GB2152774A

GB2152774A - Controller for induction motors

Info

Publication number: GB2152774A
Application number: GB08333058A
Authority: GB
Inventors: Reginald Hartill; Donald Hyde; Jack Mills
Original assignee: Pikrose & Co Ltd
Current assignee: Pikrose & Co Ltd
Priority date: 1983-12-12
Filing date: 1983-12-12
Publication date: 1985-08-07
Also published as: GB8333058D0

Abstract

A controller for an induction motor employs transductors 2 to 5 or phase shifters, to effect control of the motor by varying the phase relationship between two of the supply phases to the motor 1. <IMAGE>

Description

SPECIFICATION Improvements in or relating to controllers for induction motors The invention relates to controllers suitable for variable speed control of induction motors, particularly induction motors used for hauling loads, that is, mechanical haulages.

Induction motors having ratings of tens of kilowatts are used in minesandtheliketopowerwinchesfor hauling wagons on rails laid on undulating surfaces. It is desirable to achieve a controlled start and to maintain effective speed control in such circumstances by control ofthe motortorque. Existing systems for achieving a controlled start and effective speed control of haulage motors are complex and expensive.

The present invention uses a different approach from known systems to provide a solution to the problem of controlling a haulage motor.

The present invention provides means capable of controlling a three-phase induction motor by varying the phase relationship between two ofthethree supply phases to the motor.

In one advantageous arrangement, the said control means includes first and second transductors arranged for connection in series with a first and second phase, respectively, of a three phase supplyto an induction motor, a third transductor connected between the input port ofthefirsttransductor and the output port ofthe second transductor, a fourth transductor connected between the input port of the second transductor and the output port ofthe first transductor, and a control unit capable of controlling the first and second transductors as a first pair and the third and fourth transductors as a second pair.

In controlling an induction motor to provide a positive torque, the first pair oftransductors are saturated progressively to achieve smooth forward acceleration of an induction motor, and conversely the second pair oftransductors are saturated progressive- lyto achieve smooth reverse acceleration of the motor.

In starting an induction motor with a negative torque (overhauling load), the fourtransductors are excited in a manner appropriate to providing a controlled retarding torque by phasevariation.

The control means can provide forward and reverse control with control of speed and controlled acceleration and deceleration, of an induction motor driving a winch, or similar device, which is capable of both hauling and lowering operations.

The control unit may be capable of receiving a signal indicative of the speed of a motor connected to the control means and of adjusting the saturation of each transductorasappropriateto maintain,forexample,a constant motor speed, controlled acceleration, or controlled deceleration ofthe motor.

More specifically, in one arrangement of the present invention, the said control means includes four transductors arranged as a four-arm bridge, and a control unit arranged to control, as pairs, the transductors on opposing arms of the bridge, a first corner and the opposing corner ofthe bridge, in operation, being connected to respective first and second phases of a three-phase electrical supply, and the two remaining corners ofthe bridge, in operation, being connected to two respective input phases of a three phase electrical machine,forexample, a motor.

Additionally, a fifth transductor may be included in the control means and arranged to be controlled by the control unit, the fifth transductor being, in operation, connected in series with the third phase of a three-phase electrical supply and the third input phase ofthe electrical machine referred to above.

In the operation of the four-arm bridge oftransduc- tors, the bridge acts, in effect, like a reversing switch according to which pair of opposed transductors is in the high-impedance state, thereby effecting the operation of a three-phase induction motor in either direction, and the extent of the saturation ofthe conductive pair of opposed transductors determines the speed of rotation. Where the fifth transductor is provided, its saturation is arranged to match the extent of the saturation of the conductive pair of opposed transductors.

Inorderto improvetheefficiencythefour-arm transductor bridge, referred to above, as a reversing switch, controllable switch means may be employed in series with each transductor of the bridge to facilitate improved isolation of the high-impedance transductor pair from the remaining transductors. In the operation of the four-arm transductor bridge with controllable switches, the controllable switches associated with the partly saturated pair of transductors would be switched on continuously, the actual control of voltage level being leftentirelytothe associated transductors. Alternatively, a reversing contactor, with a transductor in series with each of two phases of a three-phase supply, may be used, and the reversing contactor may be replaced by a four-arm bridge reversing switch.

In a further development of the use of a four-arm bridge as a reversing switch, the transductors may be dispensed with and both switching and voltage control effected by controllable switch means, especiallysemi-conductorswitching devices such as thyristors.

