ELECTRIC POWER CONVERTER CIRCUIT

Abstract: An electric power converter circuit for a variable speed switched reluctance motor having a main winding (A, B, C, or D), for each phase and a main thyristor (TA, TB, TC, or TD) connected in series with each main winding across the supply. Commutating means for all or a number of the main thyristors includes a common transistor (22, 23 or 24).

Description: ELECTRIC POWER CONVERTER CIRCUIT This invention relates to electric power converter circuits for variable-speed,switched, reluctance motors. Such motors, and controlled power converter .. 5. • circuits for them, are known, and reference may be had to the papers in I.E.E. Proceedings, Volume 127, Part 'B' No. 4 July, 1980, between pages 253 and 265 entitled "Variable Speed Switched Reluctance Motors", and I.E.E. Proc. Vol. 128 Part 'B* , No. 2 10. March, 1981 entitled "Inverter Drive for Switched Reluctance Motor; Circuits and Component Ratings" for a summary of what may be the state of the art at the date of this application. The second of those papers is by three Authors, two of which are the 15. Inventors in the present application. Such motors and converters involve the use of phase windings on the stator poles, which are connected in series with thyristors across a d.v. supply, with the thyristors being switched on in 20. synchronism with the movement of the rotor in relation to the stator to provide driving torque. The thyristors are switched off, or commutated, at appropriate instants during rotor rotation, and the object of the present invention is to provide 25. an improved means for commutating the thyristors. According to the present invention, an electric power converter circuit for supplying from a d.v. supply .w ic may ;be split into two portions in series -unidirectional " pulsating currents to each main 30 winding of--a variable reluctance motor having a main winding for one phase, or each of at least two phases, inc3.udes at least one main thyristor connected in series with each main winding in a circuit across the supply thro gh which the motor can receive energy from the supply,or a.portion of it, 5. means, e.g. for each main winding a circuit including that main winding*" and at least one diode, whereby each main winding can return enery to the d.v. supply, or a portion of it, -triggering-means for switching on each main thyristor in synchronism with the relative movement 10. of the motor, and commutating means including a transistor common to at least two of the thyristors for switching those thyristors off. The means whereby a main winding can return energy to the d.v. supply may include an auxiliary 15. winding closely coupled to the/or each main winding, and an auxiliary diode connected in series with the auxiliary winding in a circuit across the supply, or may-where the supply is split into two portions in series with a main winding arranged to receive energy from 20. one portion of the supply - include a circuit containing the other portion of the supply, the main winding and one or more diodes. Although the invention may have application to a single phase motor with a thyristor at each end of 25. the single main winding in series with the supply, the two thyristors being arranged to be commutated by the common transistor, the invention has particular application to a motor having pairs of phases each in series with a thyristor, there being two winding and thyristor 30. combinations connected in series across the supply for each pair of phases. Then a transistor could be arranged to commutate all the thyristors connected between one side of the supply and half of the phase windings, while another transistor could be arranged 5. to commutate the other thyristors. It is also possible by providing a pair of additional thyristors, one or other of which is switched on when a single transistor is switched on, to have only a single transistor for commutating all the thyristors. 10. The invention is also applicable to a motor with bi-filar phase windings, each main winding being closely coupled to an auxiliary winding connected in series with a diode across the supply. There will then be only one main thyristor for each main winding and when that 15. is commutated, current is transferred to the auxiliary winding and can return energy to the supply. The common transistor can .be used for commutating any or all of the main winding thyristors. The invention is also applicable to a three 20. phase motor in which each winding has two main thyristors (top and bottom thyristors) ,one connected between one end of the winding and the corresponding side of the supply. Then a single transistor could be used for commutating all the main thyristors connected 25. between one side of the supply and the corresponding ends of the main windings, and another transistor could be used for commutating the other thyristors. Once again, by use of two additional thyristors, a single transistor can be used for commutating all the main thyristors, but 30. not all at the same time; the top and bottom thyristors have to be OMPI commutated in groups in sequence. The advantage of the invention lies in the use of a single transistor for commutating a number of thyristors. Whereas a transistor may be used as the 5. main switching device for energising a winding in synchronism with rotor movement, transistors tend to be expensive, as compared for example with thyristors .and hence thyristors are preferable as the main switching devices. However thyristors require a separate commutation 10. circuit