U.S. patent number 3,579,080 [Application Number 04/876,470] was granted by the patent office on 1971-05-18 for zero deadband reversing control.
This patent grant is currently assigned to The Louis Allis Company. Invention is credited to Donald E. Vollrath.
United States Patent |
3,579,080 |
Vollrath |
May 18, 1971 |
ZERO DEADBAND REVERSING CONTROL
Abstract
In a reversible phase controlled rectifier control system, zero
deadband response is realized by generating overlapping firing
waves for forward current and reverse current conducting controlled
switch elements of the phase controlled rectifier and controlling
the direction of load current flow substantially instantaneously to
provide for current to flow alternately in both directions when the
system is in a quiescent state. An oscillator driven flip-flop
gates firing between forward current and reverse current conducting
controlled switch elements to provide immediate current direction
control and lockout. The oscillator is responsive to the state of
conduction of the controlled switch elements, permitting the firing
signals to pass to forward current conducting controlled switch
elements when reverse current conducting controlled switch elements
are nonconductive and vice versa. The oscillator drives the
flip-flop between its stable states at a high rate when all
controlled switch elements are nonconductive so that the control
system is substantially instantaneously responsive to a command for
current to flow in either direction.
Inventors: |
Vollrath; Donald E.
(Greenfield, WI) |
Assignee: |
The Louis Allis Company
(N/A)
|
Family
ID: |
25367786 |
Appl.
No.: |
04/876,470 |
Filed: |
November 13, 1969 |
Current U.S.
Class: |
363/63;
318/257 |
Current CPC
Class: |
H02P
7/293 (20160201); H02M 7/1623 (20130101) |
Current International
Class: |
H02M
7/12 (20060101); H02M 7/162 (20060101); H02P
7/18 (20060101); H02P 7/292 (20060101); H02m
007/68 (); H02p 001/00 () |
Field of
Search: |
;321/5,13,40,47
;318/257 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Beha, Jr.; William H.
Claims
I claim:
1. A zero deadband response reversible phase controlled rectifier
control system comprising
means for developing a control function indicative of desired load
operation,
timing means for developing timing functions for the controlled
switch elements of said phase controlled rectifier, each timing
function establishing the range of current conduction for each
controlled switch element responsive thereto,
the timing functions being phase displaced from one another by said
timing means with periods of overlap therebetween creating common
ranges of controlled switch element conduction such that during
substantial satisfaction of the control function by the load,
controlled switch elements capable of conducting forward load
current and controlled switch elements capable of conducting
reverse load current conduct nonsimultaneously during half cycles
of the alternating voltage,
comparing means for comparing said control function with said
timing functions to obtain a time of firing indication for each
controlled switch element,
firing means responsive to the time of firing indications from said
comparing means for triggering each controlled switch element into
conduction at the time dictated by the corresponding time of firing
indication,
and conduction direction control means responsive to the state of
conduction of the controlled switch elements to prevent conduction
of forward current conducting controlled switch elements when
reverse current conducting controlled switch elements are
conducting and to prevent conduction of reverse current conducting
controlled switch elements when forward current conducting
controlled switch elements are conducting,
said conduction direction control means being in an oscillatory
state between forward and reverse blocking when neither forward nor
reverse current conducting controlled switch elements are
conducting such that substantially instantaneous conduction
direction control is provided.
2. A control system as recited in claim 1 further including signal
inverting means for inverting the control function prior to its
being compared with the timing functions during control of reverse
current conducting controlled switch elements such that an increase
control function command advances the firing of forward current
conducting controlled switch elements and retards the firing of
reverse current conducting controlled switch elements.
3. A control system as recited in claim 2 further including bias
means applying an adjustable DC bias to said comparing means to
adjust said ranges of controlled switch element firing overlap to
maintain a continuum of conduction during the times the load
substantially satisfies the control function command.
4. A control system as recited in claim 3 wherein said comparing
means includes a comparator for each controlled switch element and
said signal inverting means inverts the control function as it is
applied to the comparators for reverse current conducting
controlled switch elements.
5. A control system as recited in claim 4 wherein said current
direction control means includes gate means in the output path of
each comparator, a bistable switch, an oscillator and conduction
detection means for determining the state of conduction of said
controlled switch elements,
said oscillator being responsive to an indication of nonconduction
of all said controlled switch elements by said conduction detection
means for oscillating and driving the said bistable switch between
its stable states at a rate high compared with the frequency of
said alternating voltage,
said gate means being controlled by said bistable switch to permit
forward current conducting controlled switch elements to be
triggered into conduction when the bistable switch is one stable
state and reverse current conducting controlled switch elements to
be triggered into conduction when the bistable switch is in the
other stable state.
