GB2293705A

GB2293705A - Switched reluctance motor

Info

Publication number: GB2293705A
Application number: GB9519504A
Authority: GB
Inventors: Takeshi Ikeyama; Graham Williams
Original assignee: Aisin Seiki Co Ltd
Current assignee: Aisin Corp
Priority date: 1994-09-30
Filing date: 1995-09-25
Publication date: 1996-04-03
Anticipated expiration: 2015-09-25
Also published as: GB9419755D0; GB9519504D0; GB2293705B; JPH08205581A

Abstract

A switched reluctance motor includes a stator (12, Fig. 1) fixed in an inner bore of a housing (11) and having pairs of opposing stator pole portions, a rotor (22) rotatably disposed in the stator and having rotor pole portions, first coils 16a, 18a, 20a, wound on pairs of the stator pole portions, second coils 17a, 19a, 21a wound on pairs of the stator pole portions, a current switching means 30 - 41 for switching on and off current which is supplied to each of the first and the second coils, and a timing shift means 70, 70' for shifting the timing of switching on and off the current supplied to the first coils relative to the timing of switching on and off the current supplied to the second coils. Vibration of the stator and housing and acoustic noise is reduced. The timing shift means 70, 70' which provides a delay, may include a counter (171, Fig. 4) or may be provided by the controller 27 (227, Fig. 6). <IMAGE>

Description

TITLE: Switched reluctance motor BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a switched reluctance motor.

2. Description of the prior art A conventional switched reluctance motor is disclosed in, for example, GB 2231214A. This switched reluctance motor includes a housing, a stator fixed in an inner bore of the housing and formed by laminating of electromagnetic steel plates and a rotor disposed in the stator and formed by laminating of electromagnetic steel plates. The rotor is fixed to an output shaft which is rotatably supported on the housing and thereby is rotatably disposed in the stator. The rotor has a plurality of pairs of rotor pole portions which project outwards in the diametrical direction and which extend in the axial direction. The stator has a plurality of pairs of opposing stator pole portions which project inwards in the diametrical direction and which extend in the axial direction.Each of the stator pole portions moves past each of the rotor pole portions as the rotor rotates and a certain clearance is maintained between stator pole portions and rotor pole portions which are opposite each other. On each of the stator pole portions, a coil is wound. The coils which are wound on each of the pairs of opposing stator pole portions are connected in series with each other and thereby a magnetic flux is generated between the pair of stator pole portions when current is supplied to the coils that are wound thereon. A magnetic attracting force results between rotor pole portions and stator pole portions which are approaching each other.

This magnetic attracting force is changed by controlling supply current by means of switching elements in response to the rotational position of the rotor and thereby motoring torque is produced.

Furthermore, a switched reluctance motor in which two independent coils are wound on each of the stator pole portions is disclosed in GB 2232305A and US 5113113.

According to this disclosure, since the magnetic flux generated between each pair of stator pole portions is increased, it is possible to increase motoring torque.

In the above mentioned prior switched reluctance motors, the current which is supplied to the coils wound on one pair or several pairs of stator pole portions being approached by one pair or several pairs of rotor pole portions is switched on and off as a pulse. In general, the current is switched on when a pair of rotor pole portions begins to be aligned with a pair of stator pole portions and the current is switched off before the pair of rotor pole portions is fully aligned with the pair of stator pole portions. Thereby, the magnetic attracting force increases while the current is supplied and disappears in a moment when the current is switched off.

On one hand motoring torque is obtained by this magnetic attracting force. On the other hand a pair of stator pole portions is attracted to a pair of rotor pole portions by this magnetic attracting force and thereby the stator and the housing are strained. When the magnetic attracting force disappears, the strain of the stator reduces suddenly and the housing is suddenly pressed outwards in the diametrical direction by the stator. This impulsive variation of the housing is generated periodically in response to the rotation of the rotor and thereby vibration of the housing generates objectionable acoustic noise.

