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Claims  |
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I claim:
1. An energy economizing AC power control system operative to reduce the
iron and copper losses in a conventional three phase AC induction motor by
varying the form and magnitude of input voltage coupled thereto,
comprising a conventional three phase AC induction motor having three
stator windings and a rotor adapted to be coupled to a load, a three
conductor three phase sine wave power source for energizing said stator
windings to effect rotation of said rotor, and a conditionally closed loop
positive feedback control means that becomes operative to control the
energization of said stator windings from said power source by reference
to a particular speed caused to be energy-efficient by properties inherent
to induction motors during a limited range of motor speeds near the full
speed of said motor, said closed loop positive feedback control means
comprising: motor load detecting means coupled to said rotor and operative
to produce a frequency modulated signal related to the load on said motor,
frequency discriminator means coupled to said load detecting means for
producing a DC control voltage which varies in magnitude in inverse
relation to the speed of said motor, emitter-follower means coupled to the
output of said frequency discriminator means to render said frequency
discriminator means insensitive to variations of subsequent circuit means
while coupling said DC control voltage to said subsequent circuit means, a
plurality of voltage coupling means coupled to the output of said
emitter-follower means for providing a plurality of DC control voltage
outputs which are isolated from each other, two wave modifier means
responsive respectively to said DC control voltage outputs and including
associated switching means disposed between two conductors of said three
phase power source and two of said three stator windings, the third stator
winding being directly connected to the third conductor of said power
source, said wave modifier means being operative to apply full sine waves
of voltage from said power source to their associated stator windings
during rotational start-up and speed build-up of said motor and, as said
motor reaches said particular speed, then being operative to substantially
maintain said particular speed by varying the electrical angle of each
cycle of said power source which is actually coupled to said two stator
windings, and therefore to said third stator winding, thereby providing
varying fractions of each sine wave of voltage from said power source to
all three of said stator windings in accordance with the energy
requirements imposed on said motor by the rotor load at any given moment
and in accordance with the inherent electromechanical properties of said
motor, whereby said closed loop positive feedback control means functions
to substantially reduce the iron and copper losses of said conventional
motor by causing the average current supplied from said power source to
said stator windings to be supplied primarily as a function of the load on
and properties inherent to said motor as the load of said motor varies
between zero and maximum rated load.
2. The control system of claim 1 wherein said switching means includes two
Triac assemblies, each having its input coupled to a separate one of the
three conductors of said sine wave power source and its output connected
to a separate one of the three inputs to said stator winding, and an
associated control circuit coupled to the control terminal of each Triac
assembly, each of said control circuits being responsive to the magnitude
of said DC control voltage for controlling the conductivity of each said
Triac during each cycle of its respective phase of said three phase sine
wave power source.
3. The control system of claim 1 including means for altering the rate of
change in power coupled from said AC power source by each of said wave
modifiers at no or light loads as compared to each other in response to a
given change in said DC control voltage coupled from said load detecting
means through said isolated coupling means to said wave modifiers.
4. The control system of claim 3 including nonlinear coupling means
interposed between said frequency discriminator and said wave modifiers,
said nonlinear means being a resistor paralleled by a zener diode, said
diode being operative to couple steeply rising DC control voltage produced
by a step-function motor load around said resistor to said wave modifiers.
5. An energy economizing AC power control system operative to reduce the
iron and copper losses in a conventional polyphase induction motor having
a plurality of different phase stator windings and a rotor adapted to be
coupled to a load, a polyphase sine wave power source for energizing said
stator windings to effect rotation of said rotor, and a nonlinear closed
loop positive feedback control means conditionally operative to control
the form and magnitude of the energization of said stator windings from
said power source above a particular reference speed dependent upon the
inherent electromechanical properties of induction motors which cause such
motors to exhibit speed related efficiency, said feedback control means
comprising: motor load detecting means coupled to said rotor and operative
to produce a frequency modulated signal related to both the load on and
speed of said motor, a nonlinear circuit coupled to said load detecting
means for producing a control signal which varies as a function of the
speed and load of said motor above said particular reference speed, at
least two wave modifier means coupled to the output of said nonlinear
circuit and responsive to said control signal, said two wave modifier
means each including switching means and being disposed respectively
between different phases of said polyphase sine wave power source and
different ones of said stator windings, the conduction time of the
switching means in each of said wave modifier means being controllable
during each cycle from the associated phase of said power source as a
function of said control signal, said two wave modifier means being
operative respectively to apply full sine waves of voltage from said power
source to their associated stator windings during rotational start-up and
speed build-up of said polyphase induction motor and, as said motor
reaches said particular speed, then being operative to vary the electrical
angle of each cycle of said power source which is actually coupled to said
stator winding to provide varying fractions of each sine wave of voltage
from said power source to said stator winding in accordance with the
energy requirements imposed on said motor by the inherent
electromechanical properties of said motor and the rotor load at any given
moment, whereby said closed loop positive feedback control means functions
to substantially reduce the iron and copper losses of said polyphase
induction motor by causing the average current supplied from said power
source to said stator windings to be supplied primarily as a function of
the load and properties inherent to said motor as the load on said motor
varies between zero and maximum rated load.