In anotheradvantageous arrangement, the said control means may include first and second controllable phase shifters arranged for connection in series with a first phase and a second phase, respectively, of a three phase supply to an induction motor, a controllable impedance (for example, a transductor) arranged for connection in series with the third phase of the three phase supply, and a control unit capable of controlling the phase shifters and the impedance.

In yet another advantageous arrangement, the control means may include delta or star connected controllable phase shift means capable of connection to the three supply ports of an induction motor, a controllable impedance (for example, a transductor) arranged for connection in series with one phase of the three phase supply, and a control unit capable of controlling the phase shifters and the impedance.

In operation ofthe control means including phase shifters, the direction ofthe motortorque is determined by the settings ofthe phase shifters and the amount of motor torque is determined bythe setting of the impedance (transductor).

In the arrangements including phase shifters, the control unit may be capable of receiving a signal indicative of the speed of a motor connected to the control means and of adjusting each phase shifter and the impedance as appropriate to maintain a variable motor speed, controlled acceleration, or controlled deceleration ofthe motor.

The present invention also provides a motorpowered winch or other haulage device including control means, according to the invention, controlling the motor.

Several motor speed controllers each constructed in accordance with the invention will now be described, byway of example only, with reference to the accompanying drawings, in which : Fig. lisa schematic representation of a first of the control means including fourtransductors and a control unit, connected to a three phase induction motor, Fig. 2 is a diagrammatic representation ofthe electrical connections between the motor and transductors of Fig. 1, Fig. 3 is a schematic representation of a second control means including two controllable phase shifters, a controllabletransductor, and a control unit, connected to a three phase induction motor, Fig. 4 is a schematic representation of a third control means including delta or star connected controllable phase shift means, a controllable transductor and a control unit, connected to a three phase induction motor, Fig. 5 is a diagrammatic representation of electrical connections between the motor, the transductor, and the phase shifters of Fig. 3, Fig. 6 is a diagrammatic representation of a fourth control means including three controllabletransductors, one transductor being located in each supply line of a three-phase induction motor, and a reversing contactorarrangementforreversing the supplies to two of the three supply lines of the motor, Fig. 7 is a diagramatic representation of a fifth control means including a combined reversing switch and voltage level control means provided by a thyristor and diodefour-arm bridge, Figs. 8a to 8dare vector diagram representations of various conditions possible with the control means of Figs. 3 and 4, and, Figs. 9a and 3b are vector diagram representations of conditions possible with the control means of Figs.

1 and 2.

Referring to Fig. 1, ofthe accompanying drawings, control means capable of controlling a three phase induction motor 1 includes a firsttransductor 2 connected in series with the red phase, say, of a three phase supply to the motor 1, a second transductor3 connected in the yellow phase, say, a third transductor 4connected between the input port of the second transductor3 and the output port ofthe firsttransductor 2, and a fourth transductor 5 connected between the input port of the firsttransductor 2 and the output port ofthe second transductor 3.

In Fig. 1, a control unit 6 is arranged to control the transductors 2,3,4, and 5, the control unit 6 in this case being directed by a manual controller7 and being capable of accepting a signal on a line8 indicative of the motor speed and of maintaining the motor at a set speed byway of the transductors 2,3,4 and 5. The speed reference signal may be taken, for example, from a winch driven by the motor. A contactor having contacts 9, 10 and 11 is shown also. Atransductor 18 is used to keep the motor 1 balanced electrically.

In the operation of the control means shown in Fig.

1, the motor is started by closing the contactorwith the fourtransductors 2,3,4 and 5 unsaturated and the motor is run up in the forward direction by increasing the saturation ofthe transductors 2 and 3 by way of the control unit 6. Alternatively, the motor is run up in the reverse direction by increasing the saturation of the transductors 4and 5, leaving thetransductors 2 and 3 unsaturated, byway of the control unit 6. A speed reference signal is applied to the control unit 6 which increases or decreases the excitation of each trans ductor pair, as appropriate, to maintain the demand speed.The control unit 6 provides variable speed by comparing the actual speed with the desired setting, the speed ramp controlling the rate of rise of speed to the set speed. Remote operation is included by means of a signal capable of indicating start, stop, and a controlled speed, forexample by radio.