6. A control system as recited in claim 5 further including logic
means responsive to the output of each of said gate means and to
the output of said conduction detection means,
said oscillator being responsive to said logic means to oscillate
and drive said bistable switch when all controlled switch elements
are nonconductive and until a fire condition indication is
permitted to pass one of said gate means, said oscillator stopping
oscillation upon receipt of an indication from said logic means
that a fire condition indication has passed so that the bistable
switch remains in the stable state which permitted the gate means
to pass the fire condition indication.
7. A control system as recited in claim 6 wherein said conduction
detection means comprises a voltage threshold detector responsive
to the voltage level across the controlled switch elements of the
phase controlled rectifier.
8. A control system as recited in claim 3 wherein said phase
controlled rectifier is a full wave rectifier, there being a pair
of controlled switch elements consisting of a forward current
conducting controlled switch element and a reverse current
conducting controlled switch element responsive to each timing
function,
and said comparing means includes a comparator for each pair of
controlled switch elements and input gate means for each comparator
permitting comparison of the control function with the timing
function for alternate control of the forward current conducting
controlled switch element and the reverse current conducting
controlled switch element,
said signal inverting means being coupled to the comparators by the
gate means during control of reverse current conducting controlled
switch elements to invert said control function.
9. A control system as recited in claim 8 wherein said current
direction control means includes gate means in the output path of
said comparators, a bistable switch, an oscillator, and conduction
detection means for determining the state of conduction of said
controlled switch elements,
said oscillator being responsive to an indication of nonconduction
of all of said controlled switch elements by said conduction
detection means for oscillating and driving said bistable switch
between its stable states at a rate high compared to the frequency
of said alternating voltage,
the input gate means for each comparator being controlled by said
bistable switch to permit comparison of the control function with
the timing function when in one stable state to control the firing
of the forward current conducting controlled switch element of each
pair of controlled switch elements and the comparison of the
inverted control function with the timing function when in the
other stable state to control the firing of the reverse current
conducting controlled switch element,
the output gate means for each comparator being controlled by said
bistable switch to permit the forward current conducting controlled
switch element to be triggered into conduction when the bistable
switch is in said one stable state and the reverse current
conducting controlled switch element to be triggered into
conduction when the bistable switch is in said other stable
state.
10. In a phase controlled rectifier reversible power supply in
which firing signals are applied to the controlled switch elements
of the phase controlled rectifier at varying phase angles to
determine the amount and direction of power flow to and from the
load, a lockout protection circuit for substantially instantly
directing firing signals between forward and reverse current
conducting switch elements comprising:
gate means capable of permitting said firing signals to be applied
to the controlled switch elements,
conduction detection means responsive to the state of conduction of
said controlled switch elements,
and bistable switch means responsive to said conduction detection
means to cause the gate means to block firing signals for forward
current conducting controlled switch elements when reverse current
conducting controlled switch elements are conducting and to cause
the gate means to block firing signals for reverse current
conducting controlled switch elements when forward current
conducting controlled switch elements are conducting,
said direction control means including an oscillator to
continuously drive the bistable switch between its stable states
when said conduction detection means indicate that all controlled
switch elements are not conducting.
11. A lockout protection circuit as recited in claim 10 wherein
said oscillator is responsive to the first firing signal passed by
said gate means to stop the bistable switch in the state which
permitted the firing signal to pass, said direction control means
thereby causing the gate means to block firing signals for the
opposite direction current conducting controlled switch elements.
Description
FIELD OF THE INVENTION
This invention relates to control circuits for phase controlled
rectifiers. More specifically, it relates to such control circuits
for reversible phase controlled rectifiers in which there is a zero
deadband response to phase advance or retard commands.
HISTORY OF THE ART
In reversible phase controlled rectifiers, time delays and/or
deadbands, i.e., regions of no response, are often experienced when
the system is commanded to change its level or reverse its
direction. One reason for such time delays and deadbands is that in
some phase controlled rectifiers a quiescent condition, i.e., a
command satisfied condition, requires all controlled switch
elements of the phase controlled rectifier to be nonconducting and
therefore a finite voltage command must appear before the circuit
can respond. Thus, there is a signal level deadband. Because
commands applied to the system are in many applications not
instantaneous but change levels at controlled rates, this signal
level deadband takes finite time to be traversed by the command and
a time deadband before the system responds is realized. In many
systems this time deadband is longer than several cycles of the AC
source being rectified.
