GB2122434A - Electric motor control - Google Patents

Abstract

A controller provides constant or variable phase relationships between the electrical input to each of the two windings (10, 12) of a single or two phase induction motor so as to enable a constantor variable frequency and a constant or variable voltage supply to be applied to the motor for thereby controlling at least one of the parameters of speed and torque of the motor. As shown, the windings are connected to a bridge of thyristors (16, 18, 20, 22). In an alternative arrangement (Fig. 2), the winding (10) is connected to the mains supply while the winding (12) is connected to the junction of two thyristors. <IMAGE>

Description

SPECIFICATION Electric motor control The present invention relates to the control of electric motors and in particular to the control of singleortwo phase induction motorsandtothe control of two orthree phase induction motors.

Traditionally single phase and two phase induction motors used commonly in inustrial and domestic applications have used a starting capacitorto create the required phase relationship between the magnetising currents ofthetwo inherentwindings.

In some types of motors two or more capacitors are used to maintain the phase relationships required at start up and normal running speed.

The use ofthese motors as variable speed controlled ortorque controlled devices has been severely restricted bythe described use of capacitors.

According to a first aspect of the present invention there is provided a controller comprising means for providing constant or variable phase relationships between the electrical input to each of the two windings of a single ortwo phase induction motor so asto enable a constant orvariable frequency and a constant orvariable voltage supply to be applied to the motorforthereby controlling at least one of the parameters of speed and torque of the motor.

Preferably, both of the parameters of speed and torque ofthe motor are controllable by the controller.

This first aspect of the invention is particularly beneficial when used in conjunction with thefollow- ing types of single ortwo phase induction motors; capacitor start induction motor, capacitor start and run induction motor, two value capacitor motor and the true "two phase" induction motor.

The controller can beutilised,togetherwithaddi- tional circuitry as necessary, so as to enable dynamic breaking and/or reversing of the motor Additionally thecontrollertogetherwith possible additional circuitry, can enable a single ortwo phase induction motorto be used without contactor isolated supplies except in the requirement for electromechanical isolation. The resultant circuitry permits a "soft start" to be employed if required as well as permitting the speed and/ortorqueto be increased ordecreased as desired and thereby also enabling the acceleration and deceleration of the motor to be effected.

Conventionally, electric motors have been used to drive mechanical devices in order to produce mechanical vibration. Such vibration may be required,for example, to drive a vibratory conveyorfortransporting certain types of product. Frequency of vibration can be controlled by varying the speed ofthe motor or with the use of a mechanical control. Amplitude of vibration is controlled by mechanical or other means.

According to a further aspect of the present invention there is provided a controllerwhich provides an input for a two orthree phase electric motor, the input having phase relationship and current characteristics which result in reciprocating angular displacement ofthe output shaft ofthe motor, the controller controlling the amplitude and/orfrequency ofthe said displacement.

The reciprocating displacement of the output shaft can be directly or indirectly applied to provide mechanical vibration or oscillation.

In the case of a two phase induction motorthe capacitor or capacitors usually required during starting and running, often in combination with a centrifugal switch our similar device, may be eliminated.

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 illustrates a first circuit arrangement of a controller, which uses an electronically generated reference phase, Figure 2 illustrates a second circuit arrangement of a controller, which uses a mains derived reference phase, and Figure 3 is a graph showing the voltages across respective phases when the circuit of Figure 1 is used to produce reciprocating angular displacement of the output shaft of a two phase induction motor.

Controllers described with reference to the Figures 1 and 2 are particularly advantageous for use with the following types of single or two phase induction motors; capacitor start induction motor, capacitor start and run induction motor, two value capacitor motor and true "two phase" induction motor.

Figure 1 illustrates a first embodiment of a controller for single ortwo phase induction motors. The circuit maintains one ofthewindings 10 ofthe motor as a reference phase and uses the other winding 12 as a variable phase. The reference phase 10 and variable phase 12 are connected to a power control unit 14 which provideseithera constantorvariablevoltage supply. Two silicon controlled rectifiers 16 and 18 are associated with the reference phase 10. Similarly, two silicon controlled rectifiers 20 and 22 are associated with the variable phase 12.

The rectifiers 16, 18,20 and 22 are of the gate turn off type. Rectifiers 16 and 18 are connected in series across the '+' and '-' d.c. supply lines provided by the power control unit 14. Also in series with each other across the d.c. supply are rectifiers 20 and 22 which are therefore in parallel with rectifiers 16 and 18. The reference phase 10 is connected between the power control unit 14 and the junction of recitifiers 16 and 18.