SUMMARY OF INVENTION It is, therefore, an object of the present invention to provide an improved switched reluctance motor which overcomes the above drawbacks.

It is another object of the present invention to provide an improved switched reluctance motor which can reduce the objectionable acoustic noise.

In order to achieve these objectives, there is provided a switched reluctance motor comprising: a housing having an inner bore extended in the axial direction, a stator fixed in the inner bore of the housing and having a plurality of pairs of opposing stator pole portions which project inwards in the diametrical direction and which extend in the axial direction, a rotor rotatably disposed in the stator and having a plurality of rotor pole portions which project outwards in the diametrical direction and which extend in the axial direction, a plurality of first coils wound on pairs of the stator pole portions, a plurality of second coils wound on pairs of the stator pole portions, a current switching means for switching on and off current which is supplied to each of the first and the second coils, and a switching timing shift means for shifting the timing of switching on and off the current supplied to the first coils relative to the timing of switching on and off the current supplied to the second coils.

BRIEF DESCRIPTION OF THE DRAWINGS Additional objects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof when considered with reference to the attached drawings, in which: Fig. 1 is a schematic view of a first embodiment of a switched reluctance motor in accordance with the present invention; Fig. 2 is a circuit diagram which shows a current switching means and a switching timing shift means of a first embodiment of the present invention; Fig. 3 is a set of graphs which show variations of torque, current and magnetic attracting force during the supply of current to first and second coils wound on a pair of stator pole portions of a first embodiment of a switched reluctance motor in accordance with the present invention;; Fig. 4 is a circuit diagram which shows a switching timing shift means of a second embodiment of the present invention Fig. 5 is a timing chart of each signal of a switching timing shift means of the second embodiment of the present invention; and Fig. 6 is a circuit diagram which shows a current switching means and a switching timing shift means of a third embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A switched reluctance motor constituted in accordance with preferred embodiments of the present invention will be described with reference to the attached drawings.

Fig. 1 and Fig. 2 show a first embodiment of a switched reluctance motor in accordance with the present invention. Referring to Fig. 1 and Fig. 2, a switched reluctance motor 10 is provided with a housing 11 which is made of aluminium. The housing 11 is composed of a cylindrical portion lla and side housings (not shown) which are fixed to ends of the cylindrical portion lla.

In an inner bore llb of the cylindrical portion lla, a cylindrical stator 12 is disposed. The stator 12 is formed by laminating electromagnetic steel plates and is fixed at its outer circumferential portion to the inner bore llb of the housing 11 by heat shrinking.

The stator 12 is provided with three pairs of opposing stator pole portions 13a, 13b; 14a, 14b; 15a, 15b which project inwards in the diametrical direction at regular angular intervals and which extend in the axial direction. On each pair of stator pole portions, for example on a pair of stator pole portions 13a, 13b, first coils 16a, 16b are wound respectively and are connected in series to each other. Furthermore, second coils 17a, 17b are wound on the pair of stator pole portions 13a, 13b respectively and are connected in series with each other. First coils 18a, 18b; 20a, 20b and second coils 19a, 19b; 21a, 21b (not shown in Fig. 1) are wound on each of the other pairs of stator pole portions 14a, 14b; 15a, 15b in the same manner and are connected in pairs in series.These coils are all connected to a drive circuit 28.

A rotor 22 which is formed by laminating electromagnetic steel plates is disposed in the stator 12. A hole which extends in the axial direction is formed on the axis of the rotor 22 and an output shaft 23 which is rotatably supported on the side housings (not shown) at both ends is fixed in the hole of the rotor 22. Thereby, the rotor 22 is able to rotate with the output shaft 23 in the stator 12. Furthermore, the rotor 22 is provided with two pairs of opposing rotor pole portions 24a, 24b; 25a, 25b, which project outwards in the diametrical direction at regular angular intervals and which extend in the axial direction. As shown in Fig. 1, each of these rotor pole portions 24a, 24b; 25a, 25b is able to be opposed to each of the stator pole portions 13a, 13b, 14a, 14b; 15a, 15b while maintaining a certain clearance therebetween as the rotor 22 rotates.

