WO1990016111A1

WO1990016111A1 - A device for controlling a reluctance motor

Info

Publication number: WO1990016111A1
Application number: PCT/SE1990/000422
Authority: WO
Inventors: Bengt Gunnar Hedlund
Original assignee: Aktiebolaget Electrolux
Priority date: 1989-06-15
Filing date: 1990-06-15
Publication date: 1990-12-27
Also published as: SE463899B; SE8902160A; AU5853390A; SE8902160D0

Abstract

A reluctance motor comprises a stator (10) having poles (11, 12, 13, 14), co-operating in pairs, disposed in diametrically opposite positions and provided with windings (15a, b; 16a, b), and a rotor (17), made of soft-magnetic material and having poles (19a, b; 20a, b), co-operating with the stator poles and disposed in pairs in diametrically opposite positions. In each stator pole pair (11, 12; 13, 14) the windings (15a, b; 16a, b) are connectable to a source of DC voltage via two separately controllable switches (T1, T2) such that one switch (T1) connects the windings to the positive terminal of said source while the other switch (T2) connects the windings to the negative terminal of said source. Two free-wheeling diodes (D1, D2) are provided to co-operate with the respective switch in order to permit a continuous flow of current through the windings when the switch opens. Control means (54, 36, 38, 39, 40, 44, 45) are provided to emit control signals to the switches (T1, T2) to turn them on and off. The control means have a design such that the switches mainly simultaneously receive a control signal (TR1, TR2) closing the same, while at turn-off the corresponding control signals (TR1, TR2) are supplied to the switches at different times.

Description

A device for controlling a reluctance motor

The present invention relates to a device for controlling a reluctance motor of the kind indicated in the preamble of appending claim 1.

An example of a reluctance motor of the kind referred to is disclosed in the Swedish Patent Application No. 8900408-9. The motor thus described is of the two-phase type having a four-pole stator and a two-pole rotor. The four stator poles are arranged so as to form a cross and the rotor poles are disposed in diametrically opposite positons. The stator poles are provided with windings and co-operate in pairs. The stator pole pairs are activated alternately in order to rotate the rotor.

10 In the known motor, via a first switch the windings of each stator pole pair are connected to the positive terminal of a source of DC voltage, said windings via a second switch being connected to the negative terminal of said source. Two free-wheeling diodes ensure the continued current flow through the windings even after the switches have switched off. During the last-men- '^■-' tioned period of time energy is being returned to the source of DC voltage.

The connection and disconnection, respectively, of the two switches in the respective phase (stator pole pair) takes place essentially at the same time. During the time of return of energy taking place core losses are generated when the flux in the stator iron core is continuously descending to zero. ™ An object of the invention is to provide a more efficient motor and, having this purpose in mind, to reduce the core losses during the period of return of energy.

Another problem touched by the invention is the control of the speed of a reluctance motor. Traditionally, in this type of motor the speed range can 2-5 be divided into three parts, viz. the low speed area, the medium speed area and the high speed area. As an example, reference is made to a paper headed "Variable Speed Switched Reluctance Motors", Written by J.P. Lawrenson, J.M. Stephenson, B.T. Blenkinsop, J. Corda and N.N. Fulton. The paper has been published in Proceedings IEE, Vol. 127, Pt. B, No. 4, July 1980, pp. 253-265.

30 With reference to Fig. 11 and the accompanying text in the paper, a chart is given wherein power has been plotted as a function of speed for a six/four pole, three-phase operated reluctance motor. For this motor, in the low speed area of 0-1000 rpm the torque remains constant and the control of the speed is per¬ formed by chopping or, alternatively, by voltage control (PWM). At medium speeds (1000 - 3000 rpm) the output power remains constant and the control of the speed is performed by control of the switching-angle. In the high-speed region (3000 - 4000 rpm) the motor works without external control and with fixed switching-angles.

For speed control, it is common pratice to combine current control in the low-speed region with switching-angle control in the medium-speed region. However, the current control results in a high sound level for the motor and in a low efficiency.

Hence, it is another object of the invention to avoid current control in the low-speed region and to replace it by switching-angle control, at least within part of the low-speed region.

The objects indicated are achieved by a reluctance motor having the characteristic features included in claim 1. Preferred embodiments appear from the accompanying sub-claims.

