GB2084359A - Apparatus and Methods for Controlling Three Phase Induction Motors - Google Patents

Abstract

When a three phase induction motor is connected by three wires only, and its power factor is controlled by triggering triacs or other thyristor arrangements in response to deviation from a required value, a triac may cease to conduct prematurely because no other triac is conducting. Also the phase lag monitoring for control purposes may be incorrect through premature triac turn-off. If a triac 50 for one phase receives a trigger signal from a generator 16 and at the same time another triac 51 or 52, depending on direction of motor rotation, also receives a trigger signal from one of gates 75 and 76, the effect of premature turn-off is overcome by a further firing. Gates 84 and 85 are used to suppress erroneous phase-lag monitor signals from the comparator 21 when current in another phase, selected according to direction of motor rotation, has ceased. Other phases have similar arrangements. <IMAGE>

Description

SPECIFICATION Apparatus and Methods for Controlling Three Phase Induction Motors The present invention relates to apparatus and methods for controlling the supply of power to a three phase induction motor.

A patent application no. 8129044 having the title "Apparatus and Methods for Controlling Induction Motors" (Inventor: P. J. Unsworth) filed on the same day as the present application claims methods and apparatus for optimising the power consumption of induction motors by varying the fraction of the mains cycle over which conductiori occurs. However, problems arise in the control of three phase induction motors connected by three wires only (the neutral connection being omitted) to a controller comprising switching means and to a supply.

The present inventor has found one of these problems to occur when whilst one switching means, such as a triac, is non-conducting the current in a second switching means also becomes zero.

With two non-conducting switching means and only a three wire connection there is no continuous path for current through the third switching means and current in this switching means also ceases.

Since all three switching means are now non-conducting the motor is switched off and even if subsequently the three switching means are individually fired in sequence there remains no current path and the motor remains off. Thus the moment two switching means become non-conducting, the third switching means is switched off and the motor ceases to operate.

This situation occurs whenever the off periods of two switching means overlap during reduction of power to the motor by phase control such as is described in the above mentioned patent application and also in other known methods of induction motor control. The situation can also occur if one switching means fails to conduct just before a second switching means becomes non-conducting, for example due to a momentary interruption of the supply from a sliding pick-up contact.

The present inventor has also discovered that another of the above mentioned problems is due to circumstances which may arise and cause erroneous measurement of phase lag between the supply voltage and supply current. Variations in phase lag accompany changes in the motor load and are used to determine when the switching means should be triggered to meet motor load variations.

These circumstances arise when two switching means turn off, since then the resulting turn off and cessation of current in the third switching means does not occur at the natural phase lag value.

Rather, it is imposed by the off state of the other two switching means and leads to inaccurate phase lag measurement and subsequent failure of control circuits.

According to a first aspect of the present invention there is provided a power controller for three phase induction motors supplied by way of three wires only, comprising three switching means, each for a respective supply phase and being, in operation, connected between that phase of a three phase supply and a respective terminal of a star or delta connected induction motor, each switching means being capable of passing current from the moment when a primary or secondary trigger signal reaches that switching means until current applied thereto ceases, monitor means for generating a monitoring signal representative of the interval between a zero in the voltage of at least one phase of the supply and the next final cessation of current in that phase before current reversal therein, control means for generating primary trigger signals for the switching means of respective supply phases, the control means being responsive to the monitoring signal to change the time relationships between the supply waveform and the primary trigger signals in that sense which shortens the conduction period of the switching means when the said interval tends to increase and vice versa, and the control means also being constructed to generate secondary trigger signals at the same time as primary trigger signals, each secondary trigger signal being applied to that switching means which received the last primary trigger signal.

According to a second aspect of the present invention there is provided a method of controlling a three phase star or delta connected induction motor when the motor is supplied by way of three wires only, comprising the steps of supplying the induction motor from a three phase alternating current electrical supply by way of three switching means, each for a respective supply phase and connecting that phase to a respective terminal of the star or delta connection of the motor when primary or secondary trigger signals are applied thereto, each switching means conducting from the moment when a primary or secondary trigger signal reaches that switching means until current applied thereto ceases, generating a monitoring signal representative of the interval between a zero in the voltage of at least one phase of the supply and the next final cessation of current in that phase before current reversal therein, generating primary trigger signals for the respective switching means in the reverse phase sequence to that in which the supply voltages attain maximum magnitude, in response to the monitoring signal to change the time relationships between the supply waveform and the primary trigger signals in that sense which shortens the conduction period of the switching means when the said interval tends to increase and vice versa, and generating secondary trigger signals for the switching means of a supply phase at the same time as the primary trigger signal for the switching means of the next phase in the said reverse phase sequence.

The first of the above mentioned problems is overcome by generating a secondary trigger signal each time a primary trigger signal is generated. In this way a path is created for current through the two switching means receiving the simultaneous primary and secondary trigger signals. The switching means which receives the secondary triggering signal is that switching means which received the previous primary trigger signal.

