. Since there is a relatively low storage of heat in utensil
17 itself, the temperature to which the utensil is heated can be
changed rapidly, as from boiling to simmering to warming
temperatures.
A new feature in an electric range or a portable counter-top
warming or cooking appliance made possible by the induction cooking
principle is the inclusion in the appliance of a food warming
opening and tray beneath the induction heating coil. Referring to
FIG. 2, an alternating magnetic field is also produced at the
bottom of air-core induction heating coil 15 as well as the top,
and a portion of this magnetic flux is available for warming or
heating food in a closed metallic container 22 (shown in dotted
lines) supported on a non-metallic tray 23. Frozen foods in a
closed aluminum foil container can also be defrosted in this
manner. To provide the food warming facility in an electric range,
the upper portion of the range (FIG. 4) has a food warming cavity
24 that extends beneath all four coils of the four induction
surface units. Tray 23 is preferably made of plastic and is
slidable in and out of the food warming cavity. A variety of
thermoplastic and thermosetting plastics, with decorative surface
designs if desired, can be used to make tray 23 and utensil support
16. As previously mentioned, this is because no point in the system
needs to be in excess of a maximum temperature of about 450.degree.
F. Suitable plastics that can be used are Textolite (trademark of
the General Electric Company), epoxy, silicone, polyimides, and
others. These plastic materials can also be used in the fabrication
of induction heating coils 15 and 15'.
The embodiment of power converter circuit 12 illustrated in FIG. 5
is characterized by a relatively simple and inexpensive inverter
that uses only one thyristor and gating circuit, and employs
induction heating coil 15 in a dual function to couple power to the
utensil (load) and to provide commutating inductance in the
thyristor commutation circuit. To gain commercial acceptance, the
ultrasonic frequency converter must be relatively low cost and
reliable under both load and no-load conditions. The power
converter input terminals 27 and 28 are adapted to be connected
across a 120 volt, 60 Hz source of a-c supply voltage. The power
supply section of the converter circuit includes a full wave diode
bridge rectifier 13 and a filter capacitor 29 connected between the
rectifier output terminals, thereby providing a pair of direct
current supply terminals 30 and 31 for inverter 14. Inverter 14 is
a one-thyristor series capacitor commutated or series resonant
inverter that generates damped sinusoidal pulses. The power circuit
comprises essentially induction heating coil 15 connected in series
circuit relationship with a commutating capacitor 32 and a
unidirectional conducting thyristor 33 between the d-c power supply
terminals 30 and 31. A diode 34 to conduct power current in the
reverse direction is connected across the load terminals of
thyristor 33. A series RC circuit 33' is usually connected across
the load terminals of thyristor 33 for dv/dt protection to limit
the rate of reapplication of forward voltage to the device. The
power circuit also includes a reset inductor 35 connected directly
across commutating capacitor 32. As is explained in detail later,
the function of reset inductor 35 is to recharge commutating
capacitor 32 between complete cycles of operation when both
thyristor 33 and diode 34 are non-conductive.
Thyristor 33 is more particularly a silicon controlled rectifier,
but other solid state switching devices including a pair of
transistors connected to operate in thyristor fashion can also be
used. Power diode 34 can also be replaced by a thyristor controlled
to be conductive for a complete half cycle. In this low cost
circuit, however, the combination of the inverse-parallel connected
silicon controlled rectifier and diode are clearly preferred. Only
one gating circuit is required since diode 34 becomes forward
biased and conducts when the current in the series resonant circuit
reverses in the negative polarity half of the cycle. Gating circuit
36 for thyristor 33 includes a series RC timing circuit comprising
a fixed resistor 37, an adjustable resistor 38, and a timing
capacitor 39 connected between the anode of the power device and
negative d-c supply terminal 31. A voltage sensitive signal level
semiconductor such as Shockley diode 40 and a series resistor 41
are connected between the junction of resistor 38 and timing
capacitor 39 and the gate of thyristor 33. Timing begins when both
power devices 33 and 34 have ceased conducting, and the next gating
pulse is generated by the charging of timing capacitor 39 to a
predetermined voltage which casues Shockley diode 40 to break over
and conduct.