A well known rotation sensor 26, such as an encoder or a resolver, is disposed on the end (not shown) of the output shaft 23 in order to detect the rotation position of the rotor 22. The rotation sensor 26 is connected to a controller 27 and a position signal and an angle signal detected by the rotation sensor 26 are transmitted to the controller 27.

The controller 27 is electrically connected to the drive circuit 28 to which the first and second coils 16a, 16b; 17a, 17b; 18a, 18b; 19a, 19b; 20a, 20b; 21a, 21b wound on each of the stator pole portions 13a, 13b; 14a, 14b; 15a, 15b are connected. The controller 27 has a control map for determining a switching on-off timing of the current supplied to the first and second coils in response to the rotational speed and the torque (which is equal to the output demand) and transmits a switching on or off signal to the drive circuit 28 with the determined timing in response to a position signal and an angle signal of the rotation sensor 26.Namely, the controller 27 determines the relative position of the rotor 22 with respect to the stator 12 when the current begins to be supplied to the first and second coils wound on each of the stator pole portions and when this supply of the current is stopped.

When the rotor 22 is in these positions, the controller 27 transmits a switching on or off signal to the drive circuit 28. Furthermore the control map of the controller 27 determines a comparison current value for limiting the current that flows in each of the first and second coils and transmits a comparison current value signal to the drive circuit 28. Now, in a modification of the above controller 27, it is possible to determine the switching on or off timing by calculation from the value of the rotational speed and the torque (= the demand of output).

The drive circuit 28 (Fig. 2) is composed of an inverter using switching elements such as transitors 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, current limit circuits 60 and delay circuits 70, 70'. In this embodiment, the inverter, the current limit circuits 60 and controller 27 correspond to a current switching means of the invention and the delay circuits 70,70' correspond to a switching timing shift means.The upper transistors 30, 34, 38 (emitter terminals) are connected in series with the lower transistors 31, 35, 39 (collector terminals) through the first coils 16a, 16b; 18a, 18b; 20a, 20b, respectively and the upper transistors 32, 36, 40 (emitter terminals) are connected in series with the lower transistors 33, 37, 41 (collector terminals) through the second coils 17a, 17b; l9a, l9b; 21a, 21b, respectively. In order to detect the value of the current flowing in each of the first and second coils, current sensors 42, 43, 44, 45, 46, 47 are interposed in series between the first and second coils 16a, 16b; 17a, 17b; 18a, 18b; 19a, l9b; 20a, 20b; 21a, 21b and the lower transistors 31, 33, 35, 37, 39, 41, respectively.A collector terminal of each upper transistor 30, 32, 34, 36, 38, 40 is connected with a battery 29 and the emitter terminal of each lower transistors 31, 33, 35, 37, 39, 41 is connected with the ground. Lines between the collector terminals of the lower transistors 31, 35, 39 and the current sensors 42, 44, 46 are connected with the battery 29 through diodes 48, 49, 50 so as to prevent the current from flowing from the battery 29 to the current sensors 42, 44, 46 and the first coils 16a, 16b; 18a, 18b; 20a, 20b, respectively. Lines between the collector terminals of the lower transistors 33, 37, 41 and the current sensors 43, 45, 47 are connected with the battery 29 through diodes 51, 52, 53 in the same manner.On the other hand lines between the upper transistors 30, 34, 38 (the emitter terminals) and the first coils 16a, 16b; 18a, 18b; 20a, 20b are connected with the ground through diodes 54, 55, 56 so as to prevent the current from flowing to the ground. The lines between the upper transistors 32, 36, 40 (the emitter terminals) and the second coils 17a, 17b; 19a, 19b; 21a, 21b are connected with the ground through diodes 57, 58, 59 in the same manner. Each base terminal of the upper and lower transistors 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 is connected with the controller 27 so that a switching on or off signal is transmitted to each of the transistors. For simplicity, only the connections between transistors 30, 31, 32, 33 and the controller 27 are shown in Fig. 2.

