EP0429494A1 - Switched reluctance motors - Google Patents

Abstract

A switched reluctance motor control system (10), provided with phase windings (12) and switching members (21-23), is provided with a regulating device (15) which controls the switching members (21-23). In a first mode, the regulating device (15) controls the switching members (21-23) to determine in the respective windings (12) sequential conduction periods serving for the normal rotation of the system (10). In a second mode, the regulating device (15) controls the switching members (21-23) to determine in the respective windings (12) simultaneous conduction periods making it possible to brake the system (10) or to keep it at 'stop.

Description

SWITCHED RELUCTANCE MOTORS

This invention relates to switched reluctance motors and more particularly to a switched reluctance motor drive system.

Increasingly electric motors for use in motion control applications such as robotics or computer peripheral equipment are of a brushless design. Such motors use a ultipole stator wound with copper wire and fed by an electronic converter. With such motors a frequent control application requires the motor to be brought to a rapid and controlled stop from any speed within its range. Known braking systems employ either regenerative braking or a form of reverse drive wherein the stator winding current is commutated in antiphase to the rotor position. Such braking systems are relatively slow to bring the rotor to rest and once the -rotor has stopped there is no residual holding torque present to maintain the rotor in the stationary position without the addition of some form of external energisation.

It is an object of the present invention to provide a switched reluctance motor drive system having an improved braking system.

According to the present invention there is provided a switched reluctance motor drive system comprising a switched reluctance motor having a plurality of stator phase' windings, a d.c. supply and a motor drive circuit, said motor drive circuit having a phase winding control circuit interconnecting the stator phase windings and the d.c. supply and arranged to provide each of said phase windings with a conduction period, current control means connected to said phase windings and arranged to control the level of current traversing the phase windings during each said.^conduction period, and setting means arranged to operate slid phase winding control circuit, said setting means having a first condition wherein the phase winding control circuit provides the phase windings with sequentially occurring conduction periods and a second condition wherein the phase windings are provided with simultaneously occurring conduction p%riods.

By virtue of the present invention when the setting means is in the second condition the motor is held by a holding torque in a stationary position if previously the motor was at rest, and the motor is rapidly decelerated by a braking torque if previously the motor was rotating. Conditioning of the setting means as between its first and second conditions may be effected manually or automatically.

The current control means may comprise a semi-conductor switch connected in common to the plurality of phase windings and arranged to operate in a chopping mode by a control logic. Alternatively, the current control means may comprise a plurality of semi-conductor switches respectively connected to the plurality of phase windings each such switch being arranged to operate in a chopping mode by a control logic. The chopper switch or switches and the switches of the phase winding control circuit may be the same or different devices such as bipolar junction transistors, field effect transistors, gate turn-off thyristors or any other functional equivalent.

The system may also have energy recovery means for recovery of energy from the stator phase windings of said motor. The energy recovery means may comprise a storage capacitor in a current recirculation path defined by diodes.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:-

Fig. 1 is a block diagram of a switched reluctance motor drive system according to the present invention;

Fig. 2 illustrates a first form of part of the Fig. 1 system in circuit diagram form; Figs. 3-6 are typical current waveforms in a phase leg winding of the Fig. 1 system during use of the present invention;

Fig. 7 illustrates a second form of part of the Fig. 1 system in circuit diagram form;

Fig. 8 illustrates a part of the Fig. 7 circuit applied to an alternative form of the Fig. 1 system; Fig. 9 illustrates a modification of the Fig. 7 circuit; and

Fig. 10 illustrates a modification of the Fig. 8 circuit.

A switched reluctance motor drive system 10 shown schematically in Fig. 1 comprises a switched reluctance motor 11 having a plurality of stator phase windings shown generically at 12, a d.c. supply 13 and a motor drive circuit 14. The motor drive circuit 14 has a phase winding control circuit 16 interconnecting the stator phase windings 12 and the supply 13 so as to provide each of said windings with a conduction* period, and a setting means 15 to control the phase winding control circuit 16. The setting means 15 can either be in a first condition when the windings 12 conduct sequentially or in a second condition when the windings 12 conduct simultaneously.

Circuit 16 contains commutating switches connected in series with the respective windings 12 and when the setting means 15 is in its first condition these commutating switches are sequentially switched on and off by a slotted disc 9 mounted on the rotor shaft 8 and intercepting an emitter/receiver device the outputs of which are delivered by line 17 to the setting means 15. Accordingly in the first condition of the setting means 15 the motor 11 will maintain its previous rotation or accelerate from rest to achieve its normal rotational condition.

In accordance with the present invention when the setting means 15 is placed in its second condition all phase windings 12 conduct simultaneously, notwithstanding the signals (if any) delivered by line 17 and either a braking torque is rapidly applied to the motor 11 causing it to decelerate to rest (if the motor was previously rotating), or a holding torque is applied to the motor causing it to be held in its stationary position (if the motor was previously at rest) . It will be appreciated that line 17 need not have its inputs supplied by the emitter/receiver device and slotted disc arrangement described but may alternatively be supplied by an electronic timing device, as is known per se.