6. The control system of claim 5 wherein the switching means in each of
said wave modifier means includes a Triac assembly having its input
coupled to an associated phase of said sine wave power source and its
output connected to said associated stator winding, and a control circuit
coupled to the control terminal of said Triac assembly and responsive to
said control signal for controlling the conductivity of said Triac
assembly during each cycle of said associated phase of said sine wave
power source.
7. The control system of claim 6 wherein said control circuit comprises
trigger pulse generators operative selectively to produce trains of
trigger pulses, means responsive to said control signal for controlling
the operation of said trigger pulse generators, and amplifier means
coupling trigger pulses from the output of said trigger pulse generators
to said control terminals of said Triac assemblies.
8. The control system of claim 7 including rectifier means responsive to
the voltage zero crossings of two phases of said sine wave power source
for controlling the starting and stopping of said trains of trigger
pulses.
9. The control system of claim 8 wherein each of said rectifier means is
connected respectively to the base of a transistor and operates to keep
said transistor cut off except during said zero crossings of the
associated phases of said sine wave power source, said transistors being
connected respectively to a further transistor which is connected to said
trigger pulse generators, said further transistors being operative as
switches to turn said trigger pulse generators on and off.
10. The control system of claim 9 including capacitor means coupling the
output of said trigger pulse generators to said further transistors and
operative to feed the starting pulses from said output to said further
transistors to accelerate the turning on and pulse switching of said
trigger pulse generators.
11. The control system of claim 7 wherein each of said trigger pulse
generators comprises a normally inoperative multivibrator, and means
repponsive to said control signal for controlling the time at which said
multivibrator is rendered operative in respect to the beginning of each
voltage cycle of the associated phase of said sine wave power source
thereby to control the time at which said Triac assembly is rendered
conductive in relation to the beginning of each said cycle.
12. The control system of claim 5 wherein said load detecting means
comprises a comparatively small AC generator coupled to said rotor for
rotation with said rotor, said generator being operative to
electromechanically produce a frequency modulated AC signal having a
plurality of frequencies which vary with variations in the speed of and
load on said motor, an amplitude limiter, a signal-biased nonlinear DC
amplifier, and frequency discriminator means coupled through said
amplitude limiter to said AC generator and to said DC amplifier for
converting said frequency variations to amplitude variations of said
control signal.
13. The control system of claim 12 wherein said frequency discriminator
means is connected between the output of said AC generator and the input
of said DC amplifier, said DC amplifier including means for varying the
signal derived forward bias amplitude of said DC amplifer as a function of
the instantaneous frequencies of said AC signal.
14. The control system of claim 5 wherein said load detecting means
comprises a reference signal derived from the rotor action of said rotor,
said rotor action being operative to modulate a characteristic of said
signal derived therefrom in accordance with variations in the load on and
the speed of said rotor, and demodulating means operative to convert said
generated signal modulation to command variations of said control signal.
15. The control system of claim 14 including commandable control circuit
means adjustably responsive to said control signal command variations
above said particular efficiency-related reference speed.
16. The control system of claim 5 wherein at least one of said stator
windings is connected directly to one of the phases of said power source.
17. The control system of claim 5 wherein said polyphase induction motor is
a three phase motor having three stator windings, two of said stator
windings being connected to two phases of said power source via the
switching means in said two wave modifier means respectively, and the
third stator winding being connected directly to the third phase of said
power source.
18. The control system of claim 17 wherein one of said two wave modifiers
includes means operative to produce a phase angle delay in the turn on of
its associated Triac at zero and very light motor loads in response to
said control signal from said load detecting means to each of said wave
modifiers.