The current limit circuits 60 are interposed between the controller 27 and the upper transistors 30, 32. The current limit circuits 60 are composed of AND gates 61, 64, comparators 62, 65, and schmitt trigger circuits 63, 66 respectively. The comparator 62 compares the current value of the first coils 16a, 16b detected by the current sensor 42 with a comparison current value which is determined by the controller 27 and which is converted by a D/A converter 67. The output signal of the comparator 62 is fed to the AND gate 61 through the schmitt trigger circuit 63. The switching on or off signal from the controller 27 is fed to the AND gate 61 and the output signal of the AND gate 61 is fed to the base terminal of the upper transistor 30.The comparator 65 compares the current value of the second coils 17a, 17b detected by the current sensor 43 with the comparison current value which is determined by the controller 27 and which h is converted by the D/A converter 67. The output signal of the comparator 65 is fed to the AND gate 64 through the schmitt trigger circuit 66. The switching on or off signal from the controller 27 is fed to the AND gate 64 and the output signal of the AND gate 64 is fed to the base terminal of the upper transistor 32. It is the purpose of these current limit circuits 60 to prevent the switching on signal from the controller 27 from being fed to the base terminals of the upper transistors 30 and 32 respectively in the case that the current values of the first and second coils 16a, 16b, 17a, 17b are larger than the comparison current values.Similarly, current limit circuits (not shown) are interposed between the controller 27 and the base terminals of the upper transistors 34, 36, 38, 40 in the same manner.

The delay circuit 70 is interposed between the controller 27 and the AND gate 61 and is composed of a resistance 71 and a condenser 72. Accordingly, in response to the magnitude of the resistance 71 and the capacity of the condenser 72, the delay circuit delays the timing of the switching on or off signal which is fed to the AND gate 61 for a certain amount of time. The delay circuit 70' is interposed between the controller 27 and the base terminal of the lower transistor 31 and is composed of a resistance 74 which has the same magnitude as the resistance 71 and a condenser 75 which has the same capacity as the condenser 72. Thereby, the delay circuit 70' delays the timing of the switching or or off signal which is fed to the base terminal of the lower transistor 31 for the same amount of time. In this embodiment, schmitt trigger circuits 73, 76 are interposed between the delay circuit 70 and the AND gate 61 and between the delay circuit 70' and the base terminal of the lower transistor 31 in order to avoid the influence of noise.

The same delay circuit (not shown) and same schmitt trigger circuit (not shown) are interposed between the controller 27 and each of the AND gates (not shown) connected with each of the base terminals of ,the upper transistors 34, 38 and between the controller 27 and each of the base terminals of the lower transistors 35, 39 in the same manner.

The above described embodiment of the switched reluctance motor 10 operates as follows: when it is detected by the rotation sensor 26 that the rotor 22 is in a position determined by the controller 27 as the position when the current begins to be supplied to one of the first and second coils 16a, 16b, 17a, 17b; 18a, 18b, 19a, 19b; 20a, 20b, 21a, 21b wound on stator pole portions 13a, 13b; 14a, 14b; 15a, 15b, the controller 27 transmits a switching on signal to the drive circuit 28.For example, when it is detected by the rotation sensor 26 that one of the two pairs of rotor pole portions 24a, 24b; 25a, 25b is in a predetermined position with respect to the stator pole portions 13a, 13b, at which the respective pole portions begin to be opposed, the controller 27 transmits a switching on signal to each of the base terminals of the upper and lower transistors 30, 31, 32, 33. Simultaneously, the controller 27 determines a comparison current value and transmits it to the D/A converter 67. Since the detector current value of the current sensor 43 is smaller than the comparison current value, the switching on signals from controller 27 are fed to the base terminal of the upper transistor 32 through the AND gate 64 as well as to the base terminal of the lower transistor 33.Thereby, both the upper and lower transistors 32, 33 become switched ON and current is supplied from the battery 29 to the second coils 17a, 17b, so that the stator pole portions 13a, 13b are magnetized and a magnetic flux is generated between the stator pole portions 13a, 13b. A magnetic attracting force results between the rotor pole portions and the stator pole portions 13a, 13b which are opposite to each other and a torque acts on the rotor 22 by a component of the magnetic attracting force tending to pull the rotor pole portions into alignment with the stator pole portions 13a, 13b.When the detected current value of the current sensor 43 is larger than the comparison current value, the current limit circuit 60 prevents the switching on signals from controller 27 from being fed to the base terminal of the upper transistor 32 through the AND gate 64 and thereby the upper transistor 32 becomes switched OFF so that current is not supplied from the battery 29 to the second coils 17a, 17b.