Likewise, variable phase 12 is connected between the power control unit 14 and the junction of rectifiers 20 and 22.

A respective snubber circuit 24 is connected in parallel with each of the rectifiers 16, 18,20 and 22. The gate of each ofthe rectifiers 16,18,20 and 22 is connected to a common digital control and pulse generator 26 by respective drive circuits 28. The digital control and pulse generator 26 receives input from a digital and analogue signal processing and phase control module 30 which derives its power supply and signal input from the power control unit 14. The module 30 receives inputfrom a manual control 32 or from an external orfeedback input 34. The variable phase 12 is controlled so as to have the desired phase relationship with the reference phase 10.The phase relationship ofthe variable phase 12 with respect two the reference phase 10 is controlled electronically.

A graph showing the voltage across the reference phase 10 (phase 1) for one cycle is included in Figure 1.

Agraph showing the voltage across thevariable phase 12 (phase 2) during the same cycle is also included in Figure 1. These graphs show a phase relationship, which is about 90 , when the motor has obtained normal operating conditions. The phase relationship can bevaried and the square waveform for both phases in illustrated.

Thetorqueofthemotoriscontrolled byproducinga phase slip between the voltages across the two windings and this provides particularly smooth operation, especially at slow speeds. The speed of the motor is controlled byvarying the applied frequency and/or by employing feedback with control to adjust the available torque as described.

Figure 2 illustrates an alternativecircuitarrange- mentwhich is suitable for providing a variable torque control, which with feedback may be used to control the speed ofthe output shaft. This circuit uses a mains derived reference phase. The variable phase 12 is connected in the same fashion as the Figure 1 arrangement. Gate turn off rectifiers 16 and 18 together with their snubber circuits 24 and drive circuits28 are connected as in the first embodiment.

The Figure 2 circuit differs in that the reference phase 10 is connected across the mains inputto the power control unit 14 as it is not accompanied by an associated circuitry such as the Figure 1 rectifiers 20 and 22. Consequently, the variable phase 12 is controlled with a fixed frequency which is synchronised with the mains input. Figure 2 includes graphs showing the voltage across the phases. Again a 90" phase relationship is shown although the value may be varied. The phase 1 voltage, reference phase, is sinusoidal whereas the phase 2 voltage, variable phase, has a square waveform.

In both the Figure 1 and Figure 2 arrangements the respective supply may be derived from a multiphase system. In the arrangement shown in Figure 1 the single phase supply of 240 volts may be replaced by a supply of a different voltage and/orfrequency. Similarly in the Figure 2 arrangementthe single phase supply may be replaced by a supply of a different voltage and frequency.

Sincethetorqueversesspeed relationshipatstart up is differentto that required atthe normal running speeds either a static speed monitor or a centrifugal switch would conventionally be used to switch the value ofthe capacitors, thewindings themselves, or other passive devices, to effect a change in the phase relationship between the two windings.

The described embodiments permit two phase induction motors used on single phase supplies, that is single phase induction motors, to be manufactured without the centrifugal switch or speed monitor. The use of capacitors which are usuallyfittedtotheframe or case ofthe motor is eliminated. These features enable morecompactand less expensive motorsto be manufactured.

In both of the illustrated circuits the voltage applied to either the reference phase 10 and/or the variable phase 12 may be controlled in order to limitthe current passing through the winding orwindings,thereby maintaining such currents within the required levels.

Alternatively, Pulse Width Modulation may be employed so that switching ofthe GateTurn-offthyristors achieve the same result.

The advent of the gate turn-offthyristor facilitates implementation of the present invention and the use of such device is shown in Figures 1 and 2. However, the use of devices otherthan gate turn-offthyristors for switching the phases as shown in Figures 1 and 2 is not excluded from the scope ofthis invention. Gate turn-off thyristors will produce phase voltages and currents which are generally rectangular in waveform and these waveforms may or may not be modified using active or passive components or circuitry.

An electronic means of controlling the speed and/or torque of single or two phase induction motors is achieved. One of the resultant advantages is that single ortwo phase motors may be easily speed and torque controlled using either a variable frequency electronically generated variable or constant phase supplywith fixed or variable voltage output or a mains derived fixed frequency supply with one phase electronicallyvariableordifferentin its phase relationshiptothe reference phase.

The components 14,24,26,28 and 30 of Figures 1 and 2 are in themselves of conventional structure as will be appreciated by those skilled in the art.

The circuit of Figure 1 can be used to control a two phase electric motorso as to provide reciprocating angular displacement ofthe output shaft of the motor. A similar circuit may be employed forthree phase motors.