A primary problem with phase controlled rectifiers is the need to
avoid turning on both forward current and reverse current
conducting controlled switch elements at the same time. If this
condition does exist even momentarily, the power source is short
circuited. A solution to this problem, as mentioned above, is to
require that all controlled switch elements are nonconducting
before the system will respond to a command requiring current
reversal. In such systems another time delay is realized by the
method of determining when all controlled switch elements are
nonconducting.
The nonconduction of the controlled switch elements of a phase
controlled rectifier is often detected by observing load current.
When the load current is substantially zero, it is assumed that all
controlled switch elements are off and it is safe to turn the
opposite current conducting elements on. This is not a completely
reliable assumption, however, when the switch elements are silicon
controlled rectifiers where the holding current, i.e., the lowest
current that a controlled rectifier will conduct in the "on" state,
in some cases approaches the level of the device's leakage current,
i.e., the current that flows when the device is "off." Accordingly,
even when a substantially zero load current level is detected, a
delay is employed prior to turning on the controlled rectifiers to
give added insurance that all controlled rectifiers are off.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a
control circuit for a reversible phase controlled rectifier in
which delay times in response are virtually eliminated. Another
object is to provide a control circuit for a reversible phase
controlled rectifier having zero deadband response to phase advance
or retard commands. In general, these objects are realized by
providing a range of control about each quiescent point (when the
parameter in the load being controlled substantially satisfies the
command), where zero current to the load is achieved by the average
of nonsimultaneous current conduction in both directions during
each half cycle of the alternating voltage being rectified. Such a
range of either direction conduction is achieved by phase overlap
of the firing waves for pairs of forward and reverse current
conducting controlled switch elements of the phase controlled
rectifier. Once it is made possible to conduct in either direction,
the opportunity for a line to line short by having simultaneous
conduction of a forward current conducting controlled switch
element and a reverse current conducting controlled switch element
and a reverse current conducting controlled switch element becomes
much more likely. Accordingly, rapid response current direction
control is also required. This is provided by an oscillator driven
flip-flop which oscillates between a state which inhibits forward
current conduction to a state which inhibits reverse current
conduction. To avoid the delays of the methods normally employed
for zero current detection, absolute information is provided by a
voltage threshold detecting circuit which responds to the voltage
across the individual controlled switch elements. Voltage detection
avoids the guess between holding current or leakage current. If the
voltage is below the threshold level, the controlled switch element
is conducting. If the voltage is above the threshold level, the
controlled switch element is blocking. The information is therefore
accurate and no delays are needed before it can be accepted.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects and advantages of the present invention will be better
understood from a detailed description of exemplary embodiments
thereof as shown in the drawings, in which:
FIG. 1 is a schematic diagram of a phase controlled rectifier
suitable for use with the system shown in FIG. 4;
FIG. 2 is a schematic diagram of a voltage detector employed to
detect the state of conduction of the controlled switch elements of
the FIG. 1 power circuit;
FIG. 3 is a truth table showing information obtained from the
voltage detector of FIG. 2;
FIG. 4 is a block diagram of a control system in accordance with
the present invention;
FIG. 5 is a line diagram illustrating the conduction cycle of one
controlled switch element in response to various control
conditions;
FIG. 6 is a line diagram illustrating the conduction cycle of
controlled switch elements in the power circuit in response to
various control conditions; and
FIG. 7 is a block diagram of an alternative embodiment of current
direction control.
DETAILED DESCRIPTION
A typical phase controlled rectifier power circuit 10 of the prior
art is shown in FIG. 1. Here four controlled rectifiers SCR1, SCR2,
SCR3 and SCR4 apply full wave single phase reversible rectified
current to a load 5 which is shown as the armature of a direct
current motor. A transformer T1 applies alternating voltage to the
bridge circuit from a source not shown. The primary T1P of the
transformer is connected directly to the source and the secondary
T1S is shown applying the full wave voltage to SCR1 and SCR2, which
may be designated as a group of forward current conducting
controlled switch elements, and also to SCR3 and SCR4, which are
the reverse current conducting counterparts. The designation of
forward and reverse current is completely arbitrary and is only
stated for convenience of description.
The load 5 is shown connected between the center tap 6 of secondary
winding T1S and an electrical common 7 which may be ground as
shown. This particular power circuit with the transformer input
which permits the grounding of the load and the bridge is not
necessary for the control system to be described. It does, however,
provide the single access points 8 and 9 for detection of the
voltage across the controlled rectifiers by a relatively simple
voltage threshold detector such as shown in FIG. 2 and is therefore
preferred.
While silicon controlled rectifiers (SCR's) are shown in the power
circuit of FIG. 1 and the term SCR employed throughout the
description, this is only one kind of controlled switch element
capable of blocking or passing current in response to a control
signal. The term controlled switch element is used to represent all
such devices which make up a phase controlled rectifier including
transistors and all controllable thyristors.
