GB2085204A

GB2085204A - Motor control system

Info

Publication number: GB2085204A
Application number: GB8130032A
Authority: GB
Original assignee: GEC Elliott Automation Ltd
Current assignee: Roadside Technology Services Ltd
Priority date: 1980-10-06
Filing date: 1981-10-05
Publication date: 1982-04-21
Also published as: GB2085204B

Abstract

A motor control system including an A.C. supplied input convertor 1 and a motor convertor 16 interconnected by a D.C. link 3, the motor convertor being controlled in dependence upon the motor speed or torque and the input convertor being controlled to maintain a constant reactive input current. The result is constant KVAR drawn from the supply, and consequently an improved power factor, whilst still maintaining constant speed torque of the motor. <IMAGE>

Description

SPECIFICATION Motor control system This invention relates to motor control systems and particularly to such systems for feeding a motor from an A.C. supply system.

In motor control systems employing static convertor equipment to feed a motor from an A.C. power supply network, the power factor of the load on the A.C. system is commonly dependent upon the operating conditions of the controlled motor.

For example, in a naturally commutated thyristor convertor fed D.C. variable speed drive the power factor of the input current will be approximately proportional to motor speed, i.e. approximately 0.9 at full speed, zero at standstill, 0.45 at half speed etc.

The magnitude of the current will be directly proportional to the motor armature current which is dictated by the motor torque demanded by the mechanical load.

This variation of the power factor down to very low values is undesirable and is particularly bad if the load is subject to cyclic variation.

It is therefore an object of the present invention to limit the variation in the power factor.

According to the present invention, a motor control system comprises an input convertor connected to feed a D.C. link which in turn is connected to feed a motor controller, means for controlling the input convertor to draw a constant value of reactive currentfrom an A.C. supply, and means for controlling the motor controller in dependence upon a required motor speed and/or torque indication.

The motor controller may include a static motor convertor operating by natural or forced commutation.

A number of different motor control systems will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a block diagram of a basic arrangement; Figures 2 to 7 are block diagrams of various alternative arrangements according to Figure 1 shown in rather more detail; Figure 8 is a schematic circuit diagram, being an expansion of the arrangement of Figure 3; and Figure 9 is a vector diagram of input conditions at the connection of the control systems to an A.C.

supply.

Referring to Figure 1, this shows a basic arrangement of an input convertor 1 connected to threephase A.C. supply terminals 2 and feeding a D.C. link 3. The D.C. link in turn feeds a motor controller 4, although the motor itself is not in general an essential part of the invention. Control inputs 6 and 8 control the operation of the convertor 1 and motor controller 4 in accordance with the invention.

Figure 2 shows an embodiment of the invention in which the input convertor is a naturally commutated three-phase thyristor bridge. The D.C. link 3 feeds the armature 10 of a D.C. motor with a separately excited field 12. The field excitation is controlled by a field control circuit 14 in response to a control signal on control input 8.

In Figure 3 the D.C. link 3 feeds a naturally commutated three-phase thyristor bridge convertor 16 which supplies a three-phase synchronous motor 18. In this case the control input 8 is applied to control the firing angle of the thyristors in the motor convertor 16.

The operation of the arrangement of Figure 3 will be explained subsequently, in further detail, with reference to Figure 8.

In Figure 4 the same input convertor arrangement supplies a D.C. linkwhich again supplies a naturally commutated three-phase thyristor bridge convertor 20. In this case, although the motor convertor 20 feeds a similar synchronous motor 22, it is the field excitation which is controlled, by way of a field control circuit 24, by the control input 8. The motor convertor is set at a fixed firing angle chosen to facilitate the motor field control.

Figure 5 shows the application of the invention to an induction motor 28 fed from a three-phase forced commutation thyristor convertor 26. Natural commutation implies triggered firing of a thyristor and cessation of conduction caused by the anode/ cathode voltage falling to zero, although conduction will continue after the zero transition with an inductive load. Forced commutation on the other hand, involves the firing of one thyristor so as to reverse the voltage across a thyristor already firing and thus switch it off before it would naturally have ceased conduction, if indeed it would have ceased naturally.

The control input 8 in Figure 5 is applied to the motor convertor 26 to determine the periods of conduction.

The arrangement of Figure 6, like that of Figure 3, employs a synchronous motor 30 with constant field excitation. The motor convertor 32 again employs a forced commutation motor convertor which is controlled by the control input 8.

Finally, in Figure 7 a D.C. motor 34 with fixed, separate excitation, is supplied by a thyristor 'chopper' controller 36 which effectively controls the average level of the voltage applied to the motor armature. The chopper controller is itself controlled by the control input 8.

The system of Figure 3 will now be explained more fully with reference to Figures 8 and 9.