Thereby, the field energy which is stored in the second coils 17a, 17b is circulated in the closed loop constituted by the lower transistor 33, the diode 57 and the second coils 17a, 17b. Therefore, the stator pole portions 13a, 13b are maintained in the above magnetized condition so far as the stored field energy is decreased.

When the stored field energy is decreased by the inductance of the second coils 17a, 17b and the detected current of the current sensor 43 is smaller than the comparison current value, the switching on signal from the controller 27 is fed to the base terminal of the upper transistor 32. Thereby, the upper transistor 32 becomes switched ON and current from the battery 29 is supplied to the second coils 17a, 17b again. When the detected current value of the current sensor 43 is larger than the comparison current value, it is prevented by the current limit circuit 60. While the switching on signals are output from the controller 27, the above mentioned operation of the current limit circuit 60 is repeated and the current supplied to the second coils 17a, 17b is controlled.

On the other hand, the switching on signals from the controller 27 are not immediately fed to the base terminals of the upper and lower transistors 30, 31 but are delayed for a certain amount of time by the delay circuits 70, 70' and then the switching on signals from the controller 27 are fed to the base temrinals of the upper and lower transistors 30, 31. When the certain amount of time has elapsed, the switching on signal is fed to the base terminal of the lower transistor 31.

Simultaneously, since the detected current value of the current sensor 42 is smaller than the comparison current value, the switching on signals from controller 27 are fed to the base terminal of the upper transistor 30 through the AND gate 61. Thereby, the upper and lower transistors 30, 31 become switched ON and current is supplied from the battery 29 to the first coils 16a, 16b so that the stator pole portions 13a, 13b are further magnetized and a magnetic flux is generated between the stator pole portions 13a, 13b to add to the magnetic flux generated by the second coils 17a, 17b. As a result, the magnetic attracting force generated between the rotor pole portions and the stator pole portions 13a, 13b which are opposite to each other is increased and the torque acting on the rotor 22 is increased.When the detected current of the current sensor 42 is larger than the comparison current value, the current limit circuit 60 prevents the switching on signals from controller 27 from being fed to the base terminal of the upper transistor 30 through the AND gate 61 and thereby the upper transistor 30 becomes switched OFF so that current is not supplied from the battery 29 to the first coils 16a, 16b. Thereby, the stored field energy which is stored in the first coils 16a, 16b is circulated in the closed loop constituted by the lower transistor 31, the diode 54 and the first coils 16a, 16b. Therefore, the stator pole portions 13, 13b are maintained in the above magnetized condition by the first coils 16a, 16b so far as the stored field energy is decreased.When the stored field energy is decreased by the inductance of the first coils 16a, 16b and the detected current value of the current sensor 42 is smaller than the comparison current value, the switching on signal from the controller 27 is fed to the base terminal of the upper transistor 30. Thereby, the upper transistor 30 becomes switched ON and the current from the battery 29 is supplied to the first coils 16a, 16b again. When the detected current value of the current sensor 42 is larger than the comparison current value, the current limit circuit 60 prevents the switching on signals from controller 27 from being fed to the base terminal of the upper transistor 30 through the AND gate 61.While the switching on signals are output from the controller 27, the above mentioned operation of the current limit circuit 60 is repeated and the current supplied to the first coils 16a, 16b is controlled.