Referring now to FIG. 2, there is shown a voltage threshold
detector used to determine the state of conduction of the SCR's in
the power bridge of FIG. 1. FIGS. 2 and 3 are considered together
for ease in understanding the operation of the voltage detector. It
should be understood that input point 9 of the circuit shown in
FIG. 2 is access point 9 of the power bridge. The circuit shown is
identical in all respects to its twin which monitors SCR1 and SCR3
from access point 8. Likewise, while FIG. 3 shows the voltage
conditions for varying inputs at point 9, the same points in the
circuit monitoring access point 8 would have duplicate information
at the various points shown in the table of FIG. 3.
The primary purpose of the circuit shown in FIG. 2 is to determine
when all SCR's are in the off or blocking state. This information
is obtained from the output 22 of the voltage detector 23 and
output 22' of voltage detector 23'. The three conditions that can
be presented to circuit 23, as shown in the first column "9" of
FIG. 3, are voltage less than the threshold level (shown as 0);
voltage more positive than the threshold level (shown as+); and
voltage more negative than the threshold level (shown as -). These
three distinct input conditions produce acceptable "high" or
positive outputs for voltage more positive than the threshold
level, signifying that SCR4 is blocking and for voltage more
negative than the threshold level signifying that SCR2 is blocking.
The "low" output signifies that either SCR4 or SCR2 is conducting.
A "high" will appear as a positive voltage to which AND gate 45 in
FIG. 4 will respond, and a "low" will appear as substantially zero
voltage, i.e., positive logic is assumed throughout the
description.
The primary elements of conduction detector 23 are NAND gates 10
and 11 and Zener diodes 13 and 14. The Zener diodes establish the
threshold level of the circuit and directly respond to the input
voltage. When the input voltage at 9 is zero, i.e., less than the
Zener 13, Zener 14 breakdown voltage, no current i.sub.1 flows due
to the blocking action of either Zener diode 13 if the input
voltage is positive or diode 15 if the input voltage is negative,
and a current i.sub.2 will flow from the NAND gate 10 input 17
through resistor 16 to the common. Current i.sub.2 is of small
value and the input to NAND gate 10 is considered "low." The
inverting action of NAND gate 10 and the isolation provided by
diode 21 cause both inputs 18 and 19 to NAND gate 11 to be "high."
Output 22 of NAND gate 11 is accordingly "low."
When a positive voltage sufficient to overcome the value of Zener
diode 13 appears at input 9, a current i.sub.1 will flow through
resistor 12, Zener diode 13, diode 15 and resistor 16 to ground.
This will establish a positive voltage or "high" at NAND gate input
17. The inverting action of NAND gate 10 will produce a low voltage
at input 18 to NAND gate 11 and output 22 will reflect this input
as a "high."
When the input voltage 9 goes negative and exceeds the breakdown
voltage of Zener diode 14, current will flow from input 19 of NAND
gate 11 through diode 21, Zener diodes 14 and 13 and resistor 12 to
the input, resulting in a signal logic "low" at the input 19 to
NAND gate 11. This "low" input to the NAND gate produces the
inverted "high" output at point 22. The table in FIG. 3 can be
consulted to recognize all of the logic conditions of this
threshold circuit. Note that Zener diode 20 is not necessary for
operation but merely protects both NAND gates from overvoltage.
It should be noted that the full wave power bridge with a common
ground point shown in FIG. 1 permits easy access to the SCR's and
the single input circuit of FIG. 2 can be employed. This is not the
only voltage threshold detector which can be used with the FIG. 1
power circuit, nor would it be used in the form shown with a power
circuit not employing a common ground point. Many other voltage
threshold detector circuits are suitable for this application.
Turning now to FIG. 4, there is shown a system for controlling a
reversible phase controlled rectifier with zero deadband response.
The phase controlled rectifier may be the full wave bridge
rectifier power circuit shown in FIG. 1 or, with slight
modification, any single or multiphase full or half wave controlled
rectifier. While FIG. 1 shows a motor as the load and motor speed
as the load parameter being controlled, it is intended that the
concept of zero deadband response and the techniques for achieving
it can be obtained regardless of the load or parameter being
controlled.
FIG. 4 shows a command from speed reference 30 being summed with a
speed feedback indication at speed node 31. The speed reference
block 30 represents a source of voltage such as may be derived from
a potentiometer and a direct voltage source, the output of a
tachometer coupled to another motor, or any other source of direct
voltage. A control function is developed at the speed node 31 which
is the difference between the actual value of the parameter being
controlled (indicated by the speed feedback line, which may be from
a tachometer generator coupled to the motor), and the desired value
of that parameter as generated by speed reference circuit 30. The
control function is amplified by the speed error amplifier 32 and
is shown being applied to current summing node 33 as a current
reference. A current feedback signal derived from a monitoring
device in the load circuit (not shown) is summed with the current
reference and the resulting current error is amplified by current
error amplifier 34.
