GB2073921A - Motor power control circuitry - Google Patents

Abstract

To control the power supplied to a three-phase induction motor as a function of the motor load, the primary winding of a current transformer T1, T2, T3 is included in each phase input line to sense the phase currents, and the three line- to-line voltages are sensed by voltage transformers. Square waves in phase with the sensed currents and voltages are summed at a point S4 in a circuit producing firing signals FIRE A, FIRE B and FIRE C for controlling triacs connected respectively in series with the current transformer primary windings. The signal at S4 is integrated by an operational amplifier A4 and the integration signal supplied to three differential amplifiers A5, A6, A7. The voltage square waves VsA, VsB and VsC are also used to produce sawtooth signals RAMP A, RAMP B and RAMP C with a repetition rate of twice mains frequency which are respectively summed with the integration signal at inputs of the differential amplifiers A5, A6, A7. The FIRE A, FIRE B and FIRE C signals are applied to blocking oscillators for controlling the triacs. The conduction angles of the triacs are thus controlled in dependence upon motor load. <IMAGE>

Description

SPECIFICATION Motor power control circuitry This invention relates generally to an input power controller for electric motors, and, more particularly, to a control circuit which varies the input power supplied to and used by an AC induction motor as a function of the loading applied to the motor.

The induction motor is probably the most commonly used electric motor and may be, as well, the most rugged of the existing motors.

The induction motor runs at a substantially constant speed which is essentially independent of load and/or applied voltage. For efficient operation, the applied (or line) voltage should be a function of the load. In the past, this functional relationship has not been readily accomplished as a practical matter inasmuch as the line voltages are supplied externally and are difficult to control. The line voltage may vary by plus or minus 10% and, moreover, may not even be constant during any service period. Consequently, many motors are designed to deliver the rated load while operating with an undervoltage condition. However, this motor design is wasteful when the line voltage is normal or in the overvoltage range. Furthermore, induction motor current drops with load but not proportionately therewith.Thus, motor efficiency is reduced when the motor is not driving its full capacity load. Thus, in the instance where the consumer employs a motor which is overrated for the specific application or in a situation where a variable load is applied to the motor, a very inefficient situation develops and electrical power is wasted. Moreover, each of these deficiencies, i.e., loss of efficiency of the motor and/or waste of electrical power, is a cost factor which is undesirable to the consumer or to those to whom the cost is passed on. Moreover, when the motor is subjected to loads less than full capacity of the motor, phase angle shifts take place. These shifts can be as great as 80 , thereby causing severe power factor deterioration.

The most pertinent prior art known to the applicants is U.S. Patent 4,052,648 which relates to a single phase induction motor power factor controller.

The present invention is defined by claim 1 hereinafter.

A motor power controller embodying the present invention in a preferred form employs a suitable switching element, such as a triac, and a cuttent-sensing element in the power circuit of an electric motor. In the case of three-phase motor, a switching element and a sensing element are connected in series in each of the three motor power circuits. Control electronics continuously monitor and load by sensing motor current and provide feedback command signals to the switching element, thereby altering the firing angle of the switching element as a function of load. In particular, a switching pulse is applied to the switching element which acts as two back-toback uni-directional switches wherein the current in each direction is automatically turned off as the current goes through zero due to polarity reversal thereof.At that instant, current in the other direction is turned on by means of another pulse to the switching element. Power is conserved by monitoring the load conditions and delaying the turn-on pulses to the switching elements as a function of and in proportion to the decrease of the load. The delay provides compensation for phase angle shift (deterioration) caused by reduced motor loading while simultaneously reducing the RMS power consumption of the motor during these underloaded periods. That is, as load is reduced the turn-on pulses are delayed longer. The delay results in short periods of non-conduction during each half cycle of the line voltage which in turn results in reductions in both RMS voltage and current.

The invention will now be described in more detail, solely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an embodiment of the present invention, Figure 2 is a diagram of the voltage and current waveforms for an electric motor, with and without using a controller embodying the invention, Figure 3 is a chart showing the relationship between power and load as applied to the motor of Fig. 2, Figure 4 is a schematic diagram of a power supply and voltage sense circuit portion of a preferred embodiment of the invention, Figure 5 is a schematic diagram of a current sense circuit, and error amplifier and output modulator portions of the embodiment of Fig. 4, Figure 6 is a schematic diagram of a ramp generator portion of the embodiment of Figs.