According to a third aspect of thepresent invention there is provided a power controller for a three phase induction motor supplied by way of three wires only, comprising three switching means, each for a respective supply phase and being, in operation, connected between that phase of a three phase supply and a respective terminal of a star or delta connected induction motor, each switching means being capable of passing current from the moment when a trigger signal reaches that switching means until current applied thereto ceases, monitor means for generating a monitoring signal representative of the interval between a zero in the voltage of at least one phase of the supply and the next final cessation of current in that phase before current reversal therein, control means for generating trigger signals for the switching means, the control means being responsive to the monitoring signal to change the time relationships between the supply waveform and the trigger signals in that sense which shortens the conduction period of the switching means when the said interval tends to increase and vice versa, and means for suppressing generation or use of the monitoring signals whenever current through a switching means ceases but next flows again in the same direction.

According to a fourth aspect of the present invention there is provided a method of controlling a three phase star or delta connected induction motor when the motor is supplied by way of three wires only, comprising the steps of supplying the induction motor from a three phase alternating current electrical supply by way of three switching means, each of which when triggered by a trigger signal connects one phase of the supply to a respective terminal of the star or delta connection, each switching means being capable of passing current from the moment when a trigger signal reaches that switching means until current applied thereto ceases, generating a monitoring signal representative of the interval between a zero in the voltage of at least one phase of the supply and the next final cessation of current in that phase before current reversal therein, generating trigger signals for the respective switching means in response to the monitoring signals to change the time relationship between the supply waveform and the primary trigger signals in that sense which shortens the conduction period of the switching means when the said interval tends to increase and vice versa, and suppressing the generation, or the use, of the monitoring signals whenever current through a switching means ceases but next flows agains in the same direction.

Use of the second or third aspects of the invention allows the second problem to be overcome since erroneous phase lag measurements are suppressed.

Preferably, one monitoring signal is generated for each supply phase, and then the generation, or the use, of the monitoring signal for each respective phase is suppressed when the preceding phase in the sequence in which the phase supply voltages go positive, is not conducting.

Although power controllers and methods of control according to the invention are suitable for three wire only connections to motors, they may be used in four or six wire connections.

Certain embodiments of the invention will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 is a block circuit diagram of a power controller according to the invention, Figures 2a to 2j are waveforms used in explaining the operaton of Figure 1, Figures 3a to 3d show various ways in which an induction motor can be supplied from a three phase supply, Figures 4a to 41 show waveforms which arise in operating an induction motor connected as shown in Figure 3c, Figure 5 is a block diagram of another controller according to the invention, Figure 6 is a circuit diagram of an alternative arrangement for the amplifier 1 9 and the sample circuit 23 of Figure 1, and Figure 7 is a circuit diagram of threshold detectors which may be used in a modified form of Figure 5.

Before describing an embodiment of the present invention a description of a power controller to which the invention can be applied is first given.

In Figure 1 an induction motor 10 is supplied from an a.c. mains supply 11 by way of a triac 12.

One side of the motor 10 is connected by way of a switch 1 3 to a terminal 14 of the power supply 11. The other terminal 1 5 of the power supply 11 is connected to the triac 1 2.

Trigger pulses are supplied to the triac 12 by a trigger pulse generator 1 6 which generates trigger pulses when the output of a comparator 1 7 indicates that the voltage from a ramp generator 1 8 has just exceeded the output voltage of a differential amplifier 19. The trigger pulse generator 16, the comparator 17 and the ramp generator 18 are commercially available in a number of integrated circuits designed for triggering triacs in power controllers.

It is required to delay trigger pulses when the power factor of the motor falls and this is achieved by means of a comparator 21, a monostable circuit 35, a sample and hold circuit 22 and the difference amplifier 1 9. The comparator 21 detects the time instant X (see Figure 2c) when the triac 12 ceases to conduct. The lag 0 between the supply voltage falling to zero at Z (see Figure 2a) and the point X depends on the load on the motor and on the firin'g instant F. If the triac is fired earlier, the cessation of current at X occurs later and vice versa. Also increase in load on the motor reduces the lag and vice versa.

The voltage waveform across the triac (see VT in Figure 2e) is used to determine the time instant X. The voltage across the motor is shown in Figure 2d and it is the same as the supply voltage when the triac is conducting. At other times the voltage does not fall to zero because the motor is still rotating and a voltage is induced in the motor stator. At such times the voltage across the triac VT (see Figure 2e) is equal to the difference between the supply voltage and the induced voltage. It will be seen from the waveform of Figure 2e that steps occur in the triac voltage when conduction commences and ceases. The latter of these steps is used to indicate phase lag 0 (see Figure 2b) in the way which is now described.

The voltage across the triac is applied to the comparator 21 giving an output for the comparator 21 as shown in Figure 2f. Each time the output of the comparator 21 changes from positive to negative the monostable circuit 35 generates one of the sampling pulses shown in Figure 29 and the sample and hold circuit 22 samples the output voltage of the ramp generator 1 8 (shown in Figure 2h).