The control function appearing at the output of amplifier 34 is
applied to individual comparators 1, 2, 3 and 4 for each of the
corresponding SCR's in the power circuit. This control function is
a variable direct voltage having a level proportional to the change
required in the power applied to the load to correct the parameter
being controlled to the desired value. The comparators convert the
control function into a firing time for firing the SCR's. While the
control function is shown as a resultant error signal in a closed
loop system, it is to be understood that the zero deadband control
circuit could respond directly to the command generated by
reference circuit 30 in an open loop system.
Along with the control function, a timing function is applied to
the comparators. This timing function is a signal that relates the
control function to the alternating voltage being rectified so that
the firing of the SCR's is synchronized with the alternating
voltage applied thereto. Dual ramp generator 35 generates this
timing function. The timing function, as can be noted from curves
61 and 62 in FIG. 6, is made up of two sawtooth voltages or ramp
functions. Each ramp extends for 360.degree. or one complete cycle
of the alternating voltage being rectified and the two different
functions, 61 for SCR1 and SCR4 and 62 for SCR2 and SCR3, are
180.degree. out of phase. A more appropriate observation is that
the two ramp functions overlap by 180.degree.. It is noted that at
least 270.degree. of each 360.degree. ramp is needed to control
each SCR and that each ramp is 90.degree. out of phase with the
related alternating voltage so that each ramp ends at the end of
the period of possible SCR conduction. It is desired to have
control throughout the rectifying range, which extends 180.degree.
in advance (to the left) of the zero crossing of each AC cycle, and
throughout the regenerative or inverting range, when the counter
electromotive force of the motor is negative, which extends
90.degree. behind (to the right of) the zero crossing point.
Dual ramp generator 35 is made up of components well known in the
art. It includes a zero crossing detector, i.e., a polarity or
voltage threshold sensitive circuit, responsive to the zero
crossings of each of the two phases of the alternating voltage
applied to the SCR's by transformer T1.sub.1 to synchronize each
ramp with the alternating voltage applied to the SCR, two sawtooth
waveform generators and two 90.degree. phase shift circuits. The
two outputs from dual ramp generator 35 are indicated as 35a and
35b in FIG. 4. These two outputs are phase displaced 180.degree.
and as such are ramp function 61 as shown in FIG. 6, applied to
comparators 1 and 4 from output 35a, and ramp function 62 applied
to comparators 2 and 3 from output 35b.
It is noted that the control function from current error amplifier
34 is applied directly to comparators 1 and 2 and inverted by
inverting amplifier 36 prior to being applied to comparators 3 and
4. The inversion permits a phase advance command to advance the
firing of the forward current conducting SCR's and to retard the
firing of the reverse current conducting SCR's a like amount. This
will be more readily apparent by reference to FIG. 6 and the
description of that FIG. below.
A source of direct voltage bias from supply 37 is also applied to
each of the comparators. Thus, the direct voltage bias, the control
function and the timing function constitute the three inputs to
each of the comparators. Each comparator may consist of a summing
network and an amplifier responsive to the polarity of the summed
voltage at its input. The amplifier saturates to produce a low
voltage output when the input sum is of one polarity and is
nonconductive to produce a higher voltage output when the input sum
is of the opposite polarity. Such circuits are well known in the
art as switching amplifiers. Such amplifiers may also be used with
a voltage threshold device at the input so that they respond to
level change above or below the threshold instead of polarity
change.
Other level sensitive devices of the prior art could also be used
for the comparators. For example, each comparator could comprise a
differential amplifier with the DC bias and control function summed
at one input and the timing ramp function applied to the other
input so that an output is generated when the level of the timing
function exceeds the level of the combined control function and
bias.
Before continuing with the description of the system shown in FIG.
4, the function performed by the comparators should be well in
mind. In passing, however, it should be noted that the outputs of
the comparators are applied to pulse generators 46, 47, 48 and 49
to fire the SCR's in the power bridge when permitted by AND gates
38, 39, 40 and 41. Reference is had to FIGS. 5 and 6 for a graphic
showing of the operation of the comparators and the relationship of
the DC bias, the control function and the timing function to
produce zero deadband firing control of the SCR's of phase
controlled rectifier 10.