Although utensil 17 is referred to loosely as the inverter load, it
is more accurate to say that the inverter load is the electrical
loss in the utensil. With respect to the utensil load, induction
heating coil 15 functions as the primary winding of an air-core
transformer. Utensil 17 functions as a single-turn secondary
winding with a series resistance 17r representing the resistive
part of the I.sup.2 R or eddy current losses, and hysteresis losses
where applicable. The currents and voltage induced in utensil 17
when the induction surface unit is in operation are determined
essentially by the transformer laws. The physical equivalent for
utensil 17 in the form of a single turn winding and resistive
losses 17r is given in FIG. 5. FIG. 6 shows the equivalent
electricl circuit for coil 15 and utensil 17. Coil 15 is
represented by a series connected inductance 15i and resistance
15r, and these are in turn in series with the parallel combination
of an inductance 17i and resistance 17r representing the utensil.
This electrical equivalent circuit is based on conventional
transformer equivalent circuit analysis and has been found to
reasonably agree with experimental results.
With the utensil load in place, the commutating inductance for the
series resonant circuit comprising coil 15 and commutating
capacitor 32 is composed of both the coil inductance 15i and the
reflected utensil inductance 17i. Under no-load conditions, with
the utensil removed from the induction surface unit, the amount of
commutating inductance increases. This causes a change in the
resonant frequency of the series resonant circuit, so that there is
a slight decrease in the inverter output frequency. With an average
or selected utensil load in place, the series resonant circuit is
tuned to resonance at a resonant frequency higher than the highest
desired output frequency. The ultrasonic output frequency range of
interest is between approximately 18 kHz and 40 kHz. The upper
limit of this frequency range is determined largely by economic
considerations, in conjunction with the high frequency limitations
of available thyristor devices.
The operation of an induction cooking appliance employing the
static power conversion circuit illustrated in FIG. 5 is as
follows. The appliance is turned on by means of an on-off switch 42
shown only in FIG. 4 on the rear control panel, at the back of the
electric range. Switch 42 is preferably combined with adjustable
resistor 38 in gating circuit 36 to control the gating signal
repetition rate and therefore the cooking temperature. When the
circuit is initially energized thyristor 33 is nonconducting and
diode 34 is reverse-biased, but a circuit is completed through coil
15, commutating capacitor 32, and reset inductor 35 to the series
RC timing circuit in gating circuit 36. Adjusting resistor 38
changes the time constant of this timing circuit. After the
selected time delay, timing capacitor 39 charges to the
predetermined voltage at which Shockley diode 40 breaks over and
conducts. A gating pulse is supplied to the gate-cathode circuit of
power thyristor 33, turning it on, and at the same time timing
capacitor 39 is reset. A positive polarity half-sinusoidal current
pulse flows through induction heating coil 15 and theoretically
charges commutating capacitor 32 to a value approaching twice the
supply voltage V.sub.dc. Because of the resistive coil and utensil
losses, the circuit energized is more exactly a damped series R-L-C
oscillatory circuit. At the end of the half cycle, the current
drops below the holding current of thyristor 33, so that it begins
to turn off, and upon the current reversal diode 34 conducts and
aids the turn-off of thyristor 33 by applying reverse voltage to
the device. At the end of the negative polarity half cycle (see
FIG. 7a) the current through induction heating coil 15 remains at
zero since the next gating pulse is not applied to thyristor 33 at
this time. While power devices 33 and 34 were conducting, the
current in reset inductor 35 on a steady state basis was increasing
due to the net positive d-c voltage on commutating capacitor 32
during the conduction cycle. During the circuit off-time reset
inductor 35 discharges, thereby leaving commutating capacitor 32
negatively charged at the end of the circuit off-time. Voltage is
supplied to gating circuit 36 during the circuit off-time, and at
the end of the predetermined time delay determined by the setting
of adjustable resistor 38, a gating pulse is produced to render
conductive thyristor 33 and begin the next cycle of operation.