When the rotor 22 has been rotated by the torque and it is detected by the rotation sensor 26 that the rotor 22 is in a position determined by the controller 27 as the position when current should cease to be supplied to one pair of the first and second coils 16a, 16b, 17a, 17b; 18a, 18b, 19a, 19b; 20a, 20b, 21a, 21b wound on stator pole portions 13a, 13b; 14a, 14b; 15a, 15b, the controller 27 transmits a switching off signal to the drive circuit 28. For example, when it is detected by the rotation sensor 26 that one of the two pairs of rotor pole portions 24a, 24b; 25a, 25b is positioned in a predetermined position with respect to the stator pole portions 13a, 13b, the controller 27 transmits a switching off signal towards each of the base terminals of the upper and lower transistors 30, 31, 32, 33.

Thereby, the upper and lower transistors 32, 33 become immediately switched OFF and the current supplied to the second coils 17a, 17b is stopped. Thereby, the magnetic attracting force by the second coils 17a, 17b disappears.

On the other hand, it is delayed for a certain amount of time by the delay circuits 70, 70' that the upper and lower transistors 30, 31 become switched OFF. Thereby, even though the switching off signal is transmitted to the drive circuit 28, the magnetic attracting force does not disappear in a moment. When the certain amount of time has elapsed, the upper and lower transistors 30, 31 become switched OFF and the current supplied to the first coils 16a, 16b is stopped. Thereby, the magnetic attracting force by the first coils 16a, 16b disappears.

Now, the supply current which is supplied to the first and second coils 18a, 18b, l9a, 19b; 20a, 20b, 21a, 21b wound on the stator pole portions 14a, 14b; 15a, 15b is controlled by the controller 27 and the drive circuit 28 in the same manner. As mentioned above, the current which is supplied to the first and second coils wound on a pair of the stator pole portions opposing a pair of rotor pole portions is switched on and off such as a stepped shaped pulse and a certain motoring torque is obtained by the action of the above magnetic attracting force. Fig. 3 shows variations of the torque, the current and the magnetic attracting force when current is supplied as described above to the first and second coils which are wound on a pair of stator pole portions.

A pair of magnetized stator pole portions which is opposite a pair of rotor pole portions is attracted to the opposing rotor pole portions by the magnetic attracting force and thereby the stator 12 is strained.

For example, in Fig. 1, a pair of stator pole portions 13a, 13b which is opposed to a pair of rotor pole portions l9a, 19b is magnetized by supplying current to the first and second coils 16a, 16b; 17a, 17b and is attracted to a pair of rotor pole portions l9a, 19b. As a result, the stator 12 is strained so that the diameter of the stator 12 is shortened in the vertical direction in Fig. 1. When the magnetic attracting force disappears by the switching off of the current, the strain of the stator 12 is reduced. At this time, according to this embodiment, since the magnetic attracting force disappears in steps as shown in Fig. 3, the strain of the stator 12 is also reduced in steps. Thereby, the impulsive variation of the staor 12 is relieved and therefore the vibration of the housing 11 caused by the vibration of the stator 12 is reduced.Accordingly, the objectionable acoustic noise caused by the vibration of the housing 11 is reduced.

Fig. 4 shows a switching timing shift means of a second embodiment of the present invention. This switching timing shift means is a delay circuit 170 which is comprised of a counter 171 (eg type, ttl 74161) and flipflop circuit 172 (eg type 7474). This delay circuit 170 can replace the delay circuits 70, 70' in Fig. 2. In Fig.

4, the same parts as compared with Fig. 2 are identified by the same reference numerals. Referring to Fig. 4, the switching on signal from the controller 27 is fed to a data terminal 172a of the flip-flop circuit 172.