19. The control system of claim 5 wherein said wave modifiers are operative
to couple continuous sine waves of power to their respective stator
windings when the average speed of said rotor is below said particular
speed reference.
20. The control system of claim 19 wherein said three phase motor is
operated in a partial sine wave single phase mode by said wave modifier
means in response to said control signal to maintain the particular rotor
speed at which said motor achieves its most energy-efficient conversion of
electrical energy to mechanical energy at zero and very light mechanical
loads. |
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Claims |
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Description |
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CROSS-REFERENCE TO RELATED APPLICATIONS
Parker and Hedges prior copending U.S. Application Ser. No. 917,698, filed
June 21, 1978, now U.S. Pat. No. 4,190,793 for Energy Economizer for
Induction Motors, which is a continuation-in-part of U.S. Application Ser.
No. 839,945, filed Oct. 6, 1977, now abandoned, discloses various
structural and operational aspects of the present invention as employed,
however, in a single phase motor system, and the disclosure of said prior
copending Application Ser. No. 917,698, U.S. Pat. No. 4,190,793 is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
Conventional induction motors maintain the full sine wave of voltage across
the stator winding regardless of the load in the motor. In those cases
where the load varies within wide limits e.g., when the motor is used for
hoisting operations, most of the time the motor is not expanding its full
rated load. In such cases, the iron losses in the stator are substantially
the same when the motor is operating below full rated load as is the case
when the motor is operating at full rated load; and, due to low power
factor in such cases, the stator current is high and the copper losses are
also substantial.
When a conventional polyphase induction motor is operating below its full
rated load, a fraction of the sine wave of voltage or operation on a
single phase would satisfy the actual load requirement imposed on the
motor. Such cutting in part in the sine wave voltage or operating on a
single phase would result in considerably less iron and copper losses and
less heating of the stator. The resultant lower operating temperature
further reduces the copper losses in the motor due to lowered ohmic
resistance. These factors combine to effect a significant reduction in the
energy which is consumed by the motor, with a consequent conservation in
available energy sources and a reduction in operating costs.
The present invention is based upon a recognition of the foregoing factors,
and provides a simple yet reliable mechanism operative to cause the
electrical energy supplied to the stator and the stator flux density of a
standard-unmodified-AC polyphase induction motor to become a function of
its load demand at any given moment. The invention accomplishes this by
permitting a greater or smaller portion of the sine wave of voltage from a
power source to enter the stator as a function of the percentage of slip
of the motor or by operating on a single phase. In other words, the sine
wave of the voltage supplied to the motor's stator is modified to suit
existing load conditions. This results in the reduction of iron and copper
losses.
SUMMARY OF THE INVENTION
In accordance with the present invention, a standard three phase motor has
only one input to its stator connected directly to the three phase AC
power source. The other two stator inputs are coupled to said power line
through triacs which begin conduction continuously when input power is
first applied to assure undiminished torque during motor start-up. After
motor start-up, said triacs become part of a conditionally operative
nonlinear positive feedback loop and may be phase angle controlled by wave
modifier means responsive to a frequency modulation signal generated by
motor load detecting means, with the result that each triac conducts all
or a fraction of its sine wave power input in accordance with the actual
load demand placed on the motor. In this way, motor speed is kept at a
substantially constant particular speed under all load conditions within
the rating of the motor, and full power is supplied at greater loads which
assures undiminished overload performance.
In order to achieve optimum efficiency and smooth operation, each triac
couples a different fraction of source power to the stator at light to
moderate loads, and one triac is open-circuited at very light and zero
loads causing the motor to operate in a partial sine wave single phase
mode.