FIG. 7a shows in full lines the sinusoidal induction coil current
for two complete cycles of operation separated by a time delay 43.
The corresponding commutating capacitor voltage is shown in full
lines in FIG. 7b. At the end of the first conduction cycle,
capacitor 32 has a net positive d-c voltage. The action of reset
inductor 35 during the time delay period 43 is to change the
capacitor voltage almost linearly as indicated at 44, leaving the
capacitor negatively charged at the end of time delay interval 43.
Due to this extra charge on commutating capacitor 32, the peak
capacitor voltage 45 during the next cycle of operation is higher
than in the dampened oscillatory case since the system energy is
replenished. The effect of shortening time delay 43 by adjusting
resistor 38 is to increase the inverter output frequency and
therefore the current and power supplied to utensil 17. Increasing
the inverter output frequency also has the beneficial result of
increasing the amplitude of the sinusoidal current pulses. This is
illustrated in FIG. 7a by the second cycle dashed line current
waveform. By advancing the thyristor gating pulse, the ratio of
conduction time to capacitor recharge time increases, thereby on a
steady state basis increasing the average current in reset inductor
35. The result is that, referring to the dashed line capacitor
voltage characteristic in FIG. 7b, commutating capacitor 32 is
charged negatively to a higher negative voltage during the circuit
off-time as indicated by 44' so that the peak capacitor voltage 45'
during the next conduction cycle is higher than the peak voltage 45
for the lower inverter frequency case. A limit on inverter
frequency is reached when the value of the current in reset
inductor 35 becomes significant in comparison to the high frequency
reverse current pulse in coil 15 and commutating capacitor 32,
since this in effect reduces the commutation time available to the
thyristor. In summary, there are two effects that increase the
power in watts supplied to utensil 17 when the inverter frequency
is increased. There are larger, more frequently applied current
pulses in induction heating coil 15.
The static power conversion circuit illustrated in FIG. 8 shows
several modifications of the FIG. 5 converter. The full wave diode
rectifier 13 is replaced by a phase controlled rectifier simply by
substituting thyristors for two of the diodes in FIG. 5 to thereby
provide a variable source of d-c voltage for inverter 14. The
filter additionally includes filter inductor 29'. Suitable phase
controlled gating circuits for the thyristors are described in the
General Electric SCR Manual, 4th edition, copyright 1967, available
from Electronics Park, Syracuse, New York. Adjustable components in
these phase controlled gating circuits can be connected to be
adjustable by rotation of on-off switch 42 (FIG. 4). The four
mechanisms that have been described for achieving variable power
control of the static conversion circuit can be used in any desired
combination. These are, briefly, the mechanical method of raising
and lowering the induction heating coil (FIG. 3) to change the gap
spacing, the power circuit technique of varying the inverter output
frequency to change the frequency and magnitude of the current
pulses supplied to the coil, and the control circuit technique of
using a phase controlled rectifier to adjust the d-c supply
voltage. Still another mechanism disclosed in the first
cross-reference application is to step change the commutating
circuit capacitance and inductance parameters. To obtain a large
range of power output, such as a 30:1 ratio, combinations of these
techniques undoubtedly are required.
Gating circuit 36' in FIG. 8 is a modified form of the gating
circuit disclosed in FIG. 5, which times the delay interval 43
between the end of one cycle and the beginning of the next. The
disadvantage of this arrangement is that it is load dependent. The
preferred timing arrange-ment obtained by gating circuit 36' is to
time from the beginning of one cycle to the beginning of the next
cycle. To this end, resistor 37 in the timing circuit is connected
through an on-off switch 42', which can be identical to the switch
42 in FIG. 4 or operated concurrently with it, to the positive d-c
supply terminal 30. Also, a constant source of voltage for
adjustable resistor 38 and timing capacitor 39 is provided by
connecting a Zener diode 46 in parallel with both these components.