When the motor 1 is used to power a winch which is lowering a load, the saturation of the appropriate transductor is so adjusted thatthe motor holds the load being lowered. The saturation is reduced to allow speed to increase, and, if the load condition demands, drive is provided by increasing the saturation ofthe othertwo transductors.

Referring to Fig. 3, the control meansforcontrolling a three phase induction motor 1 includes first and second controllable phase shifters 12 and 13 connected in series with the red and yellow phases, respectively, ofthe supply, a controllable transductor 14 connected in series with the blue phase ofthe supply, and a control unit 15 arranged to control the phase shifters 12 and 13 and the transductor 14. A reversing contactor 16 is included in the supply circuit to the motor.

In operation ofthe control means shown in Fig. 3, means are provided to shifttwo phases ofthe supply to the squirrel cage induction motor 1,thereby advancing one phase, and retarding a second phase in orderto change the phase sequence and provide a countertorque, the amount of torque being controlled by controlling the voltage atthe third supply phase.

The unbalanced three phase supply applied to the induction motor includes positive and negative sequ ence components which result in positive and nega tive rotating magnetic fields which are controllable.

The variation in the direction ofthe motortorque is determined by the phase change which can be accompiished bythe phase shifters 12 and 13 in conjunction with the reversing contactor 16.

The required speed torque characteristicforthe drive is provided with the aid of an amplified speed signal which is used to adjustthe phase shift means 12 and 13 andthetransductor 14. The amplified speed signal is provided by means of a speed signal generator 17.

Referring to Fig. 4, the phase shift means 19 is arranged to influence all phases ofthe motor supply and a transductor 18 is included in one phase ofthe motor supply.

In operation of the system shown in Figs. 3 and 4, the contactor 16 is closed, the saturation ofthe transductor 14 is increased, and the phase shift is reduced, to increasethe motor speed. The speed detector (part of the speed signal generator 17) compares the actual speed with a preset speed ramp and adjusts the phase shift and the saturation ofthe transductorto provide the required speed at a controlled rate. Variable speed is obtained by comparing the actual speed with a desired setting, the speed ramp controlling the rate of rise of speed to the set speed.

Referring to Fig. 5, a star-connected motor 1 is shown connected to phase shifters 12 and 13, and an adjustabletransductor 14 in the manner of the Fig. 3 arrangement.

Referring to Fig. 6, a fourth control means includes a reversing contactor 30 arranged to be capable of reversing the electrical supply to the red (R) and yellow (Y) phases of a three-phase electric motor 35 under the control of a control unit 34. The Blue (B) phase is not reversed. The red (R) phase has an in-series transductor31, ,the yellow (Y) phase has an in-series transductor 32, and the blue (B) phase has an in-seriestransductor33. Thetransductors 31,32, and 33 are controllable by means ofthe control unit 34 which provides a controllable d.c. excitation effective tovarythe inductance of eachtransductoratthe operating frequency ofthe system.The d.c. excitation of the control unit 34 is obtained by transforming, rectifying, and smoothing powerfrom the same electrical power source as is used by the motor 35.

In operation of the control means represented by Fig. 6, the contactor30 is operated as necessary by the control unit 34 to provide either positive or negative sequence drive to the motor according to whether positive or negative torque is required. Atthe same time the d.c. excitation of each ofthe transductors 31 and 32 is set by the control unit according to thetorque required by the operator to achieve a particular operating speed forthe motor 35. Also, the d.c.

excitation ofthe transductor 33 is set by the control unit 34to maintain a balanced th ree-phase supply to the motor 35.

Referring to Fig. 7, a fifth control means includes a four-arm bridge of thyristor and diode pairs, connected in parallel and cathode to anode, providing both supply reversal and variation underthe action of a control unit 44. A balancing thyristor and diode pair may be provided in the third supply phase to the motor.

It will be evidentto those skilled in the art that various combinations of contactors and transductors, ortransductors and controllable semiconductor switches, or contactors and controllable semiconductor switches, may be employed to provide both reversal and variation ofthe electrical supply to a three-phase induction machine as described above.

The control means described above is particularly useful in controlling a three-phase electric motor used for hauling rail cars along an undulating track as is found in mines, for example. In the case where a rail car is being drawn uphill, positive sequence drive is applied to the electric motor and thetorque set by reducing the saturation ofthetransductors (in the case wheretransductors are used) to achievethe desired rate of uphill movement of the rail car. When, subsequently, the rail car starts rolling downhill, negative sequence drive is applied to the torque set to achieve controlled braking of the rail car during its downhill run. The control means is also effective in avoiding snatch ofthe drive cable employed for hauling rail cars in mines.

