GB2293694A

GB2293694A - Vibration damping in switched reluctance motors

Info

Publication number: GB2293694A
Application number: GB9419753A
Authority: GB
Inventors: Anthony David Wearing; Xuefeng Wu; Takeshi Ikeyama
Original assignee: Aisin Seiki Co Ltd
Current assignee: Aisin Corp
Priority date: 1994-09-30
Filing date: 1994-09-30
Publication date: 1996-04-03
Anticipated expiration: 2014-09-30
Also published as: GB2293694B; GB9419753D0; JPH08182276A

Abstract

A switched reluctance motor includes a housing 11 having an inner bore 11a extended in the axial direction, a stator 12 fixed in said inner bore of said housing and having a plurality of pairs of opposing stator pole portions 13 - 15 which project inwards in the diametrical direction and which extend in the axial direction, and a plurality of gaps 21 - 23 extending in the axial direction, a gap being gap being formed between said housing and an outer circumferential surface of said stator at each part of said stator where one of said stator pole portions is formed. As well as reducing strain in the stator and housing the gaps may carry coolant. <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 the 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 which 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.

The current which is supplied to the coil 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 such 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 a pair of rotor pole portions is fully aligned with a 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 or several pairs of stator pole portions are attracted radially 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 simultaneously the housing is 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 THE 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 switch reluctance motor comprising: a housing having an inner bore extended in the axial direction, a stator fixed in said inner bore of said 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 said 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 coils, wound on said pairs of stator pole portions, and a plurality of gaps extending in the axial direction, one of said gaps being formed between said housing and an outer circumferential surface of said stator at each part of said stator where one of said stator pole portions is formed.

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 perspective view of a housing, a stator and a rotor of a first embodiment of a switched reluctance motor in accordance with 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 a coil of a first embodiment of a switched reluctance motor in accordance with the present invention;; Fig. 4 is a top plan view of a model of a housing and a stator which is used for Finite Element Method of a first embodiment of a switched reluctance motor in accordance with the present invention; Fig. 5 is a graph which shows the radial movement of the outer circumferential surface of a housing of a model shown in Fig. 4 as a result of Finite Element Method; and Fig. 6 is a perspective view of a housing, a stator and a rotor of a second embodiment of a switched reluctance motor in accordance with 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 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 cylindrical housing 11 which is made of aluminium. In an inner bore lla of the housing 10, a cylindrical stator 12 is disposed. The stator 12 is formed by laminating of electromagnetic steel plates and is fixed at its outer circumferential portion to the inner bore lla of the housing 121 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, coils 16a, 16b are wound respectively and are connected in series with each other. Coils (not shown) 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 26.

In this embodiment, grooves 21a, 21b, 22a, 22b 23a, 23b extending in the axial direction are formed on an outer circumferential surface of the stator 12 in areas where the stator pole portions 13a, 13b; 14a, 14b; 15a, 15b; are formed. Namely, each of the grooves 21a, 21b, 22a, 22b, 23a, 23b is formed so as to be aligned radially with each of the stator pole portions 13a, 13b; 14a, 14b; 15a, 15b, respectively. Further, the width of the grooves is greater than the width of the bases of the stator pole portions in the circumferential direction.

A rotor 17, which is formed by laminating of 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 17 and an output shaft 18 which is rotatably supported on side housings (not shown) at both ends is fixed in the hole of the rotor 17. Thereby, the rotor 17 is able to rotate with the output shaft 18 in the stator 12. Furthermore, the rotor 17 is provided with two pairs of opposing rotor pole portions 19a, 19b; 20a, 20b 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 19a, 19b; 20a, 20b 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 17 is rotated.

A well known rotation sensor 24, such as an encoder or a resolver, is disposed on the end (not shown) of the output shaft 18 in order to detect the rotation position of the rotor 17. The rotation sensor 24 is electrically connected to a controller 25 and therefore a position signal and an angle signal detected by the rotation sensor 24 are transmitted to the controller 25.

The controller 25 is electrically connected to the drive circuit 26 to which the coils wound on each of the stator pole portions 13a, 13b; 14a, 14b; 15a, 15b are connected and transmits an output signal to the drive circuit 26 in response to a position signal and an angle signal of the rotation sensor 24. The drive circuit 26 is composed of an inverter using switching elements, such as transistors or thyristors and supplies current, which may be in the form of a pulse, to each of the coils in response to the output signal of the controller 25.