Once gating circuit 36' is energized by closing switch 42', gating
pulses for thyristor 33 are produced at an approximately constant
repetition rate depending on the setting of adjustable resistor 38.
One possible shortcoming of the FIG. 5 power circuit is that the
return path for high frequency current pulses is through filter
capacitor 29, which is ordinarily an electrolytic capacitor
normally having a high impedance at ultrasonic frequencies. The
power circuit modification of FIG. 8 eliminates this problem and
has improved performance. In this power circuit, the resonant
circuit comprising coil 15 and commutating capacitor 32 is
connected between the anode of thyristor 33 and negative d-c supply
terminal 31. Reset inductor 35 is now connected in series with
thyristor 33 between d-c terminals 30 and 31, the latter normally
at ground potential. The impedance of reset inductor 35 is
sufficiently large to supply relatively constant current to the
resonant circuit.
With thyristor 33 or diode 34 conducting, the ultrasonic frequency
return path is to ground rather than through filter capacitor 29.
The operation of this modified power circuit is believed to be
obvious from the previous discussion of FIG. 5. Reset inductor 35
discharges during the circuit off-time through coil 15 to charge
commutating capacitor 32 positively. As the inverter output
frequency increases, the average current in reset inductor 35 also
increases, thereby resetting capacitor 32 to a more positive level
in each cycle.
A second modification of the one-thyristor series resonant power
circuit is given in FIG. 9. This is similar to FIG. 5 with the
exception that reset inductor 35 is connected in parallel with both
coil 15 and commutating capacitor 32. This allows the discharge
reset current of reset inductor 35 to circulate through induction
heating coil 15 (as in FIG. 8), thereby increasing the power for a
given set of circuit parameters. For a given ultrasonic output
frequency, the FIG. 8 configuration gives the highest output power
and accordingly is preferred.
By way of example of a specific induction cooking appliance
suitable for use as an induction surface heating unit in an
electric range, the required maximum power output is 1,500 watts
and the ultrasonic frequency operating range is about 18 kHz to 25
kHz. Using a 120 volt, 60 Hz a-c source, the peak input d-c voltage
is 150 volts. Induction heating coil 15 has an outside diameter of
seven and one-half inches and an inside diameter of one and
one-half inches. Coil 15 more specifically has 43 turns of No. 11
braided ribbon conductor having a cross section of about
five-sixteenths inches by less than one-sixteenth inch with a
turn-to-turn spacing of at least 10 mils. The combined equivalent
coil inductance 15i and utensil conductance 17i (see FIG. 6) is
typically 150 microhenries, and the combined equivalent series coil
resistance 15r and utensil resistance 17r is 15 ohms. For a
counter-top warming or cooking appliance a lower maximum output
power is satisfactory, such as 200-400 watts.
The outstanding advantage of the cooking appliance power conversion
circuits shown in FIGS. 5 and 8, and of the modification of FIG. 9,
is that these circuits are relatively simple and economical though
yet being satisfactory for a full range of cooking tasks. A minimum
of power circuit components are employed, due particularly to the
use of the thyristor-diode combination, and to the dual function of
induction heating coil 15 as a substantial part of the commutating
inductance and as a transformer to couple power to the utensil
load. Furthermore, only one relatively simple gating circuit for
thyristor 33 is required. This one-thyristor series resonant power
circuit operates in a full satisfactory manner under both load and
no-load conditions. Although not optimized for every conceivable
type of available pot and pan, a variety of ordinary utensils that
couple well or reasonably well to the coil can be used. This
inverter is appropriate for operation by non-technical persons
since a minimum number of user controls are needed. There is no
failure due to shoot-through when adjustable resistor 38 is
slewed.