The invention will now be described more in detail in connection with a few embodiments and with reference to the accompanying drawings.

Fig. 1 shows, schematically, a reluctance motor having four stator poles and two rotor poles.

Fig. 2 shows, schematically, the directions of magnetic flux in the motor of Fig. 1.

Fig. 3 is a circuit diagram for a power stage for the supply of one motor winding. Fig. 4 is a circuit diagram for a logic circuit arrangement for one motor phase (one stator pole pair) without feedback.

Fig. 5 is a diagram showing the waveforms appearing in the circuit of Fig. 4.

Fig. 5A shows, in two diagrams, the motor current plotted as a function of the angle of rotation of the rotor in a known embodiment and in an embodi¬ ment according to the invention, respectively.

Fig. 6 is a circuit diagram for a logic circuit arrangement with feedback controlling the time of turn-on.

Fig. 7 is a circuit diagram of a logic circuit arrangement with feedback controlling the time of turn-off of that switch in Fig. 3 which is to be turned-off first. (Tl). Fig. 8, finally, is a circuit diagram of a logic circuit arrangement with feedback controlling the time of turn-on of the switches in Fig. 3, where turn-off of the first one (Tl) of said switches takes place at a predetermined position of the rotor and where the angle between the rotor position of turn-on and the rotor position of turn-off of the second switch (T2) is invariable.

The following detailed description of a few embodiments will be carried out in connection with a reluctance motor of the two-phase type. The motor comprises a stator 10 having four- poles 11, 12, 13, 14 which are disposed so as to form a cross. The poles are provided with windings 15a, b; 16a, b connected so as to co-operate in pairs generating magnetic fields which co-operate. A rotor 17, made of soft-magnetic material, is arranged to rotate in the air gap between the stator poles. The rotor is provided with two poles 18a, b; 19a, b disposed in diametrically opposite positions. The pole parts 18a, 19a extend in a circumferential direction so as to completely equal the corresponding ex- tension of the stator poles. The pole parts 18b, 19b have a corresponding ex¬ tension which, when the pole parts 18a, 19a are situated just opposite the poles of one of the stator pole pairs, fill the space to the poles of the other stator pole pair, as seen in the direction of rotation. In addition, the pole parts 18b, 19b have a greater air gap to the stator poles and will be referred to as starting poles in the following. These starting poles ensure that torque be generated in all positions taken by the rotor. A disadvantage, however, is that the design of the rotor, just described, admits rotation in one direction only.

In order to turn around the rotor the stator pole pairs 11 , 12 and 13, 14, respectively, are magnetized alternately, thereby causing the rotor poles alter- nately to align with the poles of the respective stator pole pair. In Fig. 2, schematically by flux arrows 20, 21, the directions of flux are shown when the stator pole pairs 13, 14 and 11, 12, respectively, are being magnetized. It is not necessary to alternately switch the direction of magnetization in order to have the rotor complete one turn. Hence, the direction of the magnetic flux in the respective stator pole pair lacks importance, which simplifies the con¬ struction of a control device for the control of the motor, not described in detail here.

For the indication of the rotor aligning with either of the stator pole pairs, there is provided a rotor position sensor. In principle, the sensor generates a signal controlling the switching from one stator pole pair to the other. The sensor, known per se and not shown in detail in the drawings, has been designed to generate a square-wave output voltage, shown in the uppermost line of Fig. 5. However, the sensor only gives information as to when the rotor poles and stator poles align and if there is a desire of information about the rotor movement also during the remaining parts of the turn, separate measures have to be taken. In the example, the problem has been solved in the same way as described in the Swedish Patent Application No. 8900408-9, referred to above, i.e. by gene- ^■5 rating a sawtooth voltage being resetted by the sensor signal upon the appearan¬ ce of a positive flank in said signal. This is illustrated in line 2 in the diagram of Fig. 5.

Below, an end stage will be described which is intended for the supply of the two windings of one phase, for example windings 16a, b of Fig. 1. The

10 windings are connected in parallel between the emitter of an upper transistor Tl and the collector of a lower transistor T2. The transistors shown are of the bipolar design, however, other switches can be used as well, for instance field effect transistors of the MOS type. The collector of the transistor Tl is con¬ nected to the positive terminal of a DC voltage source while the emitter of

^*---' the transistor T2 is connected to the negative terminal of said source. In the usual way, a reservoir capacitor C is connected across the terminals of the voltage source. In addition, free-wheeling diodes Dl, D2 are connected between the emitter of transistor Tl and said negative terminal and the collector of transistor T2 and the positive terminal of said source, respectively. The purpose " of the free-wheeling diodes is to permit a continued flow of current through the motor windings even after the respective transistor has turned-off, resulting in a current decay without the generation of voltage transients.