The load detecting means and wave modifiers may be as described in the
aforementioned Parker et al U.S. Application Nos. 839,945 and 917,698.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an energy economizer system constructed in
accordance with the present invention; and
FIG. 2 is a schematic diagram of a preferred circuit of the type shown in
block diagram form in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in FIG. 1, a standard 3.phi. motor 10 is provided with stator
windings 2a, 2b and 2c, e.g., which can be connected to a 3.phi. AC power
source 3, as shown by lines 4a, 4b and 4c, to energize the stator windings
thereby to effect rotation of rotor 12. In accordance with the present
invention, power source 3, instead of being coupled directly to each of
the motor stator windings, are connected as shown in FIG. 1. One
conductor, designated for descriptive purposes as 4c, is connected
directly to stator winding 2c in the normal fashion. Conductors 4b and 4a,
however, are connected respectively to one side of two solid state
switches 5 and 6, the other sides of which are connected as at 5a and 6a
to the inputs of windings 2a and 2b of the motor stator. Therefore, the
average energy coupled to all windings from power source 3, including
stator winding 2c, becomes a function of the conduction duration of
switches 5 and 6. Switches 5 and 6, which may comprise, for example, Triac
assemblies, form a part of wave modifiers 7 and 8 whose operation is
controlled by a DC control voltage appearing on line 16b at the output of
emitter-follower 11. Said DC control voltage magnitude corresponds to
frequency modulation developed by a comparatively small AC generator
mechanically coupled to motor 10.
The generator 18 of FIG. 1 comprises a toothed wheel 18a which is mounted
at 13 on the shaft of induction motor 10. Toothed wheel 18a and its
associated stator comprise a small AC generator whose output frequency
modulation is determined by the nature of its rotational speed and the
number of teeth on wheel 18a. The mean output frequency of the generator
is an integral multiple of the mean speed of rotation of induction motor
10 and may, for example, be approximately 60 times the RPS of the motor.
Generator 18's output, which by choice is of a much higher frequency than
that of power source 3, is an electromechanically produced AC signal with
two forms of load-related frequency modulation. A first form of rotor
produced frequency modulation in generator 18 is inherent to all induction
motors and is caused by the change in rotor slip that results from a
change in motor mechanical load. More particularly, the mean (or average)
frequency of the AC signal (generated by motor shaft rpm) changes in
direct proportion to load related rpm changes. Both polyphase and single
phase motors produce this form of load-induced frequency-shift modulation.
Less obvious rotor movements of a three phase motor operated in a single
phase or three phase partial sine wave mode produce another form of
frequency modulation in the output of generator 18. There are natural
torque variations in single phase motor operation due to the sinusoidal
current flowing through its stator windings passing through a zero value
twice in each cycle causing minor speed variations. Thus, with a 60 Hertz
AC power source, single phase operation produces 120 load-responsive
torque and speed variaions per second which said generator experiences.
Likewise, operating a three phase motor on 60 Hertz partial sine waves
produces 360 such torque and speed variations which frequency modulate
said generator's output. Therefore, as motor 10's load increases (from
zero load), the signal from generator 18 will consist of a mean frequency
proportional to its average speed with variations above and below this
mean frequency occurring first at a rate of 120 times per second during
single phase operation, changing to to 360 times per second as partial
sine wave, three phase operation begins.
This FM/FM modulated AC signal is applied to the input of frequency
discriminator 17 which converts said frequency modulation appearing on
line 20 to a corresponding DC control voltage or signal on line 16 whose
magnitude is a function of the total frequency modulation of said signal
at motor speeds above about 95% of synchronous speed.
For purposes of the subsequent description, it will be assumed that
generator 18 has 60 teeth and the induction motor rotates at a speed of 30
RPS so that the AC output from the small generator connected thereto
includes 1800 Hz., and these parameters will be referred to hereinafter
for purposes of illustration.
The output signal from AC generator 18 is applied across an LC circuit 32
as shown on FIG. 2 which is broadly resonant at the generator frequency to
obtain a more nearly sinusoidal waveform output. The resultant AC signal
is applied via capacitor 33 to the base of transistor 34 whose collector
is energized via resistor 35 from the positive side of DC supply 31. Any
amplitude variations of the signal from the AC generator 18 are eliminated
by the clamping actions of a diode 36 and the limiter action of the
base-emitter junction of transistor 34. Consequently, transistor 34 acts
as a limiter amplifier.
Due to the positive and negative clamping actions, the waveform at the base
and collector of transistor 34 becomes a flat top wave. These flat top
pulses are fed to and excite a high Q resonant circuit 37 which is tuned
above one of the frequencies of generator 18 to approximately 1850 Hz. The
output signal of the AC generator operates on the slope of the resonance
curve of circuit 37, so that said circuit 37 acts, in effect, as a
frequency discriminator, i.e., the voltage appearing across circuit 37
varies in amplitude in accordance with the frequency modulation of the
signal which is supplied thereto from transistor 34.