4 and 5, Figure 7 is a schematic diagram of switching element circuitry of the embodiment of Figs. 4 to 6, and Figure 8 is a schematic diagram indicating specific interconnections between input and output wiring of switching elements of the embodiment of Figs. 4 to 7.

Referring now to Fig. 1, there is shown a block diagram of controller circuitry embodying the present invention. In particular, a motor 20 is a three-phase motor, typically an induction type motor. The motor would normally be connected directly to the power source comprising lines A, B and C in a threephase operation. The controller circuitry indicated within dashed outline 25 is connected between the power source and the motor 20. The controller circuitry 25 includes a current sense circuit 22 which is connected in series with suitable switching devices such as, but not limited to, triacs 21. The series combination of the current sensing circuit 22 and the trials 21 is connected between the power source and the motor 20.A power supply 23 and control electronics 24 are connected to receive power from the power source and signals from the current sense circuit 22. The power supply 23 supplies appropriate voltages to the control electronics 24. The control electronics 24 applies control signals to the triacs 21 in order to control the operation thereof.

Fig. 2 shows the motor voltage and current waveforms (A)V, BE (C)V and (D)I which may be found on the motor side of the controller circuitry 25. In partioL'ar, the waveforms (t and lB)I represent the voltage and current waveforms, Thespectively, psOdi5Cd when the controller ch:cuitr-y is nct used. Conversely, the wave-:rms (Cj'J1 and (1:))11 represent the voltage and current waveforms, respectively, when the controller circuitry 25 is utilized. In point of fact, the voltage and current waveforms (A)V and (113)1 are applied to respective input lines such as A, B, C in Fig.

4. In this case, it is seen that the current (Bbl lags the voltage (ss.V by approximately go degrees which is typical in an induction motor arrangement. When the controller circuitry 25 is utilized, the control electronics 24 (see Fig.

1) operates to sense the load condition and to control the application of triggering or firing signals to the triacs 21. When the triacs are turned off, no voltage or current is supplied to the motor 20 from the power source. Consequently, as shown by Fig. 2 the voltage waveform (C)V' is discontinuous and includes portions thereof where the voltage applied to the motor is zero. Likewise, as shown by the corresponding current waveform (D)I the current which is supplied to the motor is rendered zero when the triac is off and, effectively, the current is delayed. That is, until the triac is turned on the current remains zero and the start of the current pulse is delayed or retarded. This delay is proportional to the percentage of load since the triac trigger signals are controlled by the load percentage as described hereinafter.That is, the zero portions of the waveforms of (C)V' and/or (D)l1 will expand or contract depending upon the load conditions at the motor. For a fully loaded motor, the waveforms (C)V' and (D)l1 will be identical to the waveforms (A)V and (B)l.

While it is conceivable that an extensive delay could cause he motor oo be, effectively, turned off, this would be an unusual situation and, in fact, the motor would probably be in a "locked rotor" condition or the motor may merely be not PJnring. Of course, when the voltage and/or current are in the zero condition, the power supplied to the motor 20 is also zero whereby a power saving is achieved.

Fig. 3 is a diagram which represents a plot of the percentage of full power which is applied to a motor against the percentage of full load. In this case, a line 10 represents the situation wherein an induction motor is used without the motor power controller circuitry embodying the present invention. As load decreases from 100% toward 0%, the power reduces from 100% to about 35%. Conversely, a line 1 2 illustrates the situation wherein the controller circuitry 25 described herein is employed with a motor.This is, as the load decreases from 100% to 0%, the power requirement reduces from 100S'o to about 1 0%. The cross-hatched area 11 between the lines represents the amount of power which is saved when the controller circuitry embodying the present invention is utilized. This saving can be significant for any motor where saving is multiplied in multimotor applications.

Referring now to Fig. 4, there is shown a schematic diagram of the power supply and voltage sensing circuitry of a preferred embodiment which is similar to the embodiment of Fig. 1. In particular, the circuitry includes power transformers T4, T5 and T6 which are also shown in Fig. 8. The primary windings of the power tranformers T4, T5 and T6 are connected together in a Delta fashion and to three-phase input lines A, B and C as shown in Figs. 4 and 8 so that each transformer effectively "sees" a voltage of one phase, line-to-line. Each terminal of each primary winding is connected to one primary winding terminal of an adjacent transformer.