Furthermore, the switching on signal from the controller 27 is fed to an exclusive OR gate 175 directly and through a delay circuit composed of a resistance 173 and a condenser 174. Thereby, the output signal becomes High for a certain amount of time which is determined by the resistance 173 and the condenser 174. The output signal of the exclusive OR gate 175 is reversed and then is fed to a load terminal 171a, a clear terminal 171b and an exclusive OR gate 176. The reversed output signal of the exclusive OR gate 175 is fed to the exclusive OR gate 176 both directly and through a delay circuit which is composed of a resistance 177 and a condenser 178. The reversed output signal of the exclusive OR gate 176 is fed to an OR gate 179. A rotation signal from the rotation sensor 26 is fed to the OR gate 179.The output signal of the OR gate 179 is fed to the clock terminal 171c of the counter 171. Terminals A, B, C, D are preset terminals for presetting a 4 bit counter value. A clock terminal 172b of the flip-flop circuit 172 is connected with a RCO terminal 171d of the counter 171 and the flipflop circuit 172 outputs the input signal of the data terminal 172a as an output signal of Q terminal 172c when an overflow signal of the counter (= delay signal) is fed from the RCO terminal 171d to the clock terminal 172b.

The output signal of the Q terminal 172c of the flip-flop circuit 172 is fed to the AND gate 61 in Fig. 2.

According to this embodiment, as shown in Fig. 5, when the switching signal from the controller 27 is fed to the data terminal 172 and to the exclusive OR gate 175, and the time determined by the resistance 173 and the condenser 174 has elapsed, the reversed output signal of the exclusive OR gate 175 (counter reset signal) becomes High and this High signal is fed to the exclusive OR gate 176. However, the reversed output signal of the exclusive OR gate 176 (counter enable signal) becomes Low for a certain amount of time determined by the resistance 177 and the condenser 178. Therefore, the output signal of the OR gate 179 (delay count) corresponding to the rotation signal is fed to the clock counter 171c.The counter 171 continues to count until the counter enable signal becomes High. therefore, it is necessary to increase the time constant of the resistance 177 and the condenser 178 so as to maintain the counter enable signal in the Low condition for a necessary delay time. When the counter overflows, the counter overflow signal is fed to the clock terminal 172b and then the switching on signal is fed to the AND gate 61 in Fig. 2. As shown in Fig. 5, in case of the switching signal, the supply timing is delayed in the a same manner.

As mentioned above, in this embodiment since one of the switching on-off timings for the first and second coils is delayed as against the other by the counter 171 and the flip-flop circuit 172 and therefore the magnetic attracting force disappears in steps, it is possible to obtain the same effects as the above first embodiment.

Furthermore, in this embodiment, it is possible to change the delay time at will by DIP switches or by the signal of the controller.

Fig. 6 shows a current switching means and a switching timing shift means of a third embodiment of the present invention. In Fig. 6, the same parts as compared with Fig. 1 and Fig. 2 are identified by the same reference numerals. Referring to Fig. 6, a switching timing shift means of the present invention is composed of a controller 227. The controller 227 has a first control map for determining a switching on-off timing of the current supplied to the first coils 16a, i6b; 18a, 18b; 20a, 20b in response to the rotational speed and the torque (= the demand of output). Furthermore, the controller 227 has a second control map for determining a switching on-off timing of the current supplied to the second coils 17a, 17b; 19a, l9b; 21a, 21b in response to the rotational speed and the torque (= the demand of output).For example, the switching on-off timing determined by the first control map is delayed for a certain amount of time as against the switching on-off timing determined by the second control map under the same value of the parameters (the rotational speed and the torque).

According to this embodiment, when it is detected by the rotation sensor 26 that the rotor is in a position determined by the second control map of the controller 227 as the position where the current begins to be supplied to one of the second coils 17a, 17b; 19a, 19b; 21a, 21b wound on the stator pole portions, the controller 227 transmits a switching on signal towards the base terminals of the upper and lower transistors 32, 33. Thereby, the transistors 32, 33 become switched ON and current is supplied to the second coils 17a, 17b.