The low cost multi-cylinder cooking appliance power converter shown
in FIG. 10 is based on the simple one-thyristor inverter of FIG. 5.
This is a two-cylinder converter using two such inverters 14a and
14b assembled in mirror-image fashion between d-c power supply
terminals 50 and 51 and common point 54. A center-tapped type power
supply is used providing +V (such as +150 volts) at positive d-c
terminal 50 and -V (-150 volts) at negative terminal 51. Between
these terminals are a pair of voltage dividing capacitors 52 and
53, with their junction 54 connected to ground. Induction heating
coil 15 is common to both inverters and is connected in the
bidirectional conducting branch of the converter between point 54
and the junction 55 between the remaining components of the two
FIG. 5 type inverfers.
Referring also to FIG. 11, the two converters 14a and 14b can be
operated alternately with or without a time delay between
conduction cycles, or can be operated alternately in overlapping
fashion. In the first mentioned mode, thyristor 33 a and diode 34a
conduct consecutively, and then thyristor 33b and diode 34b are
rendered conductive as in the usual half bridge inverter
configuration. The induction coil currents are shown in FIG. 11.
Variable output power in watts is obtained with this circuit in the
same manner as previously discussed, by control of the frequency of
the current pulses through coil 15 by varying the repetition rate
of supplying gating pulses alternately to thyristors 33a and 33b.
The advantage of this circuit is that the high frequency
requirements of thyristors 33a and 33b are not as stringent, since
in the usual mode of operation each has more turn-off time than in
the FIG. 5 inverter. In the same manner the other components have a
lower duty cycle and are lower cost components.
FIGS. 12 and 13 show low cost inverters suitable for induction
cooking appliances characterized as one-thyristor and
one-transistor parallel resonant circuits. In these circuits
induction coil 15 and capacitor 32' are connected parallel to one
another in a tank circuit rather than in series as in the previous
inverters. In FIG. 12, thyristor 58 is connected in series with a
commutating inductor 59 and parallel-connected coil 15 and
resonating capacitor 32', this entire circuit being connected
between d-c supply terminals 30 and 31. A solid state firing
circuit 61 is connected between the gate of thyristor 58 and
negative terminal 31, and is controlled by a sense voltage derived
from the voltage across capacitor 32', thus closing the oscillatory
loop. Waveform 62 represents the capacitor voltage. Power control
is achieved by varying the voltage at which thyristor 58 fires
within the thresh-old limits indicated by arrow 63. The theory of
operation is similar to that for conventional Class C tube
oscillators. The interval of conduction of thyristor 58 is limited
to a small part of the oscillatory cycle. When the threshold
capacitor voltage at which thyristor 58 is fired is between zero
volts and the upper positive limit (the voltage on terminal 30),
the amplitude of the sinusoidal oscillations decreases, and thus
the power output decreases as the threshold voltage becomes larger.
With increasing negative threshold voltages, the power output
increases. The parallel resonant circuit comprising coil 15 and
capacitor 32' is, of course, tuned to resonate at a selected
ultrasonic frequency. Commutating inductor 59 provides a means for
turning off thyristor 58 by reverse biasing it during a part of
each cycle when the capacitor voltage resonates above the supply
voltage. Suitable firing circuits that can be used are described in
the General Electric SCR Manual.
The advantage of both the FIG. 12 and FIG. 13 parallel resonant
circuits is that the high frequency circulating currents do not
pass through the solid state power device. If the circuit has had
appreciable operating Q, the power device losses can be
considerable compared to the useful power delivered to the load. A
circuit which removes the thyristor or transistor from the
oscillating tank circuit theoretically exhibits a higher efficiency
and less of a heat disposal problem. FIG. 13 is the transistor
version of the one-thyristor parallel resonant circuit. In this
power circuit the capacitor 32'-coil 15 tank circuit is in series
with a blocking diode 64 and a power transistor 65. A commutating
inductor is not required since the transistor is turned off by base
drive circuit 66. Suitable solid state base drive circuits that can
be employed are described in the General Electric Transistor
Manual, seventh edition, copyright 1964. A feedback connection 67
senses the transistor collector voltage and is operative to turn on
transistor drive circuit 66 to supply base current to power
transistor 65 when the collector voltage is at its minimum.