Traditionally, the transistor switches Tl and T2 are being turned-on and turned-off simultaneously. However, now if has been discovered that great

25 advantage can be achieved if the switches are being turned-off at different points of time. Thus, by minimizing the basic stator flux, the core losses can be minimized and this can be performed by. permitting the current stored in the windings to free-wheel during a certain time before the feedback of energy to the voltage source is initiated. During free-wheeling, torque and copper

30 losses of normal proportions are generated but only small core losses because of the fact that the flux in the core iron is invariable at zero voltage. Due to the resistive voltage drop in the windings, a small voltage is generated and thereby a small variation of flux, resulting in core losses in the stator iron. These, however, are very small. Locally in the poles normal core losses are

^•35 generated but these losses are limited due to the limited iron volume of the poles as compared to the volume of the stator iron.

In order to be able to independently control the turn-on and turn-off of the transistor switches the respective base of the transistors Tl and T2 are connected, via conductors 22, 23, to drive circuits 24 and 25, respectively, which via conductors 26, 27, respectively, are connected to a multiplexer circuit 28, coupled as a demultiplexer, for instance of the type HEF 4953 B. Input signals to the circuit 28 appear on terminals 29, 30, 31 and 32. On the terminal 29 the sensor signal appears, which emanates from the sensor, not shown, and the wave¬ form of which is shown in the uppermost line of Fig. 5. On the terminal 30 a control signal TRl is applied which controls the turn-on and turn-off of the transistor Tl. Correspondingly, on the terminal 31 a control signal TR2 is applied which controls the turn-on and turn-off of the transistor T2. Finally, the terminal 32 is used to apply a control signal which represents the speed of the motor and which, at start and at low speeds, lets the sensor signal alone control the turn-on and turn-off of the transistors Tl and T2, resulting in that the two transistors turn-on and turn-off simultaneously. The control signal on the terminal 32 is generated in a comparator 33, having a hysteresis function, by comparing a speed signal (real value), derived from the sensor signal, with a reference value, set by a potentiometer 34. Above the low speed region the turn-on and turn-off of the transistors Tl and T2 are controlled by the control signals TRl and TR2, resulting in the transistors being simultaneously turned-on but being turned-off at different times so that Tl is turned-off first while T2 continues to conduct for a preditermined time before being turned-off.

As customary in reluctance motors of the kind described, turning-on _^of the transistor switches Tl and T2 does not take place upon the sensor signal indicating the aligning of the stator and rotor poles of a pole pair but a pre¬ determined angle of rotation of the rotor prior to said position. This has been described more in detail in the Swedish Patent Application No. 8900408-9 and the purpose is to allow, at higher speeds, the magnetic field to have sufficient time to get established so that the required torque can be generated. The control signals TRl and TR2 contain information as to the appropriate so-called pre- firing, while, in addition, signal TRl contains information as to the time of turn-off of transistor Tl and the signal TR2 contains information on the time of turn-off of transistor T2.

Below, with reference to Figs 4 and 5, a first embodiment of the invention will be described. Schematically, Fig. 4 discloses a circuit in the form of a block diagram. Starting from the sensor signal, see Fig. 5, line 1, and by assistance of three presettable reference levels, the object of the circuit is to provide the control signals TRl and TR2 of Fig. 3.

The sensor signal is led to a frequency-to-voltage (F/V) converter 54 which generates a voltage appearing on a terminal 35 and the magnitude of which corresponding to the frequency of the sensor signal. This voltage repre¬ sents the instantaneous speed of the motor and, hence, said signal constitutes a speed signal, referred to in Fig. 4 a SPEED. Said voltage is also led to a ramp generator 36 which generates a ramp of a shape shown in line 2 in the diagram of Fig. 5. Likewise appearing from the diagram, a positive flank of the sensor signal activates the ramp generator while the next positive flank to follow resets the signal to the starting level. To this end, the sensor signal is, via a conduc¬ tor 37, connected to a RESET - input of the rampgenerator.