Afterwards the current supplied to the second coils 17a, 17b is controlled as mentioned in the description of the first embodiment. When the certain amount of time has lapsed after the switching on signals are transmitted to the transistors 32, 33 and it is detected by the rotation sensor 26 that the rotor is in a position determined by the first control map of the controller 227 as the position when the current begins to be supplied to one of the first coils 16a, 16b, the controller 227 transmits a switching on signal towards the base terminals of the upper and lower transistors 30,31. Thereby, the transistors 30,31 become switched ON and current is supplied to the first coils 16a, 16b. Afterwards the current supplied to the first coils 16a, 16b is controlled as mentioned in the above description of the first embodiment.When the rotor is rotated by the torque and it is detected by the rotation sensor 26 that the rotor is in a position determined by the second control map of the controller 227 as the position when the current supplied to the second coils 17a, 17b is stopped, the controller 227 transmits a switching off signal towards the base terminals of the transistors 32, 33.

Afterwards, when the certain amount of time has elapsed and it is detected by the rotation sensor that the rotor is in a position determined by the first control map of the controller 227 as the position when the current supplied to the first coils 16a, 16b is stopped, the controller 227 transmits a switching off signal towards the base terminals of the transistors 30, 31. Now, the supply current which is supplied to the first and second coils 18a, 18b, 19a, l9b; 20a, 20b, 21a, 21b is controlled by the controller 227 and the drive circuit 28 in the same manner.

According to this embodiment, since the switching on-off timing of the current supplied to the first coils is delayed as against that of the second coils by the first control map of the controller 227, it is possible to obtain the same effects as the above first embodiment.

In the above mentioned embodiments the present invention is applied to a switched reluctance motor which includes a stator having three pairs of stator pole portions and a rotor having two pairs of rotor pole portions. However, It is possible to apply the present invention to other types of switched reluctance motors, for example a switched reluctance motor which includes a stator having six pairs of stator pole portions and a rotor having four pairs of rotor pole portions.

As mentioned above, according to the present invention, since the magnetic attracting force disappears in steps, the strain of the stator is also reduced in steps.

Thereby, the impulsive variation of the stator is relieved and therefore the vibration of the housing caused by the vibration of the stator is reduced.

Accordingly, it is possible to reduce the objectionable acoustic noise caused by the vibration of the housing.

Claims

CLAIMS:

1. A switched reluctance motor comprising: a housing having an inner bore extended in the axial direction, a stator fixed in the inner bore of the housing and having a plurality of pairs of opposing stator pole portions which project inwards in the diametrical direction and which extend in the axial direction, a rotor rotatably disposed in the stator and having a plurality of rotor pole portions which project outwards in the diametrical direction and which extend in the axial direction a plurality of first coils wound on pairs of the stator pole portions, a plurality of second coils wound on pairs of the stator pole portions, a current switching means for switching on and off current which is supplied to each of the first and the second coils, and a switching timing shift means for shifting the timing of switching on and off the current supplied to the first coils relative to the timing of switching on and off the current supplied to the second coils.

2. A switched reluctance motor as recited in claim 1, wherein said current switching means has a plurality of first switching elements each connected in series with one of the first coils and a plurality of second switching elements each connected in series with one of the second coils, and wherein said switching timing shift means has a plurality of delay circuits, each of which is connected with a respective one of the first switching elements.

3. A switched reluctance motor as recited in claim 2, wherein each said delay circuit includes a resistance and a condenser.

4. A switched reluctance motor as recited in claim 2, wherein each said delay circuit includes a counter and a flip-flop circuit.

5. A switched reluctance motor as recited in claim 1, wherein said current switching means has a plurality of first switching elements each of which is connected in series with one of the first coils, a plurality of second switching elements each of which is connected in series with one of the second coils, a first control map for determining the timing of switching on and off the current supplied to the first coils and a second control map for determining the timing of switching on and off the current supplied to the second coils, and wherein the switching timing determined by the first control map is delayed relative to the switching timing determined by the second control map.