to turn them off and a transistor and auxiliary supply used for such purpose has the advantage that it may be switched on and off to reverse voltage the main thyristor for a set time independent of the commutation current whereas a thyristor used for 15. commutation relies for its own turn off on the discharge " of a commutation capacitor giving a reverse voltage time which reduces as the commutation current increases. Furthermore, the commutation capacitor must be recharged before a further commutation of a given main thyristor 20. can take place, whereas a transistor commutation circuit requires no such resetting. Since it is unduly expensive to use a separate commutation transistor for each main thyristor, the present invention enables a single transistor to be used for commutating a number of 25. thyristors. A further advantage of using transistors for commutation is that since they are only switched on for a short time relative to their off time they may conveniently be driven via isolating pulse transformers 30. as for thyristors rather than by expensive separate base drive circuits and supplies. The invention may be carried into practice in various ways, and certain embodiments will now be described by way of example, with reference to the accompanying drawings,, of which the first six figures 5. are circuit diagrams of six power converter circuits and FIGURE 7 shows how a number of transistors can be used in combination as a single transistor common to a number of thyristors. The main and/or auxiliary d.v. supplies shown 10. for simplicity in the accompanying figures as batteries may in each case, comprise a relatively large capacitor or capacitors fed from the a.c. mains by a rectifier or controlled rectifier either directly or through an isolating transformer, or fed from an isolating d.c . 15. to d.c. convertor or may comprise a rectifier capacitor circuit whereby two series d.v. supplies may be generated from a single a.c. mains supply as disclosed in British Patent Specification No. (TASC " 2) or may consist of any know form of direct voltage supply. 20. In the case that the direct voltage supply is a battery it may be necessary to connect a capacitor across the battery so as to bypass the a.c. component which would otherwise flow through the battery, the battery thereby supplying the d.c. component. 25. n the circuit of FIGURE 1, the motor is a four-phase motor, having four main windings, A.B.C. and D. They have one end connected to a common point 11, in what may be called an 'H' configuration, each having its other end connected through a main thyristor T_ or T_, 30. to one side, or T a_ or T_ u, to the other side of a d.v. supply 12 split into two equal portions in series, the supply mid point being connected to the common point 11. The cathodes of the thyristors T «rι and T_* are 5. connected to the cathodes of respective commutating diodes D and D the anodes of which are connected together to the cathode of an energy return diode D whose anode is connected to the negative side 16 of the supply. There is a similar arrangement of 10. commutating diodes D , D_, and D for the main windings B and D and their thyristors. Two commutating d.v. supplies 17 and 18 are connected between respective sides 15 and 16 of the d.v. supply and the collector of a transistor 22 and the 15. emitter of a transistor 23 respectively. The emitter of the transistor 22 is connected to the anodes of the commutating diodes D and D and similarly the collector of the transistor 23 is connected to the cathodes of the commutating diodes D_ and D_. 20. Optionally, there are a pair of free-wheeling thyristors T„ and T ^ , and each is connected between the common point 11 and one of the pairs of commutating diodes DA_ and DC_, and Ω_B and It will be clear that when any of the main 25. thyristors is conducting, its main winding will draw current from one side or the other of the supply. In order to de-energise a main winding, it is necessary to commutate its thyristor, and that is done in the case of the thyristors T and T by turning on the transistor 30. 22 so that the commutating supply 17 is connected through the diodes D Λ and D in reverse across the thyristors T and T . The transistor 22 can then be switched off, and if it was desired to commutate only one of the thyristors T and T_, the other can be immediately 5. turned on again. If, say, the main thyristor T has been commutated, then the current in the main winding A continues to flow through the diodes D E_ and DA to return energy to the supply until the current has decayed. The circuit enables fine control to be achieved 10. because the transistor 22 can be switched as required and the commutating supply 17 is also always at the right polarity for commutating the thyristors A and T . What is more, this is achieved without having to have one transistor for each of the thyristors T and T . The. 15. transistor 23 operates in a similar manner for commutating the thyristors T and T . The free-wheeling thyristor T can be switched on if it is desired to allow the current in the main winding A or C to free-wheel when the thyristor T or 20. τ_ is switched off through a circuit including the commutating diode D A or D . The circuit of FIGURE 2. is quite similar to that of FIGURE 1, but it shows how a single transistor shown at 24 can be used for commutating all of the main 25. thyristors T , T , T , and T . There is a single commutating direct source 25 and that is connected in series with the emitter collector path of the transistor 24 and commutating thyristors and T γ across the d.v. supply 12. 