The output signal of the ramp generator 36 is led to three comparators 38, 39, 40. In addition, comparator 38 is connected to a potentiometer 41 , by means of which the level is determined at which the transistors Tl and T2 are turned-on. Correspondlingly, comparator 39 is connected to a potentiometer 42 by which a level can be set which is used for turning-off of transistor Tl. Finally, the comparator 40 is connected to a potentiometer 43 which is used for setting of a level at which transistor T2 is to be turned-off. The three levels are indicated on lines 3-5 in the diagram of Fig. 5. The output signals from the comparators are led to two flip-flops 44, 45, on the outputs of which appear the control signals TRl and TR2. Thus, comparator 38 is, via a conductor 46, connected to the SET- input of the flip-flop 44 and, via a conductor 47, to the SET-input of the flip-flop 45. The comparator 39 is, via a conductor 48, con¬ nected to the RESET-input of the flip-flop 44 and, in the same way, the com¬ parator 40 is, via a conductor 49, connected to the RESET-input of the flip- flop 45.

The circuit shown in Fig. 4 operates without feedback in the following way. When the motor has started, the sensor signal, having the shape according to line 1 of Fig. 5, appears on the input of the F/V converter 54. When the sensor signal goes high, the ramp generator starts and its output voltage rises according to the waveform in line 2 of Fig. 5. In line 3 of the diagram the turn-on level for the transistors Tl and T2 in Fig. 3, set by the potentiometer 42, is shown. When the output level from the ramp generator reaches this level, the compara¬ tor 38 changes state and sets the flip-flops 44 and 45 which causes the control signals TRl and TR2 to appear on the terminals 30 and 31 of the multiplexer circuit 28. Then, via transistor drive circuits 24 and 25, base current is supplied to the transistors Tl and T2 causing the transistors to start conducting and the accompanying stator pole pair to become activated. The level set by the potentiometer 41 is a measure of the chosen prefiring, which is fixed.

The output voltage from the ramp generator continues to rise and, as time goes, the level of turn-off of the transistor Tl, set by the potentiome- ter 42, is reached. This results in comparator 39 changing state, causing the flip-flop 44 to be reset while the flip-flop 45 remains set. Therefore, the control signal TRl ceases causing the transistor Tl to turn-off. However, the transis¬ tor T2 remains conducting, as the control signal TR2 is still present, which means that current flows through a circuit comprising the transistor T2, the windings 16a,16 b and the diodes Dl, D2. However, during this free-wheeling time period no feedback of energy to the voltage source takes place and the energy is used to a full extent for driving the motor.

When the output voltage from the ramp generator 36 has reached the level of turn-off of the transistor T2, see line 5 of ^" Fig. 5, the comparator 40 changes state, resetting the flip-flop 45 and causing the control signal TR2 to cease. Thus, the transistor T2 is cut-off and the feeding-back of energy to the voltage source is being initiated via a circuit comprising the windings 16a, 16b and the diodes Dl, D2. In this way, the current is given the opportunity to decay without causing voltage transients to be generated during the time period in which the windings are emptied of magnetic energy.

The described procedure is repeated alternately for the phase comprising the the windings 16a, 16b as well as for the opposite phase, not described in detail, having the windings 15a, 15b (Fig. 1). The improved efficiency of the motor is best shown in a diagram of the current plotted as a function of time or angle of rotation of the rotor. Two such diagrams have been shown in Fig. 5A, in one diagram of which it is shown, by dashed lines, how in the classical case the current ascends and descends. In the other diagram, by continuous lines, the current waveform is shown for an embodiment of the invention in which the current descends more slowly after turning-off of transistor Tl and, with a delay, transistor T2. The current time area is greater than in the classical case resulting in a higher efficiency.

In Fig. 4 a speed signal appears on the terminal 35. In Fig. 6 this signal is led to an adding circuit 50 via a conductor 51. Also led to the circuit 50 is a reference signal, set by a potentiometer 52, said signal corresponding to the desired speed of the motor. The output of the adding circuit 50 is conducted to a regulator 53 emitting a control voltage to the comparator 38, similar to that shown in line 3 of Fig. 5. This level can be controlled so that the speed of the motor can be kept constant. A disadvantage associated with this circuit is, however, that due to time delays caused by the free-wheeling during the period of turning-off, the full power cannot be taken out from the motor.