The above described embodiment of the switched reluctance motor 10 operates as follows: When it is detected by the rotation sensor 24 that the rotor 17 is in a predetermined position in which one of the two pairs of rotor pole portions 19a, 19b; 20a, 20b begins to be opposed to one of the three pairs of stator pole portions 13a, 13b; 14a, 14b; 15a, 15b, the controller 25 transmits an output signal responding to the detected signal of the rotation sensor 24 to the drive circuit 26. The drive circuit 26 supplies current to the coils which are wound on the pair of stator pole portions opposing the pair of rotor pole portions in response to the output signal of the controller 25.

Thereby, the stator pole portions on which these coils are wound are magnetized and a magnetic flux is generated between the magnetized stator pole portions. A magnetic attracting force results between the rotor pole porions and the stator pole portions which are opposing each other and a torque acts on the rotor 17 by a component of the magnetic attracting force tending to pull the rotor pole portions to exactly opposite the stator pole portions.

When the rotor 17 is rotated by the torque and it is detected by the rotation sensor 24 that the rotor 17 is in a predetermined position in which the pair of rotor pole portions is just before a position exactly opposite the pair of magnetized stator pole portions, that is to say, it is detected by the rotation sensor 24 that the rotor 17 is in a final effective position in which the above component force acts on the rotor 17, the drive circuit 26 stops supplying the current to the coils wound on the magnetized stator pole portions in response to an output signal of the controller 25. As mentioned above, the current which is supplied to the coils wound on a pair of the stator pole portions opposing a pair of rotor pole portions is switched on and off as a 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 during the above supply of the current to the coils which are wound on a pair of stator pole portions. The above on-off timing of the supply of the current is determined in response to the demand of the rotational speed or the torque of the switched reluctance motor.

On the other hand, when a pair of magnetized stator pole portions is attracted to an opposing pair of rotor pole portions by the above magnetic attracting force, the stator 12 and the housing 11 are thereby strained. For example, in Fig. 1, a pair of stator pole portions 13a, 13b which is opposite to a pair of rotor pole portions 19a, 19b, is magnetized by supplying current to the coils 16a, 16b and is attracted to a pair of rotor pole portions 19a, 19b. As a result, the stator 12 and the housing 11 are strained so that the diameter of the stator 12 and the housing 11 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 reduces suddenly and simultaneously the housing 11 is pressed outwards in the diametrical direction by the stator.Thereby, impulsive variation of the housing 11 is generated periodically by the magnetization of each of the plurality of pairs of stator pole portions 13a, 13b; 14a, 14b; 15a, 15b.

In this embodiment, the grooves 21a, 21b, 22a, 22b, 23a, 23b having a width that is wider than that of the base of each of the stator pole portions in the circumferential direction and extending in the axial direction are formed on an outer circumferential face of the parts of the stator 12 so as to be radially aligned with each of the stator pole portions 13a, 13b, 14a, 14b, 15a, 15b, respectively. The above mentioned impulsive variation of the housing 11 is reduced by these grooves 21a, 21b, 22a, 22b, 23a, 23b. The effect of these grooves is explained on the basis of the results of structural analysis using the Finite Element Method as follows: Fig. 4 shows a model of a housing 111 and a stator 112 used for the analysis using the Finite Element Method.As shown in Fig. 4, the model is a 2-D plane stress static structural analysis model and represents one quarter of the housing 111 and the stator 112 with some simplification. The model consists of the two same materials as the above embodiment (aluminium for the housing and steel for the stator). The stator 112 is provided with one stator pole portion 113a having a half width in the circumferential direction and one groove 121a having half width in the circumferential direction.

The width (W) of the groove 121a is equal to the width (W1) of the base of the stator pole portion 113a and the depth of the groove 121a is 1 mm. In addition to this model, two models which are provided with grooves having different widths (W=0.5W1, 1.5W1 respectively) were used for the analysis in order to determine the most effective width for the groove. The inventors of the present invention have analyzed the radial movement of the housing 111 during the application of stress. The stress was half of a stress that is applied to the stator pole portion at the switching off of the current supplied to the coil (in this analysis, 52.92N) and was applied on the point A of the stator pole portion 113a.

Fig. 5 shows the radial movement of the point (point B to C in Fig. 4) at the outer circumferential surface of the housