WO1989001260A1

WO1989001260A1 - Switched reluctance motor control system

Info

Publication number: WO1989001260A1
Application number: PCT/GB1988/000603
Authority: WO
Inventors: Timothy John Eastham Miller
Original assignee: The University Court Of The University Of Glasgow
Priority date: 1987-07-23
Filing date: 1988-07-22
Publication date: 1989-02-09
Also published as: GB8717441D0

Abstract

A switched reluctance motor is controlled by a control system (10) which has only a single chopper switch (12) for all of the phaselegs (A, B, C) which are connected in parallel with a freewheeling diode (19). Each phaseleg (A, B, C) has terminals (13, 14) for connection to a respective motor winding and a commutator switch (15). The various commutator switches (15) are operated sequentially by an arrangement (21) so that the d.c. power supply V is switched sequentially from one phaseleg to another. Recovery of enegy stored in each phaseleg (A, B, C) at the end of its conduction period is achieved without the requirement of an energy storage capacitor by an energy recovery diode (18) connected between terminal (14) and the d.c. supply V.

Description

SWITCHED RELUCTANCE MOTOR CONTROL SYSTEM

This invention relates to a switched reluctance motor control system.

Switched reluctance motors are finding increased usage because they are brushless having stator windings fed from a d.c. supply under the control of a control circuit. A major element of cost in such a system is the power semi-conductor switches and associated diodes which are required. Various forms of control system for switched reluctance motor control systems are already known but these have a variety of disadvantages and it is an object of the present invention to provide a new and improved form of switched reluctance motor control system.

According to the present invention a switched reluctance motor control system comprises a d.c. supply and a phase winding control circuit, said circuit comprising a single chopper switch connected between one of the terminals of the d.c. supply and one end of each of a plurality of parallel-connected phaselegs, each phaseleg comprising a pair of connectors for connection to a respective switched-reluctance-motor stator phase winding and connected in series with a commutator switch, an energy recovery diode being connected between the series junction connector of each phaseleg and said one d.c. supply terminal, the other d.c. supply terminal being connected to the other end of each phaseleg, a freewheeling diode connected in parallel with the plurality of phase¬ legs, means for controlling on/off operation of the chopper switch, and means for operating the commutator switches to switch the d.c. supply sequentially from one phaseleg to another.

By virtue of the present invention a switched reluctance motor having N number of phases is controlled by a control circuit having N + 1 number of switches and recovery of energy stored in each phase leg winding at the end of its conduction period is achieved without the requirement for an energy storage capacitor.

The chopper switch may be any form of semi-conductor power switch such as a bipolar junction transistor, a field effect transistor or a gate turn-off thyristor. The commutator switches may also be a semi-conductor power switch or may be an electro-mechanical switch.

The means for controlling on/off operation of the chopper switch comprises a current sensor for sensing phaseleg current and a comparator having a reference input the output of which comparator operates the chopper switch in order to effect slow speed phaseleg current level control. A high speed lock-out signal is provided to the comparator to hold the chopper switch off for a predetermined time interval during high speed running of the motor to permit complete de-energisation of the phaseleg winding by the back emf produced by that winding at the end of the conduction period and before conduction is initiated on another phaseleg. The circuit arrangement enables the chopper switch to provide for zero voltage chopping rather than reverse voltage chopping since the d.c. supply is effectively disconnected from the phaseleg winding during the off period of the chopping so that the d.c. link ripple current is relatively small and can be accommodated by a relatively low value capacitance.

The means for operating the commutator switches is preferably by means of a shaft position sensor comprising a slotted disc secured to the rotor of the motor and effecting interruption between an optical transmitter and receiver for each phase winding, the angular extent of each slot determining the conduction period for each phaseleg. Under certain circumstances the means for operating the commutator switches can be generated electronically without utilising a shaft position sensor (as is known per se) .

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawing, in which: Fig. 1 illustrates a first embodiment;

Fig. 2 is a waveform diagram illustrative of the operation of the Fig. 1 embodiment;

Fig. 3 is a second embodiment; and

Fig. 4 is a modified version of_. the Fig. 1 embodiment. A switched reluctance motor control system 10 illustrated in Fig. 1 comprises a d.c. supply V having a positive terminal 11 connected to one side of a single chopper switch 12 the other side of which is connected to three parallel connected phaselegs A, B, C, for a three- phase motor. The phaselegs are identical to each other and therefore only phaseleg A will be described.

Phaseleg A comprises a terminal 13 and a terminal 14 which are arranged for connection to a monofilar stator phase winding (shown connected in Fig. 1) and -n series with these connectors 13, 14, is a commutator switch 15 which at its other end is connected to the negative terminal 16 of the supply V via a current sensing resistor 17 which is common in this embodiment to each phaseleg. The series junction of the phaseleg formed by the connector 14 is connected to terminal 11 via an energy recovery diode 18 the anode of which is connected to connector 14. In parallel with the phaselegs A, B, C, is a freewheeling diode 19 the cathode of which is connected to terminal 13. Chopper switch 12 is controlled in on/off mode to provide pulse width modulation of the current in phaseleg A and sequentially in phaselegs B and C by means of a comparator 20 having one input connected to a reference voltage and the other input connected to current sensing resistor 17 which operates when switch 12 is on to turn switch 12 off. Re-establishment of switch 12 in the on condition is effected by a turn on device which may operate after a fixed time delay forming part of comparator 20 or otherwise controlling its output.

Commutator switch 15 is operated by a control means 21 shown schematically and which according to the relative position of the rotor and stator poles of the motor sequentially switches the various commutator switches 15 so that the d.c. supply V is sequentially switched from one phaseleg to another. Means 21 also provides a high speed lock-out signal on line 22 to the comparator 20 to hold the comparator at high motor speeds in a condition which holds chopper switch 12 in the off position at the end of the conduction period for phaseleg A for a pre¬ determined time interval sufficient to permit the current in phaseleg A to reduce to zero prior to chopper switch

12 being closed to permit current to flow in another phase- leg, say, B.

In normal operation of the Fig. 1 embodiment commutato switch 15 is switched on (i.e. the switch is closed) for the entire conduction period and when chopper switch 12 is on (i.e. closed) d.c. current flows from the supply V and builds up in the phase winding, being monitored in level by resistor 17. When chopper switch 12 is off (i.e. the switch is open) arising from operation of comparator 20, th phase winding current freewheels through the commutating switch 15 and the freewheeling diode 19 without application of the d.c. supply in reverse polarity to the phaseleg winding.

At the end of the conduction period of the phaseleg as determined by means 21 the commutator switch 15 is switched off (i.e. open) and the energy in the phaseleg winding is recovered by the winding current freewheeling through diode 18 and either through the supply V if chopper switch 12 is off or through chopper switch 12 if this is still on. In each case current flow decays quickly to zero because of the absence of forward voltage applied to the phaseleg.

Fig. 2 illustrates the phase current in a represent¬ ative phaseleg during its conduction period X and for slow speed motor running so that current at the beginning of the conduction period rapidly builds up from zero to a normal operating level when chopper switch 12 is turned off and thereafter due to slow running decays slightly. Chopper switch 12 is turned on which causes the current level to increase again following which the chopper switch is turned off and this operation of chopper switch 12 is repeated throughout the conduction period X.

Fig. 3 illustrates a second embodiment in which the commutating switches 15 are referenced to the positive terminal 11 of the d.c. supply V whilst the chopper switch 12 is referenced to the negative terminal 16.

Operation of this embodiment is the same as that for the Fig. 1 embodiment but the diodes 18, 19, are reverse poled and the comparator (not shown) requires to have its output signal level-shifted to operate chopper switch 12. Fig. 4 illustrates a modified version of the Fig. 1 embodiment in which the common current sensing resistor 17 of Fig. 1 is replaced by individual phaseleg current sensors 23 each with an associated diode 24 for delivering the sensed signals to the comparator 20. It will be appreciated that the embodiments which have been described enable recovery of energy at the end of each conduction period X to be effected without use of a storage capacitor by means of a circuit having only one switch more than the number of motor phases and having only one power diode more than the number of phases. The switched operation of chopper switch 12 effects a ripple on each phaseleg current and whilst this is not important for many forms of d.c. power supply V it is advantageous to provide the power supply with a smoothing capacitor 25 as shown in Fig. 3. Such a capacitor however need only be of small capacitance value since it is not used for energy recovery purposes. The circuit is compact due to the relatively small number of components and their mutual proximity so that with care it can be built to provide very low energy radiation, i.e. be very low in electro-magnetic noise radiation.

Claims

A CLAIMS

v^* 1. A switched reluctance motor control system comprising a d.c. supply and a phase winding control circuit, characterised in that said circuit comprises a single chopper switch (12) connected between one (11) of the

5 terminals of the d.c. supply V and one end of each of a plurality of parallel-connected phaselegs (A,B,C,), each phaseleg (A,B,C) comprising a pair of connectors (13, 14) for connection to a respective switched-reluctance-motor stator phase winding and connected in series with a

10 commutator switch (15) , an energy recovery diode (18) being connected between the series junction connector (14) of each phaseleg and said one d.c. supply terminal (11) , the other d.c. supply terminal (16) being connected to the other end of each phaseleg (A,B,C) , a freewheeling

15 diode (19) connected in parallel with the plurality of phaselegs (A,B,C) , means (20) for controlling on/off operation of the chopper switch (12) , and means (21) for operating the commutator switches (15) to switch the d.c. supply V sequentially from one phaseleg (A,B,C) to another.

2. A control system as claimed in claim 1, characterised in that the means (20) for controlling on/off operation of the chopper switch (12) comprises a current sensor (17) for sensing phaseleg current and a comparator having a

5 reference input the output of which comparator operates the chopper switch (12) in order to effect slow speed phaseleg current level control.

3. A control system as claimed in claim 2, characterised in that said current sensor (17) is common to each phaseleg (A,B,C) .

4. A control system as claimed in claim 2, characterised in that each phaseleg (A,B,C) includes a current sensor (23) and a diode (24) for delivering sensed signals to the comparator.

5. A control system as claimed in any one of claims 2-4, characterised in that a high speed lock-out signal (22) is provided to the comparator to hold the chopper switch (12) off for a predetermined time interval during high speed running of the motor to permit complete de- energisation of the phaseleg winding by the back emf produced by that winding at the end of the conduction period and before conduction is initiated on another phaseleg.

6. A control system as claimed in claim 5, characterised in that the high speed lock out signal (22) is established by the means (21) for operating the commutator switch (15) .

7. A control system as claimed in any preceding claim, characterised in that the chopper switch (12) is disposed between the positive terminal (11) of the d.c. supply V and the phaselegs (A,B,C) and the cathode of said energy recovery diode (18) is connected to said terminal (11) .

8. A control system as claimed in any one of claims 1-6, characterised in that the chopper switch (12) is disposed between the negative terminal (16) of the d.c. supply V and the phaselegs (A,B,C) and the anode of said energy recovery diode (18) is connected to said terminal (16) .