The input convertor 1 is supplied from the three phase terminals 2 by way of measuring means 40 which measures the reactive power drawn from the A.C. system. The input convertor 1 is, as mentioned previously, a three-phase bridge circuit comprising thyristors 42 which receive firing pulses from a firing unit 44 in the appropriate order in well known manner. The common firing angle of these firing pulses is, however, determined in accordance with the invention by an error signal on a lead 6 from a KVAR comparator circuit 46. This latter circuit derives a signal indicative of the consumed reactive power from the measuring circuit 40, and also a reference signal from a preset potentiometer 48.The error signal on lead 6 from the comparator is dependent on the difference between the two signals and is operative on the firing circuit 44 to shift the firing angle to tend to balance the two signals input to the comparator 46. It can be seen therefore that the potentiometer 48 determines the level of reactive power (drawn from the supply) to which the circuit is stabilised.

The input convertor 1 is coupled by the D.C. link 3 to the input of the motor convertor 16. An inductor 50 may be included to smooth the current in the D.C.

link.

The three-phase output of the motor convertor 16 is then applied to the stator of a three-phase synchronous motor 18 having a field winding 58 excited by a constant D.C. source 56.

The motor convertor 16 also consists of a naturally commutated three-phase thyristor bridge. The thyristors 52 are triggered by pulses from a firing circuit 54 timed in relation to the A.C. waveform applied to the motor 18 and thus to the motor speed. The thyristors 52 are switched off by the voltage generated in the synchronous motor. Control of this convertor is obtained by varying the phase position of the gate pulse, relative to the voltages generated by the motor. The determination of the common firing angle is then controlled in accordance with the invention, this latter function being effected in response to an error signal on lead 8.

In this embodiment it is supposed that the motor is required to be driven at a constant speed independent of the mechanical load. Aspeed measuring transducer 60 determines the motor speed and transmits a corresponding signal to a speed comparator circuit 62. A preset reference signal is then derived from a potentiometer 64. The error signal on lead 8 is then the difference between the two and this error signal is of such magnitude and direction as to tend to adjust the firing angle to bring the motor speed into correspondence with the setting of the potentiometer 64.

It may be seen therefore, that within limits, the reactive power consumed by the circuit as a whole can be controlled independently of the controlled motor speed. Equally the motor torque could be measured and maintained at a preset value.

Referring now to Figure 9, this shows the effect on the input current from the A.C. supply of maintaining the reactive power constant. The input current consists of a component in phase with the supply voltage and a reactive component in quadrature with the supply voltage. It can be seen that if the quadrature component is maintained constant, the total input current vector is constrained to move along the vertical broken line locus as the overall power factor varies with load conditions. The input convertor in Figure 8 is thus controlled to maintain the input current at a value determined by the locus and the power factor.

Further features may be incorporated in the basic arrangements described above. Thus, the input convertor may consist of a number of interconnected circuits phase displaced with respect to each other to minimise the harmonics drawn from the supply.

The input convertor may be sequence controlled so as to alter the relationship between the D.C. and A.C. currents.

The action of the reactive power control system may be relatively slow compared to that of the speed controlling system.

Claims

1. A motor control system comprising an input convertor connected to feed a D.C. link which in turn is connected to feed a motor controller, means for controlling the input convertor to draw a constant value of reactive current from an A.C. supply, and means for ccntrolling the motor controller in dependence upon a required motor speed and/or torque indication.

2. A motor control system according to Claim 1, wherein said motor controller includes a motor convertor.

3. A motor control system according to Claim 2, wherein said motor convertor is arranged to be naturally commutated.

4. A motor control system according to Claim 3, and including a synchronous motor connected to the output of said motor convertor, the motor convertor being controllable in dependence upon a required motor speed and/ortorque indication.

5. A motor control system according to Claim 3, and including a synchronous motor connected to said motor convertor, the synchronous motor having field control means controllable in dependence upon a required motor speed and/ortorque indication.

6. A motor control system according to Claim 3, and including a brushless synchronous motor connected to said motor convertor.

7. A motor control system according to Claim 2, wherein said motor convertor is a forced commutation thyristor bridge convertor which is controllable in dependence upon a required motor speed and/or torque indication.

8. A motor control system according to Claim 7, including a synchronous motor connected to the output of said motor convertor.

9. A motor control system according to Claim 6, including an induction motor connected to the output of said motor convertor.

10. A motor control system according to Claim 7, wherein said motor convertor is arranged to operate as a D.C. chopper circuit and is connected to feed a D.C. motor.

11. A motor control system according to Claim 1, wherein said D.C. link is connected to a D.C. motor having a field control circuit controllable in dependence upon a required motor speed and/or torque indication.

12. A motor control system according to any preceding claim, wherein said input convertor is a naturally commutated three-phase thyristor bridge convertor.

13. A motor control system according to any preceding claim, wherein said input convertor comprises a plurality of three-phase rectifier bridges and means for phase displacing the three-phase inputs of said bridges, whereby to reduce harmonic currents drawn from the supply.

14. A motor control system according to any preceding claim, wherein said input convertor includes sequence control circuitry adapted to control the relation between the A.C. supply current and the current in said D.C. link.

15. A motor control system substantially as hereinbefore described, with reference to Figures 1 and 9 in conjunction with any of Figures 2,3 and 8,4, 5,6and7.