The signal developed across resonant circuit 37 is fed through a variable
resistor 38 and a capacitor 39 to a signal-biased DC amplifier comprising
transistor 40. As the signal derived positive base voltage rises above its
cut-off threshold, there is a rapid, non-linear turn-on of transistor 40
collector current. Variable resistor 38 is adjusted so that said
non-linear turn-on is referenced to a particular energy-efficient speed
above about 95% of the motor's synchronous speed. Above said speed,
transistor 40's average conduction becomes gradually more linearly
responsive to said demodulated signal voltage magnitude. Transistor 40 is
signal-biased in both a forward and reverse bias direction by the charge
and discharge of capacitor 39 caused by the passage of alternating current
through said capacitor and its subsequent rectification by transistor 40.
Due to said reverse bias and transistor 40's unbypassed emitter resistor
45, the load imposed on resonant circuit 37 by transistor 40 is minimal.
Zener diode 41, connected between the anode of a diode 42 and ground,
provides a low resistance discharge path for capacitor 39 during negative
reverse bias signal alternation peaks that exceed its conduction threshold
(Zener) voltage thereby protecting transistor 40 from high negative
voltage peaks. Diode 42 prevents the positive forward bias signal
alternation from being conducted to ground through Zener diode 41.
Since the amplitude of the AC voltage across resonant circuit 37 varies in
accordance with the load-related frequency modulation of the signal
supplied thereto, the bias on transistor 40 also varies and the portion of
the signal which is effective to render transistor 40 conductive similarly
varies in accordance with the motor load at any given moment. Above the
nonlinear conduction threshold of transistor 40, an increase in the
amplitude of the AC voltage across resonant circuit 37 results in an
increase in the current flow through resistor 43, which in turn, produces
a greater voltage drop across resistor 43 and reduces the voltage at the
collector of transistor 40, and vice versa. As a result, this particular
portion of the circuit operates as an inverse signal generator, i.e., a
reverse of amplitude variation occurs between the base and collector of
transistor 40.
The collector of transistor 40 is connected to one side of a capacitor 44,
the other side of which is grounded. Capacitor 44 is charged through
resistor 43 part of the time, i.e., when transistor 40 is nonconductive,
and is discharged through transistor 40 and resistor 45 when transistor 40
is rendered conductive. The time constant of the RC circuit 43, 44, is
long compared to 1800 Hz., (i.e., the nominal output frequency of AC
generator 18) and the mean frequency ripple voltage across capacitor 44
accordingly has a very low amplitude. As a result, when motor 10 is
running near its most energy-efficient speed, the voltage across capacitor
44 decreases to a fairly steady DC potential whose average magnitude
varies in proportion to the load and speed induced frequency modulation.
Said voltage across capacitor 44 comprises the DC control voltage
appearing on line 16 (shown in FIGS. 1 and 2). In FIG. 1, the DC control
voltage line 16 is shown connected to the input of emitter-follower 11.
FIG. 2 shows that emitter-follower 11 consists of a transistor 11, an AC
bypass capacitor 80, a variable resistor 82, an output load resistor 81,
and diodes 83 and 84. Emitter-follower 11 operates in a conventional
manner and serves to isolate the DC control voltage developed at the
collector of transistor 40 from loading by the DC control voltage inputs
of wave modifiers 7 and 8 to which it is ultimately coupled.
Adjustment of variable resistor 82 determines the maximum level to which
capacitor 44 may charge during moments when transistor 40 is cut-off and,
therefore, controls the rate at which the average DC control voltage may
change with load-induced frequency modulation. In other words, resistor 82
may be employed to vary the system response (time) to a change in motor
load.
Diodes 83 and 84 couple the DC control voltage appearing on line 16b to
wave modifiers 7 and 8. Zener diode 86 conducts around resistor 87 at high
DC control voltage magnitudes, thereby providing faster wave modifier
response when (if) a heavy load is abruptly applied to motor 10.
Each of wave modifiers 7, 8 requires two control signal input. One of said
inputs is coupled from a common source, i.e., the DC control voltage
output of emitter-follower 11. The other input is an AC reference signal
which synchronizes the conduction of a zero crossing detector (transistor
52 or 52a) with the voltage zero crossing of the particular phase (from AC
power source 3) controlled by said wave modifier. In the case of modifier
7, for example, said second input control (or synchronizing signal is
coupled from phase 2.