In all cases described and illustrated, the control means can provide forward and reverse control, speed control (constant or accelerated/decelerated) of the motor and can, therefore, be applied usefully to the control of a winch or similar haulage device.

Figs. 8ato 8dillustrate in sequence the following conditions which are available forthe arrangement shown in Figs. 3 and 4: (i) The transductor 14 unsaturated (Fig. 8a) (ii)The transductor 14 saturated (Fig. 8b) (iii) Phase shift setting for no phase shift (Fig. 8c) (iv) Phase shift setting forsome phase shift (Fig. 8d) Figs. 9a and 9b illustrate for Figs. 1 and 2: (i) Transductors adjusted for positive torque (ii) Transductors adjusted for negativetorque.

The hauling speed in all the situations described above will be of the order of 5 mph.

Claims

1. A controller, for controlling an induction motor, including means, connectibletothe motorwindings, arranged to effect variation of the relationship be tween two of the supply phasesto the motor.

2. A controller, for controlling an induction motor, as claimed in Claim 1, wherein the means connectible to the motorwindings is arranged to convert a controllable proportion of the electrical supply into negative sequence energy.

3. Acontroller,forcontrolling an induction motor, as claimed in Claim 1 or Claim 2, including first and second transductors arranged for connection in series with first and second phases, respectively, of an electrical supply to the motor, a third transductor connected between the input portofthefirsttransduc- tor and the output port of the second transductor, a fourth transductor connected between the input port ofthe second transductor and the output port of the firsttransductor, and a control unit arranged to control the first and second transductors as a first pair and the third and fourth transductors as a second pair.

4. A controller, forcontrolling an induction motor, as claimed in Claim 1 or Claim 2, including four transductors arranged as a four-arm bridge and a control unit arranged to control, as pairs, thetransduc- tors on opposing arms ofthe bridge, a first corner and the opposing corner ofthe bridge, in operation, being connected to respective first and second phases of a three-phase electrical supply, and the two remaining corners ofthe bridge, in operation, being connected to two respective windings of a three phase motor.

5. Acontroller,forcontrolling an induction motor, as claimed in any one of Claims 1 to 4, including switch means arranged to permit change over ofthe connections between the motor windings and the means for effecting variation of the phase relationship ofthe motor supplies.

6. A controller, for controlling an induction motor, as claimed in Claim 5, wherein the switch means includes semi-conductor devices.

7. A controller, for controlling an induction motor, as claimed in Claim 1 or Claim 2, including first and second controllable phase shifters arrangedforconnection in series with first and second phases, respectively, of an electrical supply to the motor, a controllable impedence arranged for connection in series with a third phase ofthe electrical supply, and a control unit arranged to control the phase shifter and the impedance.

8. A controller, for controlling an induction motor, as claimed in Claim 7, wherein the phase shifters are arranged in a delta or star configuration and the impedance is arranged in series with one phase ofthe electrical supply to the motor

9. Acontroller, for controlling an induction motor, as claimed in any one of Claims 1 to 8, including input means for accepting a signal indicative of the speed of the controlled motorand arranged to adjustthe meansforeffecting variation ofthe relationship between two of the supply phases to the motor in accordance with the desired and actual motor speeds.

10. A controller, for controlling an induction motor, including means, connectible to the motor windings, arranged to effect variation ofthe relationship between two ofthe supply phases to the motor, substantially as herein described, with reference to, and as illustrated by any one of Figs. 1 to 7, of the accompanying drawings.

11. A controller, for an induction motor, including means,connectibletothe motorwindings,arranged to effect variation of the relationship between two of the supply phases to the motor, substantially as herein described, with reference to, and as illustrated by Figs.

1, 9a and 9b, or Figs. 2, 9a and 9b, ofthe accompanying drawings.

12. A controller, for an induction motor, including means, connectibletothe motorwindings, arranged to effect variation of the relationship between two of the supply phases to the motor, substantially as herein described, with reference to, and as illustrated by Figs.

3 and 8ato 8d, or Figs. 4 and 8ato 8d, ofthe accompanying drawings.

13. Amotor-powered haulage device including a controller as claimed in any one of Claims 1 to 12.