GB2084360A - Apparatus and Methods for Controlling Induction Motors - Google Patents

Abstract

When induction motors are lightly loaded their power factors and efficiency are poor but in the present invention power factor is controlled regardless of load. An induction motor 10 is connected by way of a triac 12 to a supply 11. The voltage across the triac 12 is monitored by a comparator 21 for voltage steps which correspond to current turn-off and a signal is developed at the output of an amplifier 19 which represents error from required phase lag. A comparator 17 and trigger pulse generator 16 responsive to ramp generator 18 trigger the triac 12 in accordance with the error. A circuit 31 overrides the control system during starting. A number of further induction motors may be connected in parallel with the motor 10. <IMAGE>

Description

SPECIFICATION Apparatus and Methods for Controiling Induction Motors The present invention relates to apparatus and methods for controlling a power supply circuit for an induction motor to optimise power consumption.

The power consumption W of a single phase induction motor is W=VI cos(p, where V and I are the r.m.s. supply voltage and current drawn, and ? is the phase lag of the current behind the voltage waveform. At rated load the phase lag is small so that the power factor cos (p is approximateiy equal to one but at well below rated load, the power consumption decreases partly because the motor draws reduced current but more importantly because the phase lag (p increases and reduces the power factor.

This reduces the efficiency of the motor because losses from resistive heating and hysteresis are not reduced in proportion to power consumption. Similar effects occur in three phase induction motors.

According to a first aspect of the present invention there is provided a power controller for an induction motor comprising one or more switching means for connection between an alternating current electrical supply and an indudction motor which is to be energised from the supply, there being one switching means for the, or each, phase of the supply, and the or each switching means becoming conductive when a trigger signal is applied to that switching means and remaining conductive until the current supplied thereto ceases, monitor means for deriving a monitoring signal representative of respective intervals between a zero in the voltage waveform of the, or at least one, phase of the supply and the next final cessation of current in that phase before current reversal therein, means for generating a time-reference signal representative of time elapsed since the last said zero in voltage, and control means for generating the trigger signals, the control means being responsive to a comparison between the values of the monitoring signal and the time-reference signal to change the time relationship between the supply waveforms 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.

According to a second aspect of the present invention there is provided a method of controlling an induction motor comprising the steps of supplying the induction motor from an alternating current electrical supply by way of one or more switching means connecting the motor to the supply when trigger signals are applied to the switching means, there being one switching means for the, or each, phase of the supply, the or each switching means becoming conductive when triggered and remaining conductive until current supplied thereto ceases, generating a monitoring signal representative of respective intervals between a zero in the voltage of the, or at least one, phase of the supply and the next final cessation of current in that phase before current reversal therein, generating a time-reference signal representative of time elapsed since the last said zero voltage, and generating the trigger signals, the time relationship between the trigger signals and the supply waveform being changed, in response to a comparison between the values of the monitoring signal and the time-reference signal, in that sense which shortens the conduction period of the switching means when the said interval tends to increase and vice versa.

The main advantage of the present invention is that it effectively produces the same resuit as a reduction in the supply voltage V when the motor power requirement falls so that the power factor and efficiency of the motor remain high, irrespective of motor loading. Motors have been observed to consume up to 70% less energy depending on loading, when the invention is used.

For a single phase motor the monitor means may comprise means for detecting steps in the voltage across the switching means which occur when the switching means ceases to conduct, and means for providing a signal which is representative of the time interval between a zero crossing in the supply voltage and the time at which the said voltage step occurs. A signal so provided is subtracted from a reference signal before application to the control means and adjustment of the reference signal sets the phase lag. Corresponding arrangements may be made for three phase motors to provide either a monitoring signal derived from one phase, or preferably an average monitoring signal derived from all three phases.

For single phase motors, the control means may comprise a ramp generator for generating a ramp signal having a repetition frequency equal to twice the frequency of the supply and having return waveform edges which are synchronized with the zero crossings in the supply voltage, and a comparator for comparing the ramp signal with the monitor signal or a signal dependent thereon to provide a trigger pulse when the comparator input signals bear a predetermined magnitude relationship to one another. For three phase motors one ramp generator and one comparator may be provided for each phase.