FIG. 5 illustrates how the change in DC level of the timing
function affects the firing of the SCR. This FIG. shows the
alternating voltage waveforms, 50 in solid line and 51 in dashed
line, as applied to the phase controlled rectifier power bridge by
transformer secondary winding T1S FIG. 5 is divided into three
illustrations, (a), (b) and (c), each showing sawtooth waveform
timing function 61 as applied to comparator 1 with its DC level
altered by the sum of the control function and DC bias. Horizontal
line 52 indicates the fixed threshold level of the comparator in
each of the portions of the FIG. and line graphs 53--55 show the
resulting firing periods for SCR1.
In FIG. 5a the level of the ramp 61 is such that the ramp crosses
the threshold level of the comparator, as indicated by line 52, at
point a, approximately 45.degree. in advance of zero crossing point
x of alternating voltage 50. When the ramp exceeds the threshold
level of the comparator of fire condition indication is generated
by the comparator and a pulse can be applied to fire SCR1. Line
graph 53 shows the resulting SCR1 firing period.
FIG. 5b shows a higher DC level timing ramp which crosses the
threshold level 52 sooner. The more advanced crossing at "b" causes
the conduction period of SCR1 to begin 135.degree. in advance of
the zero crossing point x. This is shown by line graph 54.
FIG. 5c shows a lower DC level timing ramp crossing the threshold
level late in the cycle of alternating voltage 50 at point c. Line
graph 55 shows that the conduction period of SCR1 now commences
45.degree. after the zero crossing point x.
Several things can be observed. Either the DC level of the timing
ramp function can change, as shown in FIG. 5, or the DC level of
the threshold level of the comparator can change. In the preferred
embodiment where the DC bias, the control function, and the timing
function are summed and applied to a switch amplifier, it is the
timing function that has its DC level altered by the bias and
control function. This command information bearing sawtooth
waveform is compared with the fixed threshold level of the
amplifier or a zero volt level which renders the switching
amplifier of the comparator polarity sensitive. Where the
comparator comprises a differential amplifier, the threshold level
is established by the changing level of the bias and control
function and the timing ramp remains constant in its DC level.
Again with reference to FIG. 5a, consider that this represents a
quiescent condition, i.e., the previous command or control
condition has been satisfied and the 45.degree. advance firing is
just sufficient to compensate for system losses. Now assume the
command calls for an increase in speed. Consider that FIG. 5b
represents the response of SCR1 to this command and FIG. 5c shows
the response of SCR4. This is an illustration of how the control
system actually responds to such a command. Because of the
inversion of the control function signal as it is applied to the
comparators for SCR3 and SCR4, the firing of these controlled
switch elements is retarded by the same amount as the firing of
SCR1 and SCR2 is advanced.
A clearer picture of the quiescent condition, zero deadband, the
effect of the DC bias, and the effect of the counter electromotive
force of the motor (CEMF) is shown in FIG. 6.
In FIG. 6 sinusoidal waveforms 50 and 51, indicating the
alternating voltage applied to the phase controlled rectifier, are
again shown. Here, for each of the four situations illustrated, the
ramp timing functions 61 and 62 are of a constant level and are
shown being compared with the combined control function and DC
bias, represented by line 64 for SCR1 and SCR2 and line 66 for SCR3
and SCR4, which serves as the threshold levels for the comparators.
The difference in level of thresholds 64 and 66 is due to the
inversion of the control function by amplifier 36 before being
applied to comparators 3 and 4. These threshold levels remain
constant throughout FIG. 6, which shows the motor load responding
to a command for increased speed, typifying the condition present
in an open loop system (no speed or current feedback provided). In
the feedback or closed loop system shown in FIG. 4, the threshold
level, a function of the control function in the example of FIG. 6,
would rise for SCR1 and SCR2 and fall for SCR3 and SCR4 as the
command is satisfied.
FIG. 6 is divided into four different illustrations of the output
of the phase controlled rectifier as the motor load passes from a
motoring condition to a command satisfying quiescent condition to a
regenerative condition in response to a command for increased speed
which becomes satisfied and then out of this state of command
satisfaction when the load becomes overhauling. The output of the
phase controlled rectifier is shown by heavy line 58 and the level
of motor CEMF is indicated by horizontal line 59. The times of
firing of the controlled rectifiers are indicated by vertical
marking lines t.sub.1, t.sub.2, t.sub.3, and t.sub.4.
The showing in FIG. 6a is the response of the system to a command
for more advanced firing. Forward current conducting SCR1 and SCR2
are shown in a state of conduction in response to the fire
condition when the timing functions 61 and 62 respectively exceed
threshold 64. Since the alternating voltages applied to SCR1 and
SCR2 are more positive than the low level CEMF, SCR1 and SCR2 are
forward biased and conduct in response to trigger pulses at times
t.sub.2 and t.sub.4. SCR3 and SCR4, which would normally fire at
times t.sub.1 and t.sub.3 when timing functions 62 and 61 exceed
threshold 66, are reversed biased and thus do not conduct.