(Alternatively, a sampling spike may be more simply obtained by differentiating the output of comparator 21 with a CR circuit). However the output voltage of the sample and hold circuit 22 is obtained by subtracting the ramp voltage (Vx) at the time the sample is taken from the reference voltage (REF) obtained from a potentiometer 24.

The output of the sample and hold circuit 22 is applied to the inverting (-) input of the differential amplifier 1 9.

In operation, when the lag 0 (between the points Z and X of Figures 2a and 2c) increases due to reduction in load on the motor, the sampled ramp voltage Vx increases in magnitude since it is taken at a higher magnitude point in the output voltage of the ramp generator 1 8 (as shown in Figure 2h). Vx is subtracted from VREF so that the error signal output of the sample and hold circuit 22 falls but this is applied to the inverting input of the differential amplifier 1 9 so that the output of this amplifier increases.Trigger pulses are thus produced later in each supply voltage cycle since such pulses are produced at the point F in Figure 2h when the ramp generator just exceeds the voltage applied by the differential amplifier 1 9. Thus the triac then fires later in the supply cycle which shortens its conduction period, reduces the power input to the motor, and opposes the increase in lag due to the change in load on the motor.This maintains the power factor and efficiency of the motor and also acts to return the power factor and the lag 4i to normal operating values which are set by VREFVX=O since any difference is amplified by the full d.c. gain of the differential amplifier 1 9. The circuit acts to stabilise the motor power factor and phase lag 9 at the phase angle where the sample voltage VX=VREF and VREF determines the operating phase angle of the motor.

A.c. response is damped by a feedback network consisting of capacitor 27 and two equal resistors 25 and 26 connected to the differential amplifier 1 9. The d.c. gain of the amplifier 1 9 is unaffected (typically 60 to 100 dB) but the A.C. gain is reduced to maintain stability of the overall feedback system and produce a smooth response to sudden changes in motor loading without hunting.

The amplifier gain at high frequency (above approximately 2Hz) is unity and gives an immediate response to sudden changes in load and motor phase angle.

The integrated circuit containing the trigger pulse generator 1 6 is selected from circuits having control logic to prevent the trigger pulse from occurring after the supply voltage crosses zero at Z in Figure 2a but before the current turns off at X in Figure 2c. Triggering must occur after the point X otherwise current will turn off at X and not be retriggered until the following half cycle. The integrated circuit may require a connection from a point, such as the output of the comparator 21, where a signal is available indicating whether or not current is flowing in the triac 12. If it is required to use an integrated circuit which doers not include such control logic, an equivalent logic circuit should be provided externally.

Further motors can be connected in parallel with the motor 10 up to a maximum of about eight and two such motors 28 and 29 are shown in Figure 1.

The circuit of Figure 1 as so far described requires modification for starting and when a number of motors are connected in parallel and started at random. If one motor is running normally at a predetermined phase angle and a second motor is then connected in parallel with the first, the circuit will sense any change in phase angle between the applied voltage and the overall current supplied to the two motors. However, connecting the second motor may increase or decrease the lag depending on the phase lag at which the circuit is operating and on the phase characteristics of the two motors.

In the case of an increase in phase lag the circuit as so far described would respond by further delaying firing of the triac and by reducing power to the motors, which might fail to start the newly connected motor, and the current drawn by the stalled motor could lead to dangerous overheating.

Even if a decrease in phase lag resulted from connecting a further motor so that the circuit responded by supplying increased power, this might still not be sufficient or rapid enough for satisfactory starting.

Thus measurement of phase lag is insufficient to guarantee reliable independent starting of motors in parallel.

To overcome this problem an override system 31 is provided and is operated by the voltage across the triac 1 2. When the triac is conducting the voltage across it is less than 2 volts and may be considered as essentially zero. When the triac is non-conducting the triac voltage equals the difference between the mains voltage and the voltage across the motor. The latter is not zero because, as stated above, although the current input to the motor stator may have ceased, current is still flowing in the rotor. As it rotates, its magnetic field induces an e.m.f. in the stator winding which appears across the motor terminals. The voltage across the triac in the non-conducting period is thus the difference between the mains voltage and the induced motore.m.f.The magnitude of this voltage depends on the rotor current and rotor slip which both depend strongly on the load on the motor and on the motor speed so that the peak value of the voltage across the triac increases immediately on sudden extra loading on the motor or if it shows any tendency towards stalling. For a single stalled motor, for example, or a motor starting from rest, there is no induced e.m.f. and the voltage across the triac equals the full mains voltage. Likewise, when a second motor at rest is connected across a running motor, the voltage across the triac rises immediately in the non-conducting period to a value close to the mains voltage because the very low impedance of the stationary motor loads the induced e.m.f. of the running motor very heavily.

The peak value of the voltage across the triac is thus a sensitive indicator for sensing the connection on-line of a motor for starting and is used to actuate a starting sequence which applies the full supply voltage for a few seconds to start the motor reliably.