The time-reference signal may be the, or one of ramp signals mentioned above and the monitoring signal may be obtained by sampling the ramp signal at the instant a said cessation of current occurs.

This method of phase lag measurement by sampling which may be used without using the voltage across the, or one of the, switching means to derive a monitoring signal representative of the said interval, has important advantages over methods in the pulse length or the mark/space ratio of a pulse signal initiaily represents the phase lag and integration or capacitive smoothing is used to provide a further signal in which amplitude represents phase lag. Methods involving integration or smoothing introduce phase lag in the feedback system formed by the power controller and the motor and this can make stabilisation of the feedback control loop impossible.Using the new sampling technique, phase compensation (mentioned below) can be used to introduce phase lead (increase of gain with frequency) in the loop at higher frequencies (for example above 6Hz) to neutralize the effects of cumulative phase lag in the response of the motor and controller.

According to a third aspect of the present invention there is provided a power controller for an induction motor comprising one or more switching means for connection between an alternating current electrical supply and an induction motor to be energised from the supply to connect the motor to the supply when trigger signals are applied to the switching means, there being one switching means for the, or each phase of the supply, and the or each switching means becoming conductive when triggered and remaining conductive until the current supplied thereto ceases, monitor means for deriving a monitoring signal representative of the interval between a zero in the voltage of the, or at least one of the supply phases and the next final cessation of current in that phase before current reversal therein, control means for generating the trigger signals, the control means being responsive to the monitoring signal to change the time relationship between the supply waveform and the trigger signals in that sense which shortens the conductive period of the switching means when the said interval tends to increase and vice verse, and override means for changing the said time relationship in that sense which lengthens the said conduction period when the peak voltage across the, or at least one of the, switching means increases to a value which indicates at least that the motor is stationary or tending to stall.

According to a fourth aspect of the present invention there is provided a method of controlling an induction motor comprising supplying the motor from an alternating supply by way of one or more switching means, the, or each of the, switching means connecting the motor to the supply when a trigger signal is applied to the switching means, there being one switching means for each respective phase of the supply, and the, or each, switching means becoming conductive when triggered and remaining conductive until the current supplied thereto ceases, deriving a monitoring signal representative of the phase angle between a zero in the voltage of the, or at least one of the, supply phases and the next final cessation of current supplied in that phase before current reversal therein, and generating trigger signals having a time relationship relative to the supply waveform which is responsive to the monitoring signal to shorten the conduction period of the switching means when the said interval increases and vice versa unless the peak voltage across the, or at least one of the, switching means increases to a value which indicates at least that the motor is stationary or tending to stall and then the said time relationship is changed in that sense which lengthens the said conduction period.

In all four aspects of the invention more than one motor may be connected in parallel to be supplied by way of the switching means. References to 'the motor' in the four aspects of the invention thus include the plural, references to time and phase relationships and phase angles are overall parameters, and references to stalling or a motor being stationary relate to any one motor where motors are connected in parallel.

The override means overrides the power reduction provided by the control means and applies the full supply voltage to a motor which is to be started and this voltage is applied while the motor speeds up. The override means thus enables a single power controller circuit to run many single phase motors in parallel and still save power. It also ensures the necessary full power for starting transients when one or more single phase motors are started from rest. Any tendency of any of the motors to stall due to sudden abnormal loading or supply voltage fluctuation causes the override means to operate and prevents stalling. Arrangements for starting a number of three phase motors in parallel are described later.

The override means may comprise a rectifying diode and a Zener diode connected to the junction between the, or one of the, switching means and the motor, and a capacitor in series with the Zener diode. When the voltage applied to the Zener diode exceeds the Zener voltage the capacitor starts to change, and the voltage across the capacitor provides the override signal. The rectifier diode prevents discharging and reverse charging of the capacitor during the reverse half cycle of the supply.

The override signal may be combined with the monitoring signal at the input to a differential amplifier, their amplified difference being applied to control the time relationship between the supply voltage and the trigger pulses.

A patent application No, 8129043 having the title "Apparatus and Methods for Controlling Three Phase Induction Motors" (Inventor: P. J. Unsworth) filed on the same day as the present application has claims which cover some aspects of the apparatus described below.

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, Figure 2a to 2j are waveforms used in explaining the operation of Figure 1, Figure 3a to 3d show various ways in which an induction motor can be supplied from a three phase supply, Figure 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.

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 13 to a terminal 14 of the power supply 11.

the other terminal 15 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 1 9. The trigger pulse generator 16, the comparator 1 7 and the ramp generator 1 8 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 19. The comparator 21 detects the time instant X (see Figure 2c) when the triac 12 ceases to conduct. The lag 97 between the supply voltage falling to zero at Z (see Figure 2a) and the point X depends on the load on the motor and in the firing 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 P (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, sampling spike may be more simply obtained by differentiating the output of the 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 (VREF) obtained from a potentiometer 24.

The output of the sample and hold circuit 22 is applied to the inverting (-) input of the differential amplifier 19.

In operation, when the lag P (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 2h the ramp generator 1 8. 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 P 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 (p 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 ? 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 19. The d.c. gain of the amplifier 19 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 does not includes 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, and 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 12. 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 motor e.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 startingistalling 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 safety margins under conditions of reduced supply voltage.

A preferred alternative arrangement to replace items 1 9, 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 noninverting 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 VRVREF 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 12 form a feedback loop there is a danger of instability in the overall response. The phase lag (p 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 6db/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 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 iags 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 (iM, Figure 2c which is the sum of the currents 1M1 and IM2 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 alone 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 controlled 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 of 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 1 9 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 arrangment 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 when 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 4g, 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 Primary triac A B C Secondary triac B C A (Supply sequence -L1 -L2-L3-L1 -) Secondarytriac 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 1 9 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 triac 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-Ll -) 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 18 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 Figure 3d 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 advangage 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 6Hz and 70Hz (gain increasing at 6db/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 occur. 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 1 9 for a suitabie inteival.

Claims (17)

Claims

1. A power controller for an induction motor comprising one or more switching means for connection between an alternating current electrical supply and an induction motor which is to be energised from the supply, there being one switching means for the, or each, phase of the supply, and the or each switching means becoming conductive when a trigger signal is applied to that switching means and remaining conductive until the current supplied thereto ceases, monitor means for deriving a monitoring signal representative of respective intervals between a zero in the voltage waveform of the, or at least one, phase of the supply and the next final cessation of current in that phase before current reversal therein, means for generating a time-reference signal representive of time elapsed since the last said zero in voltage, and control means for generating the trigger signals, the control means being responsive to a comparison between the values of the monitoring signal and the time-reference signal to change the time relationship 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.

2. A power controller according to Claim 1 wherein the monitor means monitors the voltage across the, or at least one of the, switching means to detect a step in the said voltage which indicates cessation of conduction by that switching means.

3. A power controller according to Claim 1 or 2 for a single phase induction motor comprising a single switching means.

4. A power controller according to Claim 2 for a three phase induction motor comprising three switching means one for each phase and wherein the monitoring means, in operation, detects steps in the voltage across each switching means and generates three monitoring signals each representative of the interval between a zero in the voltage of one phase of the supply and the next final cessation of current in that phase before current reversal therein, and the control means is responsive to the monitoring signals to change the time relationships between the supply waveforms and the trigger signals.

5. A power controller according to Claim 2, 3 or 4 wherein the monitor means comprises one voltage-monitor means for the, or each, switching means for detecting steps in the voltage across that switching means which occur when that switching means ceases to conduct, and one signal-deriving means for the, or 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 said voltage step occurs across the switching means for that phase before current reversal in that phase.

6. A power controller according to Claim 1 or 2 wherein the means for generating the timereference signal generates a ramp waveform which has a repetition frequency twice that of the said voltage and has minima synchronised with zero crossings in the said voltage, and the controller includes means for sampling the said ramp waveform each time a said final cessation of current occurs to provide the monitoring signal which represents the said respective intervals by its magnitude, and comparison means for generating trigger signals each time the maynitudes of the time-reference signal and the monitoring signal reach a predetermined relationship with one another.

7. A power controller according to any of Claims 2 to 5 wherein means are provided for subtracting an adjustable reference signal from the signal representative of the time interval between a said zero crossing and a said voltage step in deriving monitoring signals, whereby the reference signal can be used to set the said time relationship.

8. A power controller according to any preceding claim wherein the control means comprises a ramp generator and a comparator for each phase, each ramp generator for a respective phase, in operation, generating a ramp signal having a repetition frequency equal to twice the frequency of the supply and having return waveform edges which are synchronized with the zero crossings in the supply voltage of that phase, and each comparator for a 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 trigger pulse for the switching means of that phase when the signals applied to that comparator bear a predetermined magnitude relationship to one another.

9. A power controller according to Claim 7 including logic means for each phase for inhibiting trigger signals after one trigger signal has occurred until the next said return waveform edge occurs which is synchronized with a zero crossing in the supply voltage of that phase.

1 0. A power controller according to any preceding claim including override means for changing the said time relationship in that sense which lengthens the said interval when the peak voltage across the, or at least one of the, switching means increases to a value which indicates at least that the motor is stationary or tending to stall.

11. A power controller according to Claim 10 wherein the override means comprises a rectifying diode and a Zener diode connected to the, or at least one of the, switching means at the point where that switching means is to be connected to the motor, and a capacitor in series with the Zener diode.

12. A power controller according to Claim 10 wherein the override means comprises one override comparator for comparing the voltage across the switching means of each phase with a reference voltage, the override comparator being connected to the control means to override response to the monitoring signal when the reference voltage is exceeded.

1 3. A power controller according to any preceding claim connected to an induction motor, or a number of induction motors connected in parallel.

1 4. A method of controlling an induction motor comprising the steps of supplying the induction motor from an alternating current electrical supply by way of one or more switching means connecting the motor to the supply when trigger signals are applied to the switching means, there being one switching means for the, or each, phase of the supply, the or each switching means becoming conductive when triggered and remaining conductive until current supplied thereto ceases, generating a monitoring signal representative of respective intervals between a zero in the voltage of the, or at least one, phase of the supply and the next final cessation of current in that phase before current reversal therein, generating a time-reference signal representative of time elapsed since the last said zero voltage, and generating the trigger signals, the time relationship between the trigger signals and the supply waveform being changed, in response to a comparison between the values of the monitoring signal and the time-reference signal, in that sense which shortens the conduction period of the switching means when the said interval tends to increase and vice versa.

1 5. A method according to Claim 14 wherein the time-reference signal is a ramp waveform which has a repetition frequency twice that of the said voltage and has minima synchronised with zero crossings in the said voltage, the monitoring signal representing respective intervals by its magnitude and being derived by sampling the said ramp waveform each time a said final cessation of current occurs, and the trigger signals being generated each time the magnitudes of the time-reference signal and the monitoring signal reach a predetermined relationship with one another.

1 6. A power controller for an induction motor comprising one or more switching means for connection between an alternating current electrical supply and an induction motor to be energised from the supply to connect the motor to the supply when trigger signals are applied to the switching means, there being one switching means for the, or each phase of the supply, and the or each switching means becoming conductive when triggered and remaining conductive until the current supplied thereto ceases, monitor means for deriving a monitoring signal representative of the interval between a zero in the voltage of the, or at least one of the supply phases and the next final cessation of current in that phase before current reversal therein, control means for generating the trigger signals, the control means being responsive to the monitoring signal to change the time relationship 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 override means for changing the said time relationship in that sense which lengthens the said conduction period when the peak voltage across the, or at least one of the, switching means increases to a value which indicates at least that the motor is stationary or tending to stall.

17. A method of controlling an induction motor comprising supplying the motor from an titernating supply by way of one or more switching means, the, or each of the, switching means connecting the motor to the supply when a trigger signal is applied to the switching means, there long one switching means for each respective phase of the supply, and the, or each, switching means 3becoming conductive when triggered and remaining conductive until the current supplied thereto ceases, deriving a monitoring signal representative of the interval between a zero in the voltage of the, or at least one of the, supply phases and the next final cessation of current in that phase before current reversal therein, and generating trigger signals having a time relationship relative to the supply waveform which is responsive to the monitoring signal to shorten the conduction period of the switching means when the said interval increases and vice versa unless the peak voltage across the, or at least one of the, switching means increases to a value which indicates at least that the motor is stationary or tending to stall and then the said time relationship is changed in that sense which lengthens the said conduction period.

1 8. 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.