The secondary windings of each of the transformers T4, T5 and T6 have the center taps thereof connected together and to the positive input (+) terminals of respective amplifiers A8, A9, and A10 as described hereinafter. Each terminal of each secondary winding of each of the transformers T4, T5 and T6 is connected to a pair of rectifiers connected in head-to-tail arrangement. For example, the anode of a diode CR6 is connected to the cathode of a diode CR7 and this combination is connected to the one terminal od the secondary winding of the transformer T4. Likewise, diodes CR8 and CR9 are connected, in opposite directions, to the other end of the secondary winding of transformer T4. The cathodes of all of the diodes CR6, CR8, CR10, CR12, CR14 and CR16 are connected together. Likewise, the anodes of diodes CR7, CR9, CR11, CR13, CR15 and CR17 are also connected together.

The common terminal at the cathodes of the diodes CR6, CR8 and so forth, is connected to an output terminal at which a regulated powder supply voltage is produced.

In this embodiment, the output voltage is + 1 5 volts. In addition, this common terminal is connected to the junction of one side of a capacitor C3, one terminal of a resistor R35 and the collector electrode of a transistor Q1.

The emitter electrode of the transistor Q1 is connected to an output terminal which, in this embodiment, produces a regulated power supply voltage of + 9 volts. The base electrode of the transistor Q1 is connected to another terminal of the resistor R35 and to the cathode of a Zener diode CR18. The anode of the Zener diode CR18 and the other side of the capacitor C3 are connected to a common line COM. This line is the equivalent of a ground line for the circuit.

The other common terminal at the anodes of diodes CR7, CR9 and the like. is connected to the common junction of one side of a capacitor C4 and one terminal of a resistor R36. The other terminal of the resistor R36 is connected to a common junction of the anode of a Zener diode CR19 and one side of a capacitor C5 as well as to an output terminal at which is produced a regulated power supply voltage of - 9 volts. The other sides of the capacitors C4 and C5 and the cathode of the Zener diode Cur 19 are connected to the common line COM.

The rectifier bridges comprising diodes CR6 to CR17 in conjunction with a power supply circuit 23 shown within a dashed outline produce the regulated power supply voltages noted above. Of course, other power supply circuits can be used if other voltages are necessary or desirable.

In addition, resistors R29. R30 R31. R32.

R33 and R34 are connected to respective terminals of the secondary windings of the transformers T4, T5 and T6. In particular. the resistors R29 and R34 (connected to the transformers T4 and T6, respectively). are also connected together and to the negative Input ( - ) terminal of the amplifier A8. In a similar manner, the resistors R30 and R31 (connected to the potential transformers T4 and T5) are also connected together and to the negative input (-) terminal of the amplifier A9. The resistors R32 and R33 (associated with the transformers T5 and T6) are connected together and to the negative input (-) terminal of the amplifier A7 0.

As a result phase reference voltages are applied to the amplifiers A8. A9 and A10 whereby the amplifiers produce reference voltages VsA, VsB, and Vsc, the center taps of the secondary windings of the transformers T4.

T5 and T6 being connected in common to the positive input ( + ) terminals of the amplifiers A8, A9 and Alto. The connected pairs of resistors (for example, R34 and R29) form a set of voltage dividers between the respective Delta-connected secondary windings The voltage obtained at the junction of each voltage divider is compared with the center tap voltage so that the respective phase voltages are obtained. The amplifiers A8, A9 and A10 are typical operational amplifiers which produce output signals which are a function of the respective analog input signals. The voitage generated by each transformer and supplied to the respective amplifier is in-phase with the related input line signal The amplifiers in response to the input signals generate square wave voltages which are in-phase with their respective input phases and input lines.These square wave voltages produced by the respective amplifiers are supplied to other portions of the circuit as described hereinafter.

It should be noted that by connecting together the center taps of the respective transformer secondaries, a "phantom neutral" is created. That is, the common line or terminal is considered as a ground line inasmuch as no phase voltage is produced on the common line and is not in phase with any of the respective phase lines. The "phantom-neutral" technique permits the present controller to be used on either "Delta" or "Wye" wound three-phase motors without need of a neutral connection; something not always readily available on "Wye" wound motors without extensive electrical changes to the motor.

Referring now to Fig. 5, there is shown a schematic diagram of the current sense, error amplifier and output modulator circuit portions. In this circuit, there are shown three current transformers, T1, T2 and T3 with a turns ratio of approximately 1:200. The primary windings of the transformers T1, T2 and T3 are connected in series between the motor and the switching elements and receive input currents IA, IB and IC, as also shown in Fig.