Any rise in the peak value of the voltage VT across the triac above a predetermined value is sensed by means of a Zener diode 32. The voltage VT is applied by way of a potentiometer 33 and when the potentiometer voltage rises above the Zener conduction voltage a current flows through the Zener diode 32 and charges a capacitor 34 to create a negative voltage transient at the non-inverting input to the differential amplifier 1 9. A rectifying diode 36 prevents the capacitor 34 discharging and the charging in reverse during the reverse half cycle of the supply, and Zener diode 37 limits the override signal across capacitor 34. The effect is to reduce the output of the differential amplifier 1 9 and thus cause the triac to fire earlier in the supply cycle.Subsequently, when, for example, a newly connected motor has speeded up, VT falls to a lower value and current through the Zener diode 32 ceases. The transient then decays exponentially back to zero and smoothly reduces the motor power to the normal reduced running level. A time constant of a few seconds is suitable for discharge of the capacitor 34 through a resistor 38. A motor which is usually slow in starting results in VT remaining high throughout the starting period and this sustains the transient applied to the differential amplifier 1 9 allowing triggering pulses to be applied immediately after current ceases at X (see Figure 2c) during each supply half cycle throughout the starting period, thus giving the full supply voltage for starting.

Since VT increases with motor slip, that is decreases in rotor speed, the same circuit allows the full supply voltage waveform to be applied if motor speed drops excessively for any reason, such as abnormal load or operation at too small a lag. This effectively eliminates the possibility that power reduction by the control system may cause a motor to slow excessively or to stall. The override system 31 thus acts as a starting/stalling sensor and as a safety device to override the phase lag controller.

Instead of causing the override circuit to come into operation when the peak voltage across the triac exceeds the voltage of a Zener diode, it may be arranged for the override circuit to operate when the triac voltage exceeds the peak mains voltage less the voltage of a Zener diode. The effect of this is that if the supply voltage drops then so does the threshold triac voltage at which the override circuit operates. This ensures adequate saefty margins under conditions of reduced supply voltage.

A preferred alternative arrangement to replace items 19, 22 and 24 to 27 is shown in Figure 6.

The ramp signal from the generator 1 8 passes to a sample-and-hold circuit 23 which receives a sampling pulse from the monostable 35. The sampled output of the circuit 23 is applied to the non inverting input of a differential amplifier 19' which has a similar function to the amplifier 1 9. The reference signal VREF is applied by way of a resistor 25' to the inverting input of the amplifier 19', and the output signal of this amplifier is therefore VxVREF amplified, as is that of the amplifier 1 9 of Figure 1. In a similar way to Figure 1 the output START of the threshold circuit 31 is applied to the amplifier 1 9' by way of the resistor 25'.

Because the motor 10, the amplifier 19', the comparator 17, the trigger pulse generator 1 6 and the triac 1 2 form a feedback loop there is a danger of instability in the overall response. The phase lag 0 of motors responds in a complex way to rapid changes in load and firing point F of the supply triac, and the response also depends on motor construction. It has been found that the response of the amplifier 19' should be one of phase lag at frequencies up to about 2 Hz with gain decreasing at 6 db/octave from high value at zero frequency to around unity at about 2 Hz. The phase lag is imparted by a resistor 26' and a capacitor 27'. As is explained later a resistor 86 and a capacitor 87 are used in controllers for three phase motors.

As has been mentioned the controller can operate many motors in parallel and this is so even with different motors driving different loads such as pumps, drills, compressors and variable loads.

Identical motors driving identical loads operate with the same phase lag and the power saving is the same as operating each motor with a separate controller. Different motors or identical motors with different loads operated in parallel tend to operate with different phase lags but a triac supplying the motors will cease conduction when the total current reaches zero. The circuit described operates as though this cessation of current indicates overall phase lag and the setting of VREF determines the value of this lag.

The overall phase lag will not necessarily be optimum for each motor but by considering the waveforms for two motors in parallel (Figures 2i and 2j for a first and second motor, respectively) it is seen that the combined current (1M, Figure 2c which is the sum of the currents 1M1 and 1M2 of the first and second motors, drops to zero at the point X and conduction through the triac ceases. However, equal but opposite currents continue to flow in the two motors and as shown the first motor draws current from the second motor. In fact, when the triac does not conduct, the second motor is acting as a generator supplying current to the first motor.Thus the first motor which demands power over a greater portion of the main cycle, obtains this during the non-conductive period of the triac from the stored energy in the second motor. In operating the second motor alone, the circuit of Figure 1 would fire the triac earlier to lengthen the conduction period and cause the current to reach zero at the predetermined phase lag. For the first motor aione the circuit would act in the opposite way. When operating in parallel the requirement for different conduction periods is balanced out by the motors exchanging energy. As long as the conduction periods required are not too different, the improvement in power consumption is not very different from that when the motors are controiled individually.

It will be realised that the control system described with reference to Figures 1 and 2 can be put into practice in many other ways. For example, the circuit for sensing motor phase angle or the total phase angle of a group df motors may be constructed differently, for example by sensing the voltage and current supplied to the motors. Similarly other override circuits may be adopted.