Fig. 7 shows a circuit having a feedback loop controlling the time of turn- off of the transistor Tl. As in Fig. 6, the speed signal from the F/V converter 54 is led to an adding device, here designated by 55, to which is also led a reference signal, settable by a potentiometer 56 and constituting a measure of the desired speed. The output signal of the adding circuit 55 is led to a regulator 57, the output of which being connected to the RESET-input of the comparator 39. An disadvantage of this circuit is the bad efficiency of the motor at low load.

A particularly preferred embodiment is shown in Fig. 8. In this circuit, having a feedback-controlled turn-on as in Fig. 6, a fixed angle has been chosen for turning-off of the transistor Tl while the angle of turn-off of the transis¬ tor T2 is controlled such that a fixed angle is contained between the time of turn-on and the time of turn-off of the transistor T2. The circuit has the same appearance as the circuit of Fig. 6, however, being completed with an additional ramp generator 58 having as its input signal the output signal of the F/V con¬ verter 54. The output signal of the ramp generator 58 is, via a conductor 59, connected to the comparator 40, in which it is being compared with the level of turn-off of the transistor T2, set by the potentiometer 43. In order to achieve a constant angle between the angle of turn-on and the angle of turn-off of the transistor T2, the output signal of the comparator 38 is, via a conductor 60, connected to a RESET-input of the ramp generator 58. In this way, a reset is obtained of the ramp generator 58 at the same time as the setting of the flip-flops 44 and 45 for the generation of the control signals TRl and TR2, required for the activation of the transistors Tl and T2. Accordingly, the ramp signal from the ramp generator 58 starts rising upon turn-on to reach the level set by the potentiometer 43 after a time or an angle which is always the same. Then, the turn-off of the transistor T2 will always take place at the same angle of rotation, as counted from the angle of turn-on.

In the embodiment of Fig. 8 it is possible to use the full power of the motor at the same time as the efficiency is good even in case of low power output from the motor.

Claims

C l a i m s

1. A device for controlling a reluctance motor, said motor comprising a stator (10) having stator poles (11,12,13, 14) co-operating in pairs, disposed in diametrically opposite positions and provided with windings (15a,b; 16a,b), and a rotor (17) made of soft-magnetic material, said rotor having poles (19a,b; 20a,b) which in pairs are disposed in diametrically opposite positions and co¬ operate with the stator poles, said windings (15a,b; 16a,b) in each stator pole pair being connectable to a source of DC voltage via two separately controllable switches (T1,T2) such that one switch (Tl) connects the windings to the positive terminal of said source while the other switch (T2) connects the windings to the negative terminal of said source, and two free-wheeling diodes (D1.D2) are arranged to co-operate with the respective switch to permit a continued flow of current through the windings when the switch opens, control means (54,36,38,39,40,44,45) being provided to emit control signals (TR1.TR2) for turning-on of the switches (T1.T2) at a predetermined rotor position, and to emit a first control signal (TRl) for turning-off of one (Tl) of the switches at a predetermined first angle of rotation, following said rotor position, and to emit a second control signal (TR2) for turning-off of the other switch (T2) at a predetermined second angle of rotation, following the said rotor position, greater than the first -mentioned one, c h a r a c t e r i z e d in that the angle of turn-on is variable, that the first angle of turn-off is fixed and that the difference between the angle of turn-on and the second angle of turn-off is invariable.

2. A device according to claim 1, c h a r a c t e r i z e d in that means ar provided to emit a reference signal (line 1 , Fig. 5) at the moment a rotor pole is about to move in over a stator pole, control means (54,36,38,39,40,44,45) being provided to emit control signals (TR1.TR2) for turning-on of the switches (T1.T2) at a predetermined angle of rotation prior to the reference angle de¬ termined by the reference signal, and to emit a first control signal (TRl) for turning-off of one (Tl) of the switches at a predetermined first angle of rotation, following said reference angle, and to emit a second control signal (TR2) for turning-off of the other switch (T2) at a predetermined second angle of rotation, following said reference angle, greater than the first-mentioned one.

3. A device according to claim 1 or claim 2, c h a r a c t e r i z e d in that the control means (54,36,38,39,40,44,45) comprises a feedback loop (51,52,53) for speed control by variation of the angle of turn-on.