Fig. 2 illustrates a circuit 20, being part of the switched reluctance motor drive system shown in Fig. 1, used to apply various periods of conduction to the phase windings of a motor having three phaselegs and when desired, braking of the motor 11.

The circuit 20 comprises three stator phase windings 12A, 12B, 12C, a d.c. supply V+ (shown as 13 in Fig. 1), and the numerous components which form the motor drive control circuit 14. The motor drive control circuit 14 comprises three chopping transistors 29, 30, 31, which are respectively connected in series with the phase windings 12A, 12B, 12C, and which form current control means 28, commutation buffer amplifiers 25, 26, 27, which form part of the setting means 15 and which are connected to commutation transistors 21, 22, 23, forming part of the phase winding control circuit 16. The inputs to the buffer amplifiers 25, 26, 27, are provided by the feedback loop 17 shown in Fig. 1 either directly or via suitable combinatorial logic. The setting means 15 is conditioned by a stop switch 24 connected to the inputs of the amplifiers 25, 26, 27, and three pull-up resistors 25A, 26A, 27A, connected in parallel with the outputs of the buffer amplifiers 25, 26, 27. Phase windings 12A, 12B, 12C, are connected between the commutation transistors 21, 22, 23, and the chopping transistors 29, 30, 31. Amplifiers 25, 26, 27, may be bipolar or field effect transistors or logical gate or any other functional equivalent.

A control logic 32 drives the chopping transistors 29, 30, 31, with a high frequency chopping signal the duty cycle of which can be varied for speed and torque control.

Energy recovery means is provided by a storage capacitor 33 and various current recirculation or free-wheeling paths are defined by diode pairs 34A, 35A; 34B, 35B; 34C, 35C.

Braking is effected by simultaneously switching on all three commutation transistors 21, 22, 23, and this is achieved by closing the stop switch 24, which applies zero bias to the positive input of the (differential) commutation buffer amplifiers 25, 26, 27. Due to the action of the pull-up resistors 25A, 26A, 27A, the closing of the switch 24 causes all commutation signals 17B to go high, thereby switching on all the commutation transistors 21, 22, 23, in the phas-e winding control circuit 16.

The chopping transistors 29, 30, 31 are used to control the level of current traversing the phase windings 12A, 12B, 12C, at all times, the frequency and duty cycle of switching being determined by the electronic control logic 32. Accordingly torque is controlled during normal running, during braking, and following braking when the motor is at a standstill.

Figs. 3 to 6 show different current waveforms in a representative phase winding for four different motor running speeds in the time interval N prior to braking, T being braking time during which braking torque is applied and R being the time interval during which holding torque is applied and the rotor is at rest.

Fig. 3 is the waveform for a motor initially running at 600 rpm and with the chopper transistor operating at a duty cycle of 30%; Fig. 4 is for a motor initially running at 1200 rpm and with the chopper transistor operating at 40% duty cycle. Fig. 5 is for a motor initially running at 2000 rpm and with the chopper transistor operating at 60% duty cycle; and Fig. 6 is for a motor running initially at

3000 rpm and with the chopper switch operating at 70% duty cycle.

As can been seen from the waveforms when the motor is running in the time interval N the phase conducts in steps or pulses of phase current and the pertaining chopper transistor 29, 30, 31, is driven with a fixed high frequency and with a pulse width (or duty cycle) which is varied to control the motor speed and hence rotational energy.

The braking effect provided in accordance with the present invention during the time interval T may be understood as follows. Considering one phase in isolation, if the rotor rotates while the commutation transistor for that phase is kept continuously in the ON state, the motor current will be a series of unidirectional pulses and the torque will be oscillatory with positive excursions when a pair of rotor poles is approaching alignment with the excited stator phase, and negative excursions when the pair of rotor poles is passing away from the aligned position. If there were no resistance or other dissipative elements in the circuit, and with a purely inertial load, the integral of the positive torque excursions with respect to rotor position, that is, the work done, would be equal to the energy returned by the motor to the controller during the negative torque excursions; the average torque would be zero, but since, as must be the case, there is resistance and other dissipative mechanisms in the circuit, then average power will be dissipated at the expense of both the supply energy and the motor's kinetic energy. Since all three phases are equally supplied with voltage continuously, the stator poles exert no influence on the rotor that would cause it to rotate continuously; they merely exert a static influence that tends to capture each passing rotor pole. Continued rotation is therefore at the expense of the rotor's own kinetic energy, which is rapidly dissipated in the transitor and diode circuits, their respective resistances, and the winding resistances themselves.