The operation of only wave modifier 7 and its zero crossing detector will
be described since said operation in each wave modifier is the same. The
synchronizing reference signal (described above) is coupled by transformer
28 whose primary is connected between phase 2 and phase 3. The secondary
output of transformer 28, a low potential (e.g., 12.6 VAC) 60 cycle
voltage, is connected to full wave rectifier 51 whose output is a series
of negative going alternations with respect to ground (as at 29 on FIG. 2)
which is coupled to the base of transistor 52. The base of transistor 52
is also supplied with forward bias current through resistor 46 from DC
power supply 31. The negative going alternations from rectifier 51 keep
transistor 52 nonconductive except at voltage zero crossing. Forward bias
through resistor 46 causes collector-emitter saturation of transistor 52
near said zero crossings and, during this time, the junction of resistors
47 and capcitor 48 (i.e., the collector of transistor 52) is clamped to
the near ground potential of 0.1 vdc. More particularly, after the phase 2
reference voltage pases through zero, the voltage supplied by rectifier 51
begins falling toward a negative (peak) value of -12.6 vdc. When the
resultant voltage on the base of transistor 52 falls below approximately
+0.7 vdc, collector-emitter cut-off occurs. Transistor 52 remains cut-off
until the voltage at its base rises to +0.7 vdc due to the forward bias
supplied by resistor 46 as phase 2 closely approaches its voltage zero
crossing.
Thus, transistor 52 is cut-off most of the time during each said AC power
source cycle, and conducts only slightly before, during and slightly after
voltage zero crossings of the phase to which it is referenced. The
duration of conduction of transistor 52 (and transistor 52a) is
approximately 1 ms.
When transistor 52 is conductive, capacitor 48 discharges; when transistor
52 is cut off, as described above, capacitor 48 begins charging through
resistor 47 toward the level of the DC control voltage which is supplied
by capacitor 44. The resultant signal is supplied via resistor 54 to the
base of a transistor 55 to render transistor 55 conductive, but the
conduction of transistor 55 is delayed in accordance with the voltage
which is actually present on the positive side of capacitor 48. More
particularly, transistor 55 remains nonconductive until the voltage across
capacitor 48, which is coupled to the base of transistor 55 through
resistor 54, reaches approcimately +0.7 vdc., whereafter transistor 55
(which constitutes a trigger delay switch) begins to conduct
collector-emitter current.
Transistor 55 is connected to the emitter of a transistor 56, which
cooperates with a further transistor 57 and with a plurality of associated
capacitors and resistors, to provide a multiple gate trigger generator
which comprises an astable (free-running) multivibrator of well-known
configuration, with one exception. The exception is that, whereas the
emitter of transistor 57 is directly grounded, the emitter of transistor
56 in said multiple gate trigger generator is not grounded and, instead,
is connected to ground through transistor 55. As a result, typical
multivibrator operation of transistors 56, 57 is prevented until
transistor 55 conducts to provide a conduction path to ground for the
emitter of transistor 56. As soon as transistor 55 is brought to saturated
conduction, typical astable multivibrator operation occurs. The starting
of the multiple gate trigger generator 56, 57 is accelerated by capacitor
58 which feeds the starting pulse from the output of said generator to
transistor switch 55.
The component values of the multivibrator (or multiple gate trigger
generator) 56, 57 are selected to cause multivibrator action at
approximately 20 kHz. The output signal which is produced, when the
multiple gate trigger generator is rendered operative, takes the form of a
train of trigger pulses each of which has a width in the order of 25
.mu.s, occurring over a maximum time period of approximately 7 ms. per
alternation of the AC power source 13, or occurring during such lesser
portion of said AC power source cycle as may be determined by the time at
which transistor 55 was rendered conductive to enable operation of the
multiple gate trigger generator.
After transistor 55 is brought to saturation by the combined action of the
forward bias coupled from capacitor 48 and the positive going or forward
bias coupled to the base of transistor 55 via capacitor 58 from the output
side of the multiple gate trigger generator, transistor 55 is maintained
in this state by said combined forward biases for the remainder of the
power source voltage alternations. The positive voltage pulses appearing
at the output side of the multiple gate trigger generator are coupled via
a resistor 59 to a gate trigger amplifier comprising a transistor 60, an
associated transformer 61, and failure mode protection diodes 61a, for
conversion to higher power current pulses which, in turn, are supplied to
the gate electrode or control terminal 62 of a Triac assembly 6 connected
between phase 2 of AC power source 3 and the stator winding 2b of
induction motor 10. The protection diodes 61a prevent positive gate
current flow and limit reverse gate voltage to approximately 2 v by diode
clamping.