8. Resistors R1, R2 and R3 are connected across the secondary windings of the current transformers T1, T2 and T3, respectively.

These resistors are used to develop input voltages at respective amplifiers Al, A2, and A3. One end of each resistor Ri, R2 or R3 is connected to a respective (+) input terminal of the respective amplifier Al, 2 or A3. The (-) input terminals of the amplifiers Al, A2, and A3 are connected together and to ground. The other ends or terminals of the resistors R1,R2, and R3 are connected together, and, via a resistor B11, to ground. In addition, the common junction of the resistors R1, R2 and R3 is connected via a resistor R10 to the - 9 volt source provided by the regulated power supply shown in Fig. 4.The output terminals of the amplifiers Al, A2, and A3 are connected to summing junctions S1, S2 and S3 via resistors R4, R5 and R6, respectively.

The reference voltages VSA, Vss, and Vsc from the circuitry shown in Fig. 4 are supplied to the summing junctions Si, S2 and S3 via resistors R7, R8 and R9, respectively. The summing junctions S1, S2 and S3 are connected to a summing junction S4 via diodes CR1, CR2 and CR3. The summing junction S4 is connected to ground via a resistor R1 2, and to the (-) input of an integrating amplifier A4. In addition, the (-) terminal of amplifier A4 is connected to one terminal of a resistor R14, the other terminal of which is connected to the tap of a potentiometer R13 between ground and the - 9V source of Fig.

4. Adjustment of the potentiometer R13 controls the operation of the amplifier A4. This arrangement permits a consolidated circuit to provide balanced firing of the triacs (as described hereinafter; in that a single reference firing signal is transmitted to the modulator circuits by the amplifier A4. The advantage obtained is that balance of the motor is maintained with relatively easy control. The (+) input of the amplifier A4 is connected to the - 9 volt source of Fig. 4 via a capacitor Cl, and to ground via a resistor R16. A diode CR4 is connected in parallel with the resistor R1 6 to prevent the (+) terminal from going below ground voltage level.A feedback network comprising a diode CR5, a capacitor C2 and a resistor 1 5 is connected from the (-) input terminal of the amplifier A4 to the output terminal thereof.

The output terminal of the amplifier A4 is further connected to the (+) input terminals of amplifiers A5, A6 and A7 via resistors R17, R18 and R19, respectively. In addition, the - 9 volt source is connected to the (+) input terminal of the amplifiers A5, A6 and A7 via resistors R20, R21 and R22, respectively. The resistors R 1 7 to R22 provide a voltage divider network which supplies a reference voltage to the (+) input terminals of the amplifiers A5, A6 and A7.

The (-) input terminals of the amplifiers AS, A6 and A7 are connected together and to ground. The output terminals of the amplifiers AS, AS and A7 are connected to the ( (+) input terminals of the respective amplifiers, via feedback resistors R23, R24 and R25.

RAMP A, RAMP B and RAMP C signals are supplied by the circuit of Fig. 6 via resistors R26, R27 and R28, respectively to the (+) input terminals of the respective amplifiers.

The output terminals of the amplifiers A5, A6 and A7 provide respective output signals designated FIRE A, FIRE B and FIRE C signals.

In operation, the current supplied to the motor is, effectively, sensed at the primary windings of current transformers Ti, T2 and T3. A current signal is detected across the respective load resistor R1, R2 or R3 and applied to the respective operational amplifier Al, A2 or A3 which produces an output voltage in the form of a square wave representative of the voltage dropped across the respective load resistor. This square wave is also representative of the current in the respective transformer T1, T2 or T3, i.e., it is produced by the respective amplifier at a phase/angle which is the same as the current supplied to the primary windings of the re spective current transformer T1, T2 or T3.

Thus, a "current sense square wave is generated for each of the three phases of the motor and the input source. The output signal produced by each amplifier Al, A2 or A3 is added to the sense voltage VsA, V88 or Vsc at summing junction S1, S2 or S3 respectively, and applied via unidirectional diodes CR1, CR2 or GR3 to the summing junction S4 to produce a course current for the summing amplifier A4. The signal supplied to amplifier A4 through the diode CR1 is a function of the phase relationship between IA and VsA (only one of the phases is described). That is, inasmuch as IA and VsA are both square m/aves, they are summed at summing junction S1.If 1A is lagging Visa, the square waves are displaced from each other in terms of time. At some time during the cycle, both square waves are negative, whereby no current or signal is supplied to the amplifier in;4. At other times, the waveforms are out-of-phase with each other, with the result that the sum thereof is zero and again, this provides no current to the amplifier. Finally, in other conditions, both waveforms are positive and, therefore, supply current to the amplifier. Of course, each of the phase circuits provides waveforms or current signals to the error amplifier A4.