3°« If when ' it is desired to commutate the thyristors OMPI T_ and T„, the transistor 24 is switched on, it is A C necessary also to switch on the thyristor T , and then commutating voltage can be applied in reverse across the thyristors T ft and T from the positive side of 5. the d.v. supply2 , through the transistor 24, the diodes and D , the circuit being completed through the thyristor T , to the negative side of the supply 25. If it is one or both of the thyristors T and T , that is to be commutated, then it is necessary to fire the 10. thyristor T at the same time as the transistor. In other respects the circuit of FIGURE 1 is similar to that of FIGURE 2. It should be pointed out that in FIGURE 2, the transistor 24 is shown with a snubber circuit connected 15. across it, and the thyristors T v and T,_ are each shown as being associated with a snubber circuit, and a saturating reactor for protection against a high rate of change of voltage. These features are quite conventional with transistor .and thyristor circuits, and would normally 20. be included in the other-circuits shown in this specification although they have been omitted for clarity. It should also be pointed out that for FIGURE 2, it is not possible to commutate T Α and/or TV__ * at the same time as T and/or T . Following the firing of B D 25. say T v x as above, when sufficient time has elapsed for T Ά and/or T to turn off, transistor 24 is turned off and the current flows through the snubber diode 26 and capacitor 27 thereby charging the capacitor 27 until diode D_l becomes forward biassed. The current thereafter 30 , flows through D h_., D A_. and A and/or D C_m. and C and back through the supply to D_, thereby returning energy to the supply. E Thyristor T , having been deprived of current, turns off and only after sufficient time has elapsed for this turn off to be complete can transistor 24 be turned 5. on and thyristor fired so as to commutate T and/or T_ in similar manner. It may be inconvenient to provide a three wire supply of the kind shown in FIGURES 1 and 2 so that only a two wire supply 12 is available, as shown in FIGURE 10. 3 and there is ho centre terminal for connection to the • H' Doint 11. In many respects ; "the circuit of FIGURE 3 is very like that of FIGURE 2, and a single transistor 24 can be used for commutating all of the main thyristors T , T_, and T . The circuit has some disadvantages, 5. but nevertheless may be useful in certain applications. In the circuit of FIGURE 4, the motor is a three phase motor, with phase windings, A , B , and " C-, each of which is connected in series with a main thyristor T , T , or T_ in series across the supply 10. 32 -. Each phase winding is closely coupled in a 1:1 transformer ratio with an auxiliary winding A., A_, or A g on the same stator pole, and connected in series with a diode DA.., DA-, or DA- across the supply 32. When 15. a thyristor is on its phase winding draws current from the supply, but as soon as it is commutated by switching on a transistor TR, so that a commutating supply provided by a battery S- is connected through commutating diodes D_, D and D_ in reverse across the thyristors, 20. current in the phase windings is transferred firstly to the transistor and subsequently to the auxiliary windings and can flow to return energy to the supply 32. Once again a single transistor is used for commutating all the thyristors, and if any thyristor is required to 25. be retained on, it is merely switched on again after the transistor has been switched off. In the circuit shown in FIGURE 5, a three phase motor is shown with phase windings A , B , and C , each of which is connected across a d.v. .supply 31 in 30. series. ' with thyri * s ' tors 1 and 4,2- nd 5, or 3 and 6, at "top" and. "bottom 11 of the : . winding. Commutating diodes D_. , D„, and D_, enable the thyristors 1, 2 and 3 to be commutated from a commutating supply19 when a transistor 22 is switched on, and similar diodes D 4., D5.. and D6, are used for 5. commutating the thyristors 4, 5 and 6 from the supply 21 when the transistor 23 is turned on. Considering phase winding A ' , it can be seen that if thyristors 1 and 4 are on, the winding draws current from the source 31; if thyristors 1 and 4 10. are both off, current in the winding A can be returned to the source 31 by way of a current path including diodes D„ and D.. , the winding A and diodes D. and D„; if thyristor 1 is on, and thyristor 4 is off, current can free-wheel through a current path including 15. diodes D. and D R , and the thyristor 1. Once again a single transistor 22 or 23 is all that is supplied for commutating three thyristors 1, 2 and 3 or 4, 5 and 6. FIGURE 6 shows a motor with a single phase winding, A connected across a di-v.v supply 32 in ' ,series ' with 20. top and bottom thyristors 1 and 4-. The circuit is very similar to that of FIGURE 5, except that there is only a single phase winding, and the commutating circuit is similar to that of FIGURE 2. In this case, a transistor 24 is used in a similar manner 25. to that described with reference to FIGURE 3 for commutating both thyristors 1 and - - in dependence upon whether a thyristor T χ or a thyristor T γ is. fired at the same time as the transistor is switched on. . Where a transistor has been shown in FIGURES 1-6, 30. it may be "replaced by a combination of transistors in series and /or parallel connected to act in unison as a single transistor , for example, as shown in FIGURE 7. 5.

Claims: CLAIMS 1. An electric power converter circuit for supplying from a d.v. supply unidirectional pulsating currents to each main winding of a variable reluctance motor having a main winding for one phase, or each of at least two phases, including at least one main thyristor connected in series with each main winding in a circuit across the supply through which the motor can receive energy from the supply, means whereby each main winding can return energy to the d.v. supply, triggering means for switching on each main thyristor in synchronism with the relative movement of the motor, and commutating means including a transistor common to at least two of the thyristors for switching those thyristors off. 2. An electric power converter circuit for supplying from a d.v. supply unidirectional pulsating currents to each main winding of a variable reluctance motor having a main winding for one phase, or each of at least two phases, including at least one main thyristor connected in series with each main winding in a circuit across the supply through which the motor can receive energy from the supply, for each main winding a circuit including that main winding and at least one diode whereby each main winding can return energy to the d.v. supply, triggering means for switching on each main thyristor in synchronism with the relative movement of the motor, and commutating means including a transistor common to at least two of the thyristors for switching those thyristors off. 3. A converter circuit as claimed in Claim 1 in which the means whereby the motor can return energy to the d.v. supply, includes an auxiliary winding closely coupled to the/or each main winding, and an auxiliary diode connected in series with the auxiliary winding in a circuit across the supply. 4. A converter circuit as claimed in Claim 1 or Claim 2 in which the supply is split into two portions and in which each main winding is arranged to receive energy from one portion of the supply, and to return energy to the other portion of the supply. 5. A converter circuit as claimed in Claim 1 or Claim 2 in which the motor is a single phase motor with a thyristor at each end of the single main winding in series with the supply, the two thyristors being arranged to be commutated by the common transistor. 6. A converter circuit as claimed in any of Claims 1, 2 and 4 in which the motor has pairs of phases each in series with a thyristor, there being two winding and thyristor combinations connected in series across the supply for each pair of phases. 7. A converter circuit as claimed in Claim 6 in which the common points of each pair of phases are all connected together. 8. A converter circuit as claimed in Claim 4 or Claim 7 in which the said common points are connected to the common point of the two portions of the supply. 9. A converter circuit as claimed in any of Claims 6-8 including a first transistor arranged to commutate all the thyristors connected between one side of the supply and half the phase windings, and a second transistor arranged to commutate the other thyristors. 10. A converter circuit as claimed in Claim 5 having a single common transistor for commutating all the main thyristors, and a pair of commutating thyristors one or other of which is arranged to be switched on when the single common transistor is switched on to determine which half of the main thyristors is to be commutated. 11. A converter circuit as claimed in either of Claims 1 and 2 in which the motor is a multi-phase motor in which each winding has two main thyristors one connected between one end of the winding, and one side of the supply, and the other connected between the other end of the winding and the other side of the supply. 12. An electric power converter circuit for a variable reluctance motor arranged substantially as herein specifically described with reference to any of FIGURES 1-6 of the accompanying drawings.

Inventors: DAVIS REX MOUNTFORD, RAY WILLIAM FREDERICK, DAVIS, REX, MOUNTFORD, RAY, WILLIAM, FREDERICK
Applicants: SWITCHED RELUCTANCE DRIVES LTD
Application year: 1981
Main classification: H02M1
IPC codes: H02M7, H02P27