In FIG. 6b the CEMF level 59 has risen by the response of the motor
to the command so that SCR1 and SCR2, although still forward
biased, conduct only shortly and now SCR3 and SCR4, no longer
reverse biased, also conduct. This is an illustration of zero
deadband. Both forward current and reverse current is being
conducted within a single half cycle of the AC line. Thus, with
fast acting lockout to prevent short circuits, as will be
discussed, the control system is substantially instantly responsive
to commands for motor current reversal as well as to other advance
or retard firing commands.
FIG. 6c illustrates the range of quiescence. All controlled
rectifiers remain conductive with the small rise in the motor's
CEMF. Now SCR3 and SCR4 conduct more than SCR1 and SCR2.
In FIG. 6d the condition has changed. The level of the CEMF of the
motor has risen so that SCR1 and SCR2 are back biased into
nonconduction and only SCR3 and SCR4 conduct. Because the command
or system input indicated by threshold levels 64 and 66 has
remained constant, the condition in FIG. 6d is the system's
response to load change. Here the load has become overhauling,
i.e., the load drives the motor to supply power back to the
alternating voltage source through SCR3 and SCR4.
With zero deadband response as FIG. 6 illustrates, there is a
continuum of response from conduction in one direction through the
quiescent range of reversing overlap into conduction in the other
direction. Since the only period of nonconduction of the switch
elements is within the half cycle of the alternating voltage, no
band of nonresponse as noted in systems of the prior art is
realized. FIG. 6 shows that the system responds both to command
change and load change with zero deadband response.
When the load has satisfied the command it is necessary to insure
that a continuum of conduction is maintained. The DC bias is
provided for this purpose. It has been shown that the DC bias can
either be summed with the control function and serve as the
threshold for the comparator as illustrated in FIG. 6 or can be
summed with the control function and the timing ramp and compared
with a fixed threshold level as shown in FIG. 5. By adjusting the
value of the DC bias the amount of overlapped firing of forward and
reverse current conducting switch elements during a quiescent state
is determined. In addition, the DC bias provides for sufficient
conduction of the SCR's during the quiescent state to satisfy
system losses. 6b In order to accommodate the overlap shown in
FIGS. 6b and 6c where both forward and reverse current conducting
controlled switch elements are conducting in each half cycle of the
alternating voltage, current direction control means that are
extremely fast acting must be employed. Such means are shown in
FIG. 4 and constitute another element of the invention.
FIG. 4 shows a substantially instantaneous reacting direction
control circuit comprising a series of AND gates 38, 39 and 40 and
41, responsive to the outputs of the comparators 1, 2, 3 and 4
under the control of direction flip-flop and oscillator 43. AND
gates 38 and 39 for forward current conducting SCR1 and SCR2 are
enabled by one stable state of the flip-flop to respond to the
output of comparators 1 and 2 respectively, and AND gates 40 and 41
are enabled by the other stable state of the flip-flop. When AND
gates 38 and 39 are enabled, the state of the flip-flop is such
that AND gates 40 and 41 block any output from comparators 3 and 4.
Direction flip-flop and oscillator 43 constitute a bistable device
driven by an oscillator at a rate which is high compared to the
frequency of the alternating voltage source. The oscillator could
be of the free running type, driving the flip-flop at 20,000 hertz
which is at least two orders of magnitude greater than the normal
alternating voltage source frequency of 60 hertz. Thus, the
direction control circuit is able to substantially instantaneously
respond to either forward or reverse current conduction within a
half cycle of the alternating voltage source.
The oscillator which drives the flip-flop is activated to oscillate
when all controlled rectifiers are blocking voltage as indicated by
conduction detector 23, 23' and when there are no fire condition
indications being presented to any of the pulse generators by the
AND gates. This is logically achieved by AND gate 45 which responds
to outputs 22 and 22' from the conduction detector and to the
inverted output of OR gate 42 which in turn responds to the outputs
of AND gates 38--41. AND gate 45 controls the activation of the
oscillator of direction control circuit 43. Thus, when none of the
SCR's in the bridge are conducting as indicated by conduction
detectors 23, 23' and there is no output from the AND gates 38--41.
To render any one of the SCR's conductive, the direction flip-flop
will be driven by its oscillator between its bistable states. In
this condition of oscillation, the first fire condition indication
applied to an AND gate will pass within the microsecond of time it
takes the flip-flop to get the the proper state if it isn't already
there. The passage of this pulse changes the condition to AND gate
45 via OR gate 42; the oscillator is no longer activated; and, the
flip-flop stays in the state that passed the fire condition
indication.
The fire condition indication, once permitted to pass the AND gate,
is applied to the pulse generator for the SCR via a small input
delay. This delay allows the flip-flop to settle in the state that
permitted the pulse to pass and prevents the pulse generator from
responding to transient indications. If a fire condition indication
was generated by a comparator just as the flip-flop was changing
from one state to the other, the indication would not be of
sufficient duration to appear when the delay period transpired.
Coincident fire condition indications which can occur only during
the transition of the flip-flop between states are suppressed in
like manner because it is only the indication that remains at the
end of the delay period when the flip-flop has settled in a stable
state that is presented to the pulse generator.
The delay circuit could comprise a simple resistor-capacitor
integrator and the pulse generator would accordingly respond to a
predetermined capacitor charge level. Thus, if the fire condition
indication passed by an AND gate was of short duration, the charge
level would not be sufficient to initiate pulse generation by the
pulse generator. For this purpose, the length of the delay can be
comparatively short, i.e., several microseconds.
FIG. 7 shows an alternative embodiment of current direction
control. In this FIG. only new elements and those elements shown in
FIG. 4 necessary to understand the embodiment of FIG. 7 are shown.
The conduction detector and other control logic is the same as
shown in FIG. 4 and left out for convenience. The embodiment of
FIG. 7 takes advantage of the fact that the ramp function for SCR1
is the same as that for SCR4 and the ramp function for SCR2 and
SCR3 is the same. Thus, only one comparator 74 is time shared by
the SCR1, SCR4 circuitry and one comparator 75 is time shared by
the SCR2, SCR3 circuitry. The control function, again shown coming
from current error amplifier 34, is applied to the comparators by
gate circuits 70--73. These gates, called analog gates, are
responsive to the level change of the control function when
switched on by direction flip-flop 43. Each gate may comprise an
emitter follower transistor responsive to the control function and
a switching transistor responsive to the direction flip-flop. Any
other switch arrangement of the prior art for passing an analog
signal would also suffice.
Comparator 74 receives output 35a from dual ramp generator 35 (ramp
function 61 as shown in FIG. 6), the output from either gate 70
(the direct control function for control of SCR1) or from gate 71
(the inverted control function for control of SCR4), and the DC
bias from source 37. Comparator 75 receives output 35b from the
dual ramp generator (ramp 62), the control function from gate 72
for control of SCR2 or the inverted control function from gate 73
for control of SCR3, and the DC bias. Note that since either gate
70 or gate 71 is on a single gate which switches from a regular to
an inverted output could be used.
The output of comparator 74 is applied to AND gates 38 and 41 for
SCR1 and SCR4 and the output of comparator 75 is applied to AND
gates 39 and 40 for SCR2 and SCR3. Since AND gate 38 is controlled
by the same flip-flop output as gate 70 so that the input and
output gates for each comparator are synchronously controlled by
the flip-flop whenever the flip-flop is in the state to apply a
positive output to these two gates, a direct connection is
established between the comparator and the control function and
between the comparator and the pulse generator for SCR1. At the
same time alternate gates 71 and 41 (as well as gates 73 and 40)
are blocked by the direction control flip-flop.
It is to be understood that specific embodiments of the invention
selected as preferable for illustrating the principles thereof have
been described. Many alternatives are possible. For example, while
a control system for a single phase source is shown, the principles
of the invention are applicable to control a multiphase phase
controlled rectifier. In a multiphase system the dual ramp
generator could be replaced by means to apply phase shifted
portions of the AC wave directly to the comparators.
The system that has been described utilizes a full wave power
circuit requiring a timing function for each pair of forward and
reverse current conducting SCR's. The zero deadband control system
of the present invention is as readily applicable to a half wave
reversible phase controlled rectifier. For control of a half wave
circuit, it is noted that a timing function is generated for each
SCR.
In addition, as mentioned above, transistors could be used instead
of SCR's and are to be construed as included in the term
"controlled switch elements." Since a transistor is not a latching
device, i.e., one that remains in conduction after the triggering
excitation has been removed, such device would not be fired by a
pulse but rather by a voltage level that remains for the desired
conduction period. Thus, the pulse generators would be replaced by
voltage level switching means such as, for example, a Schmitt
trigger.
Thus, because many changes of the type that fall within the skill
of the art can be made in practicing this invention without
departing from the teachings set forth, it is intended that the
examples shown be taken as illustrating these principles only and
that the appended claims alone shall serve to define the scope of
the invention.
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