The control system can also be applied to motors connected to three phase supplies, and in one arrangement each phase has a separate circuit similar to that shown in Figure 1 except that a single common difference amplifier 19 and a single common override circuit 31 may be used. The difference amplifier is connected as a summing amplifier with three inputs from the three sample and hold circuits 22 and provides a single output signal which is applied to the three comparators 1 7. Using the single difference amplifier prevents imbalance between power supplied to the various phases. For example an increase in power to one phase may result in a reduction in power to another phase and tend to cause imbalance between the phases. The single override circuit responds to any peak triac phase voltage which exceeds the reference voltage set by the Zener diode 37.To enable grounded common connections and a single override circuit to be used, the triacs are fired via isolating pulse transformers, and the voltages VT across the triacs, from which the control voltages are derived, are transmitted to the override circuit and the comparators 21 by way of optical isolators driven from the voltages VT by respective full wave rectifiers. (Similar isolating arrangements may be used in the arrangement of Figure 1).

In another arrangement the difference amplifier may be driven by signals from one phase only but its output signal may be used for all phases by way of separate comparators 17, trigger pulse generators 1 6 and triacs 12.

Various ways of connecting a three phase induction motor are shown in Figure 3. For example in Figure 3a a star connection is shown with triacs 40, 41 and 42 connected in series with star connected induction motor windings 43, 44 and 45. Three line connections L1, L2 and L3 and also a neutral connection N are also shown. Four wires are required for connection to motor terminals 52 to 55 with use of the neutral terminal 55. In Figure 3b a six wire connection to six motor terminals 46 to 51 is used between a controller containing the triacs and the motor itself since in this case the triacs form part of the delta connection. In Figures 3a and 3b each triac conducts independently of the others; for example if the triac 40 in Figure 3a is conducting there is no need for the triacs 41 and 42 to conduct since current can pass by way of the neutral connection N.Similarly in Figure 3b if one triac conducts, a current path is established between two of the supply lines L1, L2 and L3.

In Figures 3c and 3d, three wire connections are used, with only three wires connecting to the three terminals of the motor. Two problems arise when only three supply wires are used.

The first problem is explained in more detail with reference to the waveforms of Figure 4 which are simplified waveforms showing time relationships between supply voltage waveforms and the "off" periods, firing points, turn off points and sampling points. In one condition shown in Figures 4d, 4e and 4f the non-conducting period of each triac is such that there are always two triacs conducting so that for example when phase A is conducting strongly at 60 phases B and C are each taking small amounts of current as shown at 61 and 62. If however the non-conducting periods are increased the condition shown in Figures 4g, 4h and 4i is reached where the current in waveform A falls to zero at 63 as the current in phase B ceases at 64 and phase C is fired at 65.The waveforms of Figures 49, 4h and 4i are a transition to the state shown in the waveforms of Figures 4j, 4k and 41 when the conducting periods have been reduced to the point where phase A cannot conduct during an interval 66 because the triac in phase B cease to conduct at instant 67 and the triac in phase C has not yet been fired at 68. The phase A triac therefore switches off and cannot conduct, when required at the end of the interval 66 and the motor would come to rest.

This problem is overcome by causing the triacs to fire in pairs. For example when current is to flow in phase C and the triac 52 is to be fired at point 68 then the triac 50 in phase A is also fired as indicated by the arrow 70. Similar firing pairs can be deduced from Figures 4j, 4k and 41 and are shown in Table 1.

Table 1 Primarytriac A B C Secondary triac B C A (Supply sequence -L1-L2-L3-L1;) Secondary triac C A B (Supply sequence -L1-L3-L2-L1-) The primary triac in row 1 of this table indicates the triac which would normally be fired, the secondary triac in row 2 indicates the triac which should also be fired when the supply sequence is as given in the fifth column, and the third row of the table gives the secondary triac which should also be fired when the supply sequence is reversed.

When the triacs are fired according to this scheme, it is apparent from Figures 4j, 4k and 41 that the triac currently considered as a secondary triac was that triac last considered as a primary triac.

Thus each pair of triacs fired consists of the one which would "normally" be fired plus the one which was the last to be fired "normally".

Figure 5 includes the connections necessary to implement Table 1 , where three controllers 71, 72 and 73 are shown, one for each of phases A, B and C respectively, connected to respective supply lines L1, L2 and L3. Only the controller 71 is shown in detail, the other two controllers 72 and 73 being similar. The controllers 71, 72 and 73 are each the same as the controller of Figure 1 except that a single common difference amplifier 19 and associated components, a common threshold circuit 31 and a common detector for direction of rotation 74 are provided, together with additional circuits 75.

76, 83, 84 and 85. The component circuits of the controller 71 are designated in the same way as Figure 1 where their function is the same.

The voltage output held by each sample circuit 22 is applied by way of one of three resistors: the resistor 25 and corresponding resistors for the other controllers. The average error signal is developed at the junction of the three resistors and applied to the inverting input of the amplifier 1 9.