The rate of dissipation of energy increases as the pulsating current increases. The peak current during each pulse depends on the duty cycle of the chopper transistor. For high initial speeds, the duty cycle will have been set by the control logic 32 at a high value because the speed is approximately proportional to the voltage applied to the motor windings. Therefore during braking from a higher speed, the pulsating current will be higher so that the rate of dissipation of the rotor energy will be higher. The result of this is that the increased duty cycle used to get higher speeds creates in the braking condition higher pulse currents and more rapid dissipation of kinetic energy, thereby compensating for the additional speed and tending to produce a constant deceleration time for all initial speeds. Figs. 3-6 demonstrate this effect, which is an important asset for many applications (particularly for example in pump dosing or where rapid stop may be required for safety reasons).

Since the motor can produce negative torque only during a period of decreasing inductance (poles separating), and can only produce positive torque during a period of increasing inductance (poles approaching), it must be the case during deceleration that the current peaks during braking far exceed the current peaks arising during normal running, and this is evident on close examination of Figs. 3-6. It may therefore be desirable to reduce the duty cycle just prior to entering the braking condition by providing a signal from the setting means 15 to the control logic 32, so as to limit the peak currents that flow in the transistors and diodes during the braking condition even although this will have the effect of lengthening the deceleration time.

In the braking condition during each current pulse the freewheeling diode 34A conducts intermittently from an early part of the pulse and this may be exploited to optimise the dynamic braking torque by controlling the resistance in the freewheeling diode circuit during the deceleration period. An example of a circuit that performs this function is shown in Fig. 7 where transistor 40 is normally in the fully conducting state during motoring operation but is switched off during braking so as to insert the braking resistor 44 in series with the freewheeling diodes. The resistor 44 is chosen to optimize the dynamic braking torque by maximizing the rate of dissipation of kinetic energy. Transistor 40 may be switched off by the same signal that activates the braking mode in the commutation transistors via switch 24. It may alternatively be a field-effect device that is controlled by a predetermined biasing circuit to enter a high-resistance state during braking, eliminating the parallel braking resistor 44.

Fig. 8 illustrates the transistor 40 and braking resistor 44 applied to a phase winding control circuit having only four switches for a three-phase motor. It will of course be clear that if the commutating transistors are referenced to the upper supply rail instead of to the lower supply rail the chopping transistor 29 is referenced to the lower supply rail instead of to the upper supply rail. With this arrangement the motor may have any number of phaselegs.

In Fig, 9 is shown an alternative dynamic braking arrangement using auxiliary diodes 47, 48 and 49 and^a different connection of the braking control transistor 40. During motoring operation transistor 40 is in the OFF state but in the braking mode, transistor 40 is switched ON, so that phase commutation transistors 21, 22 and 23 are bypassed. The pulsating currents in all three phases flow through their respective auxiliary diodes and through the braking resistor 45 and control transistor 40. This circuit relieves the main commutating transistors of the onerous braking duty and associated high peak currents, and passes this function to the transistor 40, which may equally will be a mechanical switch or relay contact for low cost and reliability.

Fig. 10 shows the Fig. 9 dynamic braking arrangement applied to a circuit having only four switches for a three-phase motor. As in the case of Fig. 8, the dynamic braking arrangement has no effect during normal motoring operation.

It will be appreciated that in any of the embodiments the braking time T may be preset to any required value by changing the duty cycle of the chopping frequency at or during braking and to conserve power, after the rotor has come to rest the duty cycle of the chopping switch can be reduced to a value which optimises the input power versus holding torque relationship.

Claims

1. A switched reluctance motor drive system comprising a switched reluctance motor (11) having a plurality of stator phase windings (12), a d.c. supply (13) and a motor drive circuit (14), said motor drive circuit (14) having a phase winding control circuit (16) interconnecting the stator phase windings (12) and the d.c. supply (13) and arranged to provide each of said phase windings (12) with a conduction period, current control means (28) connected to said phase windings (12) and arranged to control the level of current traversing the phase windings (12) during each said conduction period, and setting means (15) arranged to operate said phase winding control circuit (16), said setting means (15) having a first condition wherein the phase winding control circuit (16) provides the phase windings (12) with sequentially occurring conduction periods and a second condition wherein the phase windings (12) are provided with simultaneously occurring conduction periods.

2. A system as claimed in claim 1, wherein the phase winding control circuit (16) comprises one commutation switch (21, 22, 23) in each phaseleg and the setting means (15) comprises, a stop switch (24) which in its closed condition causes said commutation switches (21, 22, 23) to be simultaneously conductive.

3. A system as claimed in either preceding claim, wherein the motor drive circuit (14) includes a dynamic braking resistor (44) which is rendered operational when said setting means (15) is in said second condition and is by-passed when said setting means (15) is in said first condition.

4. A system as claimed in any preceding claim, wherein the current control means (28) comprises chopper switch means (29-31) for said phase windings (12), said chopper switch means (29-31) being driven by a control logic circuit (32) and a connection is provided between said logic circuit (32) and said setting means (15) to enable reduction of the duty cycle of said chopper switch means (29-31) immediately prior to said setting means (15) entering its said second condition.

5. A method of operating a switched reluctance motor drive system comprises sequentially enabling conduction of the respective phase windings to achieve normal running of the system and simultaneously enabling conduction of the respective phase windings to achieve braking of the system.