The inputs 32 and 34to the control module 30 will normally both be manual controls one controlling amplitude and the other controlling frequency ofthe displacement.

The amplitude and frequency ofthe displacement are thus controllable and the displacement can be used, directly or indirectly, to provide controlled mechanical vibration or oscillation. Such vibration may be employed, for example, to power a vibratory conveyorfor transporting certain types of products.

When used to provide this reciprocating angular displacement, the voltages applied to the phases 10 and 12 are as indicated in Figure 3.

The reference orfirst phase and thevariable or second phase are both connected to a variable frequency or a constantfrequency electronically generated supply. The phase relationship ofthe second phasetothefirst phase is electronically controlled.

The voltage applied to the first and second phase is shown to be adjustable in orderto limit the current flowing through thewindings and to allow the amplitude ofthe reciprocating angular displacemenu to be adjusted.

By controlling the phase relationship between the two windings, as shown, the shaft, leftfree to rotate, would tend to rotate. This is due to the phasing pattern and may be used to assist the flow of material along a vibratory conveyor directly or indirectly coupled to the motor.

The phases, voltages and currents so produced within the motorwindings are generally rectangular - in waveform but these may or may not be modified using active or passive components or circuitry using pulse width modulation circuitry.

In the case of a two phase induction motorthe capacitor or capacitors usually required during starting and running, often in combination with other components, can be eliminated.

Conventionally, mechanical vibratory conveyors are operated by establishing a mechanical oscillation.

This requires the structure associated with the conveyorto be particularly robust in order two withstand start up conditions. This invention enables the requiredfrequencyto be established with a controlled increase in amplitude ofvibration.

The illustrated circuits can be used to provide a backtension brake which may have applications, for example, in the textile industry.

As shown in Figure 3, the voltage across the variable phase is made to first lead then lag the voltage across the reference phase. However, the markto space ratio ofthevariable phase is smaller than that of the reference phase. twill be appreciated thatthe dwell time ofthevariable phase may be used to assistthe direction of movement of products on a conveyor directly driven by the controlled motor.

Claims (12)

1. A controller comprising means for providing constantorvariable phase relationships between the electrical inputto each ofthetwowindings of a single ortwo phase induction motor so as to enable a constant orvariablefrequency and a constant or variable voltage supply to be applied to the motorfor thereby controlling at least one of the parameters of speed and torque of the motor.

2. Acontroller as claimed in claim 1, wherein both the parameters of speed and torque are controllable by the controller.

3. Acontrollerwhich provides an inputforatwo ooth rev phase electric motor, the input having phase relationship and current characteristics which result in reciprocating angular displacement of the output shaftofthe motor, the controllercontrolling the amplitudeand/orfrequencyofthesaid displacement.

4. Acontrolleras claimed in any preceding claim, wherein the controller uses one phase ofthe motor as a reference phase and another phase of the motor as a variable phase, each phase having a respective pair of silicon control led rectifiers provided by the controller, each pair of rectifiers being connected across a D.C. supply ofthe controller with the respective motor phase being connected to the junction between its pair of rectifiers.

5. A controller as claimed in claim 4, wherein the gate of each rectifier is connected to a common pulse generator.

6. A controller as claimed in claim 4 or 5, wherein each silicon controlled rectifier has a respective snubbercircuitin parallel therewith.

7. A controller as claimed in any of claims 4,5 or 6, as dependent upon claim 1, wherein the voltages across the reference and variable phases have a 90C phase difference.

8. A controller as claimed in any of claims 4,5 or6, as dependent upon claim 3, wherein the voltages across the variable phase is in phase with the voltage across the reference phase but which has a mark to space ratio smallerthan that ofthe reference phase.

9. A controller as claimed in claim 1 or 2, wherein the controller uses one ofthe phases of the motoras a reference phase and a different phase of the motor as a variable phase, the variable phase having a respective pair of silicon controlled rectifiers provided by the controller which rectifer connected across a D.C. supply ofthe controllerwith the variable phase being connected to the junction between the rectifiers, the reference phase being connected across the input to the controller.

10. A controller as claimed in claim 9, wherein each ofthe rectifiers has a snubber circuit connected in parallel therewith.

11. Acontrollerforcontrolling the speed and/or torque of a motor, substantially as hereinbefore described with reference to and as illustrated in Figure 1 or 2 ofthe accompanying drawings.

12. Acontrollerforproducing reciprocatingangu- lardisplacementofthe output shaft of an electric motor, substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 3 ofthe accompanying drawings.