Table 1 shows that with the first supply sequence when the triac 50 for phase A fires it should also fire the tiac 51 for phase B. This is arranged by passing the triggering signal for the phase A triac 50 from the trigger pulse generator 1 6 to an AND gate 75. The normal direction of rotation is taken to be the sequence in the second row of Table 1 and this is detected by the direction detector 74. For the normal direction of rotation, then, the AND gate 75 is opened and the triac 51 is triggered at the same time as the triac 50. Another AND gate 76 receiving inputs from the trigger pulse generator 1 6 and the reverse terminal of the direction detector 74 is connected to trigger the triac 52 of phase C for the other supply sequence as shown in Table 1.

The other phase controllers 72 and 73 have respective pairs of gates corresponding to the gates 75 and 76 coupled to their trigger pulse generators (TPGs) and these gates have outputs to the trigger terminals of the triacs of other phases as indicated in Figure 5. The trigger pulses are applied by way of respective isolating pulse transformers (not shown) having secondaries connected to the trigger terminals of the triacs.

The direction detector circuit 74 receives two of the supply voltage waveforms. These sinusoidal waveforms are limited to produce symmetrical square waves, in phase with the respective supply waveforms. The square waves are applied to logic in order to produce the normal (N) and reverse (R) signal. The logic may comprise a D-type clocked flip-flop which is connected to receive one square wave at its data input and the other after differentiation, at its clock input.

The second problem which arises when only three supply wires are used, concerns the measurement of phase lag between the instant the supply voltage reaches zero (relative to neutral) and the instant when the triac current reaches zero. As is explained above variations in this phase lag accompany changes in motor load and are used by the controller to fire the triacs to meet motor load variations.

Unfortunately when two triacs turn off, the resulting turn off and cessation of current in the third triac does not occur at the natural phase lag value for cessation of current in this phase. Rather the turn off is imposed by the cessation of current in the other two triacs. In measuring phase lag this premature turn off must be ignored and this is achieved by omitting to sample the phase lag during periods when the corresponding triac has been prematurely turned off.

For example in Figure 4j the current waveform of phase A is prematurely terminated at the beginning of the interval 66 and since the comparator 21 uses the voltage increase across the triac 50 when it goes open circuit to cause phase measurement by sampling the waveform from the ramp 18, a false phase measurement will occur at the beginning of the interval 66.

This problem is overcome by inhibiting sampling at certain times, for example in the normal phase sequence when the triac 52 of phase C is off it inhibits sampling in the controller 71 for phase A as indicated by a broken arrow 80 in Figure 4k. The full scheme for inhibiting sampling is shown in Table 2 for both the normal direction of phase sequence (row 2) and the reverse sequence (row 3).

Table 2 Signal causing inhibiting A off B off C off Phase sampling inhibited B C A (Supply sequence -L1-L2-L3-L1-) Phase sampling inhibited C A B (Supply sequence -L1-L3-L2-L1-) However, a further problem arises in that since the beginning of an interval 81 in waveform 41 and the natural end of conduction in phase A shown in waveform 4j at point 82 coincide, the cessation of conduction in phase C which inhibits sampling in phase A would also inhibit sampling at the point 82. This problem is avoided by delaying the inhibiting waveform long enough to allow sampling to occur at the point 82 but not long enough to prevent sampling at a time such as the beginning of the interval 66.Such an arrangement functions correctly because current always ceases in the phase which is to control inhibition well before inhibition is required but ceases at the same time at which undesired inhibition could occur.

In Figure 5 an inhibiting signal for the controllers 72 and 73 is generated from the comparator 21 of the controller 71. The output of this comparator is 'low' when the triac 50 is not conducting (see the waveform of Figure 2f) and it is delayed in a resistance capacitance delay circuit 83 before being passed to the controllers 72 and 73. Corresponding inhibiting signals are received by AND gates 84 and 85 from delay circuits of the controllers 72 and 73, respectively. The gates 85 and 84 also receive enabling signals from the direction detector 74 corresponding to normal and reverse rotation respectively. The output of the monostable circuit 35 indicating when sampling of the ramp waveform from the generator 18 should take place is passed to the gates 84 and 85 and only reaches the sample circuit 22 when one of these gates is enabled.

Since intervals of non-conduction by triacs occur after they have been initially triggered, such as the interval 66 in Figure 2j, further triggering by the trigger pulse generator 66 must be prevented although the ramp voltage from the generator 1 8 exceeds the output of the amplifier 1 9. This can be achieved by including logic which inhibits the generator 1 6 after each trigger pulse until a zero crossing occurs in the power supply. Such logic is available in some of the aforementioned integrated circuits containing the generators 1 6 and 1 8 and the comparator 1 7. The logic mentioned above for inhibiting primary trigger pulses while the triac is conducting is also required.

Although the invention has been specifically described with reference to the arrangement of Figure 5 it will be clear that it can be put into use in many other ways, for example with the motor connected in three-wire delta (see Figure 3d). The connections to the triacs are then as shown in Figure 5, that is each comparator 21 is connected across a respective one of the triacs, with the trigger connected to the corresponding trigger pulse generator 16, and each ramp generator 1 8 and each input of the direction detector 74 connected to a respective one of the line terminals L1, L2 and L3.