Referring to capacitor voltage waveform 68, during the conduction
time of transistor 65 the capacitor voltage falls to zero, after
which coil 15 is charged with current until a timing circuit shuts
off the transistor. The resulting transistor dissipation is the
minimum possible with this circuit. Control of the conducting
interval of transistor 65, indicated by arrow 69, accomplishes
power output control of the circuit. Increasing the conduction
interval of transistor 65 increases the current in coil 15.
Decreasing the interval conversely decreases the current and
therefore the power delivered to the utensil. An alternative
technique for controlling power output is by varying the peak
transistor current.
FIGS. 14a and 15 show low cost two-transistor ultrasonic frequency
inverters for solid state cooking appliances. The induction heating
coil for the FIG. 14a parallel resonant circuit comprises two coil
sections 15a and 15b with the center-tap 15c connected to the
positive d-c supply terminal 30. Coils 15a and 15b are series
aiding, equal-turn coils wound two-in-hand as shown in FIG. 14b.
The free terminals of the two coils are connected to negative d-c
supply terminal 31 through the respective power transistors 72 and
73 controlled by base drive circuits 74 and 75. A tuning capacitor
76 is connected across a selected number of turns of coils 15a and
15b. In operation, power transistors 72 and 73 are rendered
conductive alternately for desired intervals of conduction. When
transistor 72 conducts current flow through coil 15a produces a
pulse of one polarity, and a pulse of the other polarity is
generated when transistor 73 conducts. Variable power control is
achieved by input voltage control, or by varying the conduction
interval of the two transistors to change the amplitude of the
ultrasonic wave.
In the low cost two-transistor series resonant sine wave inverter
shown in FIG. 15, the two power transistors 72 and 73 are connected
in series between d-c supply terminals 30 and 31, and diodes 80 and
81 are connected across the emitter-collector terminals of the
respective devices. The series resonant circuit comprising
commutating capacitor 32 and coil 15 is connected to the junction
77 between the two power transistors and to the junction between a
pair of voltage divider capacitors 78 and 79. In operation,
transistor 72 is rendered conductive during generation of the
positive polarity half sinusoidal current pulse, and diode 80
conducts when the current reverses and the series resonant circuit
generates a negative polarity half sinusoidal pulse. Transistor 73
and diode 81 conduct during the next conduction cycle. The output
power is again controlled by modulating the inverter output
frequency, or by input voltage control.
In summary, improved economical solid state induction cooking
appliances are characterized by relatively simple power conversion
circuits for use with inexpensive flat air-core induction heating
coils as well as improved ferromagnetic-core coils. The ultrasonic
frequency inverters preferably employ a single controlled switching
device, and therefore only one control circuit, and use the
induction heating coil in a dual function to couple power to the
utensil and as a commutating inductor or resonating inductor.
One-thyristor series resonant circuits, one-thyristor and
one-transistor parallel resonant circuits, two-transistor circuits,
and multicylinder series resonant circuits have been discussed. The
first is advantageous because power control to adjust the
temperature to which the utensil is heated is by the combined
effect of increasing both the amplitude and frequency of the
sinusoidal current pulses supplied to the coil. Other power control
techniques usable in different combinations are to employ a phase
controlled rectifier and to move the coil relative to the utensil
support to change the gap spacing. An additional feature of the
air-core coil is the provision of a warming cavity and tray beneath
the coil.
While the invention has been particularly shown and described with
reference to several preferred embodiments thereof, it will be
understood by those skilled in the art that various changes in form
and details may be made therein without departing from the spirit
and scope of the invention.
* * * * *