Triac assembly 6 is turned on by the arrival of the first pulse in the
train of pulses coupled to its gate electrode from the gate trigger
amplifier 60. The continuous stream of pulses thereafter supplied to the
gate electrode 62 of Triac assembly 6 assures full balance conduction of
Triac assembly 6 regardless of voltage transients which may be produced by
the varying inductive load of the motor 10 that, otherwise, might create
alternation imbalance by self-commutation at times other than the current
zero crossings conducted from the AC power source 3.
The structure of wave modifier 8 is the same as that described above with
respect to wave modifier 7, except for the addition of a resistor 85a
which is connected across the time constant charging capacitor 48a to
ground, and produces a difference in time constants and charging rate as
compared to that of wave modifier 7. Variable resistors 47 and 47a may be
adjusted at the time of manufacturing to avoid the necessity for precision
components, or component selection, to obtain individually preferred delay
vs control voltage input characteristics for wave modifiers 7 and 8 beyond
that produced by fixed resistor 85a. Resistors 47 and 47a may also be
employed to cause wave modifiers 7 and 8 to accommodate differing torque
characteristics peculiar to three phase motors of different manufacture or
construction.
The operation of the overall circuit shown in FIG. 2 will now be discussed.
Assume no load is coupled to shaft 13 or rotor 12. When stator windings 2b
and 2a of the three phase motor 10 are connected to solid state switches 6
and 5 (as shown in FIG. 2), and power from AC source 3 is applied directly
to terminal 2c of motor 10 and to said solid state switches, there will at
first be no output signal from generator 18 because motor 10 has not yet
started to rotate. This results in a maximum DC control voltage on line
16b. This high DC voltage causes both wave modifiers 7 and 8 to couple a
train of gate trigger pulses to their respective solid state switches
without delay after each phase's voltage zero crossing, which in turn
causes said solid state switches to start conducting without delay. Since
this action is continuous, said switches will pass current in both
directions, stator windings 2b and 2a will receive full waves of 60 Hertz
voltage, as does winding 2c, causing the rotor 12 to commence rotation
with full torque.
As motor 10 approaches full rotational speed, frequency modulated AC
generator 18 acting through frequency discriminator 17 will begin to
reduce the magnitude of the relatively high DC control voltage on line 16
and, therefore, on line 16b. Ultimately the decreasing DC control voltage
on line 16b will become of insufficient magnitude to fully charge
capacitors 48a and 48, through their respective charging resistors 47a and
47, before they are discharged by transistors 52a and 52, which act as
reference phase voltage zero crossing reset switches. Under this
condition, the voltages across capacitors 48a and 48 are not sufficiently
high immediately after zero crossing to reach the conduction threshold of
transistors 55a and 55, and to start operation of their multiple gate
trigger generators. The result is that solid state switches 5 and 6 will
not immediately begin conduction at the start of the sine wave cycles from
power source 3 as they did before. Fractional waveforms are actually
delivered to windings 2b and 2a of the motor stator by solid state
switches 6 and 5 and the resultant waveform at winding 2c accordingly soon
represents only a part of the sine wave power available from power source
3.
As previously mentioned, the charging time constants of the input circuits
to transistors 55a and 55, of wave modifiers 8 and 7 respectively, are
different due to the addition of resistor 85a in wave modifier 8. More
particularly, the time required to turn on transistor 55a is longer, and
the rate of charge per unit of DC voltage input is less of capacitor 48a
than for transistor 55 and capacitor 48 respectively. Therefore, as the DC
control voltage decreases in accordance with the full power to full speed
start-up of motor 10 just described, a delay in the turn on conduction
after zero crossing reset appears first with solid state switch 5
(designated .phi.2 for descriptive purposes). Under the no load condition
(previously assumed here for descriptive purposes), the DC control voltage
continues to decrease after the motor has reached full rotational speed
and soon solid state switch 6 likewise does not commence conducting to
.phi.2 immediately after its phase zero crossing.