Terminals 90, 91 and 92 of Figured are then the same as terminals 93, 94 and 95, respectively, of Figure 5.

It is an important advantage of the controller of Figure 5 that it can be used with a motor of the type shown in Figure 3d where the motor may have only three external terminals. This advantage arises because in the present invention phase lag may be measured in the supply line as opposed to the motor windings (the latter requiring four or six wire connection to the motor).

A conventional starter may be interposed between the triacs 50 to 52 and the motor windings A, B and C but in many applications the controller described makes such starters unnecessary. The controller may also be placed between a starter and the motor.

It is usually preferable to replace the threshold circuit 31 of Figure 5 by a threshold circuit of the type shown in Figure 7, which responds to the voltage across the triacs in each phase of the controller.

A photo-transistor 96 which forms part of an optical isolator in the connection between the triac 50 and the comparator 21 is connected by way of a relatively low value resistor 97 (for example 2 Kohms) to the non-inverting input of an amplifier 98 forming the comparator 21. A relatively high value resistor 99 (for example 200 Kohms) connects the non-inverting input to a positive supply and the junction between the resistors 97 and 99 is connected to earth by way of a diode 1 00. The phototransistor 96 is also connected by way of a diode 101 to a differential amplifier 102 whose input is also fed from similar circuits for phases B and C.

In normal operation the current through the photo-transistor 96 is low corresponding to the low voltage VT (see Figure 2d) across the triac 50. The voltage across the resistor 99 varies in accordance with this current since the diode 100 remains back biassed and the output of the amplifier 98 is low (negative) when the triac 50 is not conducting (see Figure 2f). However when the voltage across the triac 50 rises considerably due to, for example, a tendency towards stalling, the non-inverting input of the amplifier 98 is held to the forward bias voltage across diode 100, and voltage is developed across resistor 97 proportionate to the voltage across the triac 50.When the voltage at the junction of resistor 97 and photo-transistor 96 becomes more negative than the override threshold voltage set by a potentiometer 103, the diode 101 conducts and the output of the amplifier 102 goes negative to provide a voltage VSTART which is fed to amplifier 1 9 (Figure 6) to cause increased voltage to the motor.

Thus if a tendency to stall is indicated by any phase, a voltage is applied to the amplifier 1 9 to cause the trigger pulses to be applied immediately after a zero crossing occurs in the supply waveform.

Preferably the circuit of Figure 6 is used instead of the amplifier 1 9 and the circuits 22. To overcome instability problems in three phase systems, phase lead is advantageous and sometimes essential at frequencies between about 6 Hz and 70 Hz (gain increasing at 6 db/octave). This may be introduced by the resistor 87 and the capacitor 86.

Circuits other than those of the type shown in Figure 1 may be used for measuring phase lag.

Other switching means such as parallel connected oppositely poled thyristor pairs of a thyristor connected to the d.c. terminals of a full wave rectifier may be used instead of triacs.

The or each switching means may be in the form of a single thyristor and may then be used in parallel with a diode, so that triggering is carried out in half cycles having one polarity but current flows automatically through the diode in half cycles having the other polarity without triggering.

The invention is also applicable to three phase motors connected in parallel when all corresponding terminals of all star or delta connected motors are supplied by way of one switching means which may comprise a triac or a group of triacs in parallel all receiving triggering signals at the same time. Thus three switching means are required, for each group of parallel connected motors. If more than two three phase motors are to be connected in parallel and started independently it is advisable to ensure that for an interval when each motor is started, the triacs are fired immediately zero crossings in the power waveform occurs Motor starters usually have a pair of extra contacts which are closed on starting and these contacts in all starters may be connected in parallel to cause a voltage similar to that obtained from the threshold circuit 31 of Figure 1 to be applied to the non-inverting input of the amplifier 19 for a suitable interval.

Claims (14)

Claims

1. A power controller for three phase induction motors supplied by way of three wires only, comprising three switching means, each for a respective supply phase and being, in operation, connected between that phase of a three phase supply and a respective terminal of a star or delta connected induction motor, each switching means being capable of passing current from the moment when a primary or secondary trigger signal reaches that switching means until current applied thereto ceases, monitor means for generating a monitoring signal representative of the interval between a zero in the voltage of at least one phase of the supply and the next final cessation of current in that phase before current reversal therein, control means for generating primary trigger signals for the switching means of respective supply phases, the control means being responsive to the monitoring signal to change the time relationships between the supply waveform and the primary trigger signals in that sense which shortens the conduction period of the switching means when the said interval tends to increase and vice versa, and the control means also being constructed to generate secondary trigger signals at the same time as primary trigger signals, each secondary trigger signal being applied to that switching means which received the last primary trigger signal.

2. A power controller according to Claim 1 wherein the monitor means comprises three voltagemonitor means, one for each switching means for detecting steps in the voltage across that switching means which occur when that switching means ceases to conduct, and three signal-deriving means, one for each supply phase, for providing a signal which is representative of the time interval between a zero crossing in the supply voltage of that phase and the time at which the last voltage step occurs across the switching means for that phase before current reversal in that phase.