With the feedback network Gut5, C2 and R15 connected thereto, the amplifier A4 becomes an integrator circuit which sums the outputs of the circuits connected to the inputs thereof thereby detecting the voltage and current phases as well as the negative bias supplied by the resistor R 1 4 and the potentiometer Tri 3. The potentiometer Rl 3 is adjustable and thus permits setting of the firing angle for the switches, i.e., triacs as described hereinafter. In addition, an adjustment of the potentiometer Tri 3 keeps the motor current and voltages baianced in all three phases.

This circuit portion is a distinct advantage over prior concepts of intrnducing power factor controllers into each phase of a 3-phase system.

When the positive current from the phase detecting circuits is equal to the negative current from the voltage dividing combination of the potentiometer R13 and the resistor R14, the amplifier A4 provides a constant output. Conversely, when the output of the phase detecting circuit is greater than the negative current supplied by the voltage dividing combination, the output of the amplifier Al moves in a negative direction causing the "dead band" of the triacs to increase. That is, the delay described with reference to Fig. 2 is increased.

The amplifiers A5, A6 and A7 operate as modulators of the signal produced by the amplifier A. That is, the output signal produced by the amplifier A4 is supplied via the appropriate resistor (e.g., resistor Ri 7) and added to the ramp signal generated by the ramp generator (see Fig. 6) and supplied to the input of the respective amplifier. The ramp signal, as will be discussed infra, is in phase with the respective phase voltage. The sum of the two voltages is. compared with ground as shown (or other suitable reference potential) by means of the modulating amplifier. When the sum is greater than zero, the output of the amplifier is positive. At all other times, the output signals are negative.The feedback resistors R23, R24 and R25 are positive feedback circuits and cause a "snap action" in the output signal produced by the amplifier thereby ensuring that the waveform produced is, in fact, a squarewave.

Referring now to Fig. 6, there is shown a schematic diagram of a suitable ramp generator circuit for use in the present embodiment.

Again, the input terminals are connected to receive the reference voltages VsA, VSB and Vsc from the circuit shown in Fig. 4. Three identical ramp generator circuits are provided, and therefore only one circuit or channel is described in detail. Thus, the VsA input terminal is connected to the base of a transistor Q2 via a capacitor C6 and a resistor R40. In addition, the input terminal is connected to the base of a complementery transistor Q5 via a capacitor C9 and a resistor R46. The collector electrode of the transistor Q2 is also connected to the base of the transistor Q5 via a resistor R40.The emitter of the transistor Q2 is connected directly to the + 9 volt source and via a resistor R37 to the junction between the capacitor C6 and the resistor R40.

Similarly, the emitter electrode of the transistor Q5 is connected directly to ground and, via a resistor R49, to the junction between the capacitor C9 and the resistor R46. The collector electrode of the transistor OS is connected to the + 9 volt source via a resistor R52. The collector electrode is also connected to ground via a capacitor C12. In addition, the output terminals for producing RAMP A, RAMP B and RAMP C signals are taken from the collector electrodes of the transistor Q5 and the counterpart transistors Q6 and 07 in the other phase sections or channels. The two-transistor circuit in each phase channel generates a 1 20 Hz sawtooth waveform.The transistor OS resets the RC network directly whenever the AC waveform at the input terminal VSA changes from negative to positive. The transistor Q2 is connected to the drive transistor Os to reset the RC network whenever the input waveform changes from positive to negative. This results in the sawtooth wave being reset twice during each cycle of the power line.

Referring now to Fig. 7, there is shown a schematic diagram of the firing circuit portion of the present embodiment. This firing circuit utilizes triacs but can be used with two silicon controlled rectifiers or other similar devices instead of each triac. Again, there are three substantially identical circuits, each of which is associated with a separate one of the phases in the 3-phase system and only one circuit is described in detail. In particular, the anodes of input coupling diodes CR20, CR21 and CR22 are connected to receive the FIRE A, FIRE B and FIRE C input signals produced by the circuit of Fig. 5.The cathode of the input diode CR20 is connected via a resistor R55 to the anode of a diode CR23, one side of a capacitor C15 and to one terminal of a resistor R6 1. The other terminal of the resistor R61 is connected to one terminal of a resistor R58 and, as well, to the emitter electrode of a transistor Q8. The base electrode of the transistor Q8 is connected to a second terminal of the resistor R58 and to the cathode of the diode CR23. The other terminal or side of the capacitor C15 is connected to the common terminal COM via the series connected combination of a resistor R64 and one winding of a transformer T7. A second winding of the transformer T7 is connected between the collector electrode of the transistor Q8 and the + 1 5 volt source from the power supply of Fig. 4.In addition, the collector electrode of transistor Q8 is connected via a filter capacitor C18 to the connection of common terminal COM and the emitter electrode of the transistor Q8. A third winding on transformer T7 is connected in series with a resistor R67 and a diode CR26 between the output terminals A-COM and A-GATE. A voltage dropping resistor R7O is connected between these output terminals as well.

The basic circuit which includes the transistor Q8 (and the corresponding circuits of the counterpart transistors Q9 and Q10 respectively) operates as a blocking oscillator. This permits a relatively high current to be applied to the gate of the switching device (the triac in this example) without any undue loading of the internal power supply. That is, when a positive signal is applied at the anode of the appropriate diode (i.e., diode CR20, CR21), a blocking oscillator is turned on.The frequency of the oscillation is a function of the saturation time of the transformer as well as the RC network comprising the resistor R64 and the capacitor C15. The frequency should be sufficiently rapid for a pulse of output current to be available as soon as possible at the A GATE output terminal subsequent to the circuit requirements. However, the pulse should be of adequate width to ensure the firing of the switching device. Consequently, upper and lower frequency limits can be defined. In the embodiment described herein, the frequency limits of 5 KHz to 1 5 KHz is desirable.

The turns ratio of the output transformers (T7, T8 or T9) and the saturation current of the output transistors (Q8, Q9, Q10) determine the gate current level supplied to the switching devices. The level is readily ad justed by changing the turns ratio of the transformer or by lowering the drive to the output transistor. Capacitor C18, C19 and C20 function to prevent the fly-back voltage developed across the primary of the output tra.-sformers from exceeding the safe voltage for the output transistors and, therefore, prevent the burning-out of the transistors.

Referring now to Fig. 8, there is shown the circuitry interfacing the triacs or other switching devices, with remainder of the circuits of Figs. 4, 5 and 7. In addition, other details relating to some of the transformer constructions are observed.

The three phase line input comprises input terminals of lines A, B and C which respectively represent the 3-phases of the input line or voltage source. Each of these lines is connected to one terminal of each of two of the primary windings of respective potential transformers T4, T5 and T6 to provide the Delta windings. Each of the primary windings of the respective potential transformers is shown as having two separate windings.

These separate windings are included between terminals 1 and 2 or 3 and 4, respectively.

Depending upon the current and voltage input conditions, terminals 2 and 3 can be connected together (for 480 volt operation) or terminals 1 and 3 can be connected together and, as well, terminals 2 and 4 are connected together. This latter arrangement provides for a 240 volt operation.

In addition, each of the input lines A, B and C is connected directly to one terminal of the switching device, in this case to terminals 1 of triacs TR1, TR2 and TR3. The terminal 1 of the triac TR1 is equivalent to the terminal A-COM shown in Fig. 7. Terminal 3 of the triac TR1 (or triac TR2 or TR3) is equivalent to the A-GATE terminal shown in Fig. 7 and is connected to the junction of resistor R70 and the anode of the diode CR26.

Terminal 2 of the triacs is connected to one terminal of transformers T1, T2 or T3 as shown in Fig. 5. The other terminals of the primary windings of the transformers T1, T2 and T3 are connected to the phast A, phase B and phase C terminals iDA, ssB and fC of the motor. A snubber path comprising a resistor and capacitor is connected across each triac for protection thereof and to provide EMI/RFL protection as well.

As may be best seen in Fig. 8, when a control signal is supplied across the resistor R70 by the transformer T7, the gate electrode of the triac is appropriately driven. When the triac is turned on as a result of the signal across the resistor R70, current passes from the 3-phase line A input to the 3-phase winding of the motor. As long as the triac is operative, power is supplied to the motor.

When the triac is inoperative (i.e., not triggered on), power is not supplied to the motor.