As the DC control voltage continues to decrease, the portion of sine-wave
power coupled by action of wave modifier 8 acting upon solid state switch
5 continues to decrease. Ultimately, the control voltage charging
capacitor 48a corresponds to the (assumed) zero load condition and does
not reach the conduction threshold of transistor 55a at .phi.1 zero
crossing and, therefore, solid state switch 5 does not turn on during any
portion of the sine-wave period from power source 3. That is, winding 2a
of the motor stator is open-circuited, and motor 10, a three phase motor,
begins operating in what the art refers to as a single phase mode. It is
well known in the art that, once running, a lightly loaded three phase
motor will continue to run near synchronous speed when only two of its
three stator windings are connected to a three phase AC power source.
Thus, when there is no load on the motor, the control system, after full
power start-up, becomes controllingly operative and maintains a particular
motor speed by establishing a condition wherein winding 2a is
open-circuited and winding 2b receives electrical energy for approximately
6 ms out of the 16.6 ms (available) during each cycle of sine-wave power
from a 60 Hertz AC power source. Said particular motor speed is
established as an energy-efficient operating reference by adjusting
variable resistor 38 of FIG. 2 (in frequency discriminator 17) to the
incidental motor speed which provides maximum motor efficiency at zero
load. This otherwise incidental shaft speed is thus made particular by
generic properties of induction motors and is further particularized to
properties peculiar to said controlled motor by adjustment of resistor 38,
and possibly by adjustment of resistors 47 and 47a as well.
It has been demonstrated with a model constructed in accordance with the
teaching of this invention that after full power start-up, three phase
motor 10 will operate effectively when only a portion of AC source 3
sine-wave power is coupled by wave modifier 7 to stator winding 2b and
winding 2c is connected directly to the power source at 4a while winding
2a is open-circuited by solid state switch 5. Furthermore, motor 10
consumes substantially less energy when running in said partial sine-wave,
single phase mode, with no or very light loads, as compared to its
operation with two or all three stator windings connected directly to
power souce 3, or even when all three stator windings are receiving
partial sine-wave power.
Assume now that the load on shaft 13 of rotor 12 begins rising above zero,
referring still to FIG. 2.
As the motor load increases, the slip of motor 10 increases and its speed
drops; this reduces the mean frequency and produces load-induced frequency
modulation of generator 18 which, in turn results in proportional increase
in the DC control voltage output of frequency discriminator 17, on line
16b, which charges capacitors 48 and 48a. The increasing DC control
voltage enables transistor 55a to conduct before transistor 52a discharges
capacitor 48a at .phi.1 voltage zero crossing, and enables solid state
switch 6 to start conducting nearer the beginning of the power source 4a
voltage cycle. Solid state switch 5, previously open-circuited, starts
conduction just before the voltage zero crossing of the power source to
4b. At this moment, three phase motor 10, which had been operating in a
single phase mode, begins to operate again as a three phase motor. With
switch 6 coupling more of the AC power source 3's sine-wave to motor
stator winding 2b, and switch 5 beginning to couple AC power source 3's
sine-wave to motor stator winding 2a, unbalanced three phase power is now
being applied to motor 10.
When the load increases further, switches 6 and 5 continue to increase
their conduction duration, with switch 5 increasing conduction time at a
different rate because of the different time constants of wave modifiers 8
and 7. Three phase power unbalance is as insignificant at light loads as
is single phase mode operation at very light loads and, due to said
different wave modifier time constants, sufficient balance is quickly
achieved above light motor loads. Near full rated load and thereafter,
switches 6 and 5 are again both conducting continuously and full three
phase power is coupled to the motor stator windings.
When power is first applied, and motor 10 is coupled to a mechanical load
equal to or greater than its rated load, the DC control voltage on line
16b, being responsive to the load-induced frequency modulation of
generator 18, will not decrease to the wave modifier controlling level and
full sine-waves of three phase power will continue to be supplied after
full power start-up. If the motor load decreases, however, the slip of the
motor decreases, and its speed begins to increase; this decreases said
load-induced frequency modulation and increases the output mean frequency
of AC generator 18 which, as a result of the operations described above,
decreases the operative angle of the AC waves from power source 3 applied
to stator windings 2b and 2a and, therefore, the three phase sine-wave
power which is supplied to the motor. Ultimately, if the load continues to
decrease, solid state switch 5 will not be triggered on for reasons stated
above, although switch 6 will continue to be triggered, and motor 10 will
again assume a single phase mode of operation.
Thus, regardless of the load condition at motor start or any subsequent
moment, the present invention substantially maintains the particular speed
at which three phase motor 10 most efficiently converts electrical energy
to mechanical energy.
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