3. A power controller according to Claim 1 or 2 wherein the control means comprises three ramp generators and three comparators, one generator and one comparator for each phase, each ramp generator for each respective phase generating a ramp signal having a repetition frequency equal to twice the frequency of the supply and having return waveform edges which are synchronised with the zero crossings in the supply voltage of that phase, and each comparator for each respective phase, in operation, comparing the ramp signal from the ramp generator of that phase with the monitor signal or a signal dependent thereon to provide a primary trigger signal when the signals applied to that comparator bear a predetermined magnitude relationship to one another.

4. A power controller according to Claim 3 including a primary logic means for each phase for inhibiting primary trigger signals after one primary trigger signal has occurred until the next said return waveform edge occurs which is synchronised with a zero crossing in the supply voltage of that phase.

5. A power controller according to Claim 3 wherein the control means includes directiondetection means for indicating the sequence in which the voltages of the supply phases become positive, and secondary logic means for each phase coupled to receive input signals from the directiondetection means and from the comparator which provides primary trigger signals for that phase, the secondary logic means of each phase supplying secondary trigger signals to one of the switching means of the other phases in dependence upon its input signals.

6. A power controller according to any preceding claim including means for suppressing generation or use of the monitoring signals whenever current through a switching means ceases but next flows again in the same direction.

7. A power controller according to Claim 6 wherein the monitor means generates a respective monitoring signal for each phase, and the means for suppressing generation or use of the monitoring signals suppresses the generation or use of the monitoring signal of a phase when the switching means of the preceding phase in the sequence in which the phase supply voltages go positive, is not conducting.

8. A power controller according to Claim 6 or 7 insofar as dependent on Claims 2 and 5 wherein the means for suppressing generation or use of the monitoring signals includes suppression logic means for each phase coupled to receive input signals from the direction-detection means and, by way of delay means, from the voltage-monitor means of the other two phases, the suppression logic means of each phase being connected to suppress the output signal of the voltage monitor means of that phase in dependence upon its input signals.

9. A method of controlling a three phase star or delta connected induction motor when the motor is supplied by way of three wires only, comprising the steps of supplying the induction motor from a three phase alternating current electrical supply by way of three switching means, each for a respective supply phase and connecting that phase to a respective terminal of the star or delta connection of the motor when primary or secondary trigger signals are applied thereto, each switching means conducting from the moment when a primary or secondary trigger signal reaches that switching means until current applied thereto ceases, generating a monitoring signal representative of the interval between a zero in the voltage of at least one phase of the supply and the next final cessation of current in that phase before current reversal therein, generating primary trigger signals for the respective switching means in the reverse phase sequence to that in which the supply voltages attain maximum magnitude, in response to the monitoring signal to change the time relationships between the supply waveform and the primary trigger signals in that sense which shortens the conduction period of the switching means when the said interval tends to increase and vice versa, and generating secondary trigger signals for the switching means of a supply phase at the same time as the primary trigger signal for the switching means of the next phase in the said reverse phase sequence.

1 0. A power controller for a three phase induction motor supplied by way of three wires only, comprising three switching means, each for a respective supply phase and being, in operation, connected between that phase of a three phase supply and a respective terminal of a star or delta connected induction motor, each switching means being capable of passing current from the moment when a trigger signal reaches that switching means until current applied thereto ceases, monitor means for generating a monitoring signal representative of the interval between a zero in the voltage of at least one phase of the supply and the next final cessation of current in that phase before current reversal therein, control means for generating trigger signals for the switching means, the control means being responsive to the monitoring signal to change the time relationships between the supply waveform and the trigger signals in that sense which shortens the conduction period of the switching means when the said interval tends to increase and vice versa, and means for suppressing generation or use of the monitoring signals whenever current through a switching means ceases but next flows again in the same direction.

11. A method of controlling a three phase star or delta connected induction motor when the motor is supplied by way of three wires only, comprising the steps of supplying the induction motor from a three phase alternating current electrical supply by way of three switching means, each of which when triggered by a trigger signal connects one phase of the supply to a respective terminal of the star or delta connection, each switching means being capable of passing current from the moment when a trigger signal reaches that switching means until current applied thereto ceases, generating a monitoring signal representative of the interval between a zero in the voltage of at least one phase of the supply and the next final cessation of current in that phase before current reversal therein, generating trigger signals for the respective switching means in response to the monitoring signals to change the time relationship between the supply waveform and the primary trigger signals in that sense which shortens the conduction period of the switching means when the said interval tends to increase and vice versa, and suppressing the generation, or the use, of the monitoring signals whenever current through a switching means ceases but next flows again in the same direction.

12. A power controller according to any preceding claim connected to a three phase induction motor, or a group of parallel connected three phase induction motors.

13. A power controller and a motor or group of motors according to Claim 12 wherein connection between the controller and motor, or group of motors is by way of three wires only.

14. A power controller, or a power controller and one or more motors, substantially as hereinbefore described with reference to and as shown in Figure 1, or Figures 1 and 5, or Figures 1, 5 and 6 and/or 7 of the accompanying drawings.