Thus, the power supplied to the motor is a function of the operation of the controller circuit operation. This circuit operation is a function of the current in the primary winding of the transformer T1 (or T2 or T3) which of course is a functon of the load on the motor.

Also, it is eminently clear that the transformers provide isolation from the line to the respective circuit portions.

Thus, there is shown and described a preferred embodiment od the present invention.

In this embodiment, a 3-phase line source is electrically applied and conected to a 3-phase induction motor. By properly detecing the current in each of the phase lines, the amount (or percentage) of the load applied at the motor is detected. As a function of this percentage of load, a signal is produced which controls the operation of a switching device.

The switching device selectively interrupts the circuit path between the input line source and the motor. By interrupting the line circuits, the amount of power which is supplied to the motor is interrupted, and, therefore, reduced.

By reducing the power which is supplied to the motor as a function of the load on the motor,-the efficiency of the motor can be increased and the amount of power which is used can be reduced. The power which is, in this manner, removed from or prevented from being applied to the motor is a saving in both energy and cost. The power which would otherwise be applied to the motor would have been lost as generated heat causing reduced efficiency motor as well as very probably decreasing the lifetime and the operating characteristics of the motor. When applied to the control of a 3-phase motor, the present invention has the advantage over prior art systems which are directed to a single-phase motor and which require three such systems to control a 3-phase motor The motor power control circuitry described hereinbefore with reference to Figs. 4 to 8 constitutes a power factor control system for use with three-phase induction motors and operates by sampling the line voltage and the current through the motor in order to decrease power input to the motor as a functon of the detected phase displacement between the current and the voltage whereby lesc power is provided to and used by the motor when less load is applied to the motor.

Claims (11)

1. Motor power control circuitry comprising, power source input means for applying power to an electric motor, voltage sensing means connected to the power source input means for producing signals representative of voltage applied through the power source input means in operation, current sensing means arranged to sense current supplied through the power source input means in operation, switching means connected in the power source input means for selectively interrupting the power supplied through the power source input means in operation, and controller means connected to receive signals from the said current sensing sense means and the said voltage sensing means and such as to supply signals to the said switching means as a function the instantaneous power supplied through the said power source input means, the instantaneous power being proportional to the load on a motor arranged, in operation, to receive power through the said power source input means.

2. Motor power control circuitry according to claim 1, wherein power supply circuit is connected between the said power source input means and the said controller means.

3. Motor power control circuitry according to claim 1 or 2, wherein the said power source input means comprises a three-phase power source input arrangement.

4. Motor power control circuitry according to any preceding claim, wherein the current sensing means includes first winding means connected in the power source input means, second winding means inductively coupled to the first winding means, impedance means connected across the second winding means to establish a signal related to the signal through the first winding means, and amplifier means so connected to receive the signal established across the said impedance means that the amplifier means produces an output signal representative of current supplied through the power source input means in operation

5.Motor power control circuitry according to any preceding claim, wherein the voltage sensing means includes transformer means having the primary windings connected to the power source input means, and terminals of the secondary windings connected to one input of sensing amplifier means, center taps of the said secondary windings being so connected to another input of the sensing amplifier means that the sensing amplifier means produces an output signal representative of voltage supplied to the power source input means in operation.

6. Motor power control circuit according to claim 5, wherein the controller means includes ramp generator means connected to the sensing amplifier means to receive the output signal produced thereby.

7. Motor power control circuitry according to claim 5 or 6, wherein the controller means includes trigger circuit means connected to supply trigger signals to the said switching means to control the operating state thereof.

8. Motor power control circuitry according to claim 7, wherein the controller means in clues fire control circuit means for selectively causing the said trigger circuit to produce the said trigger signals, the fire control circuit being connected to receive signals from the said amplifier means and the said ramp generator means to produce a fire signal which is supplied to the said trigger circuit means.

9. Motor power control circuitry according to claim 8, wherein summing means are connected to receive the current representative output signal from the said amplifier means and voltage representative output signal from the sensing amplifier means, and integrator means are connected to receive signals from the said summing means and to supply signals to the said fire control circuit means.

10. Motor power control circuitry according to claim 9, wherein voltage adjustment means are connected to the input of the said integrator means to permit adjustment of the output signal produced by the said integrating means.

11. Motor power control circuitry according to claim 1 and substantially as described hereinbefore with reference to Figs. 4 to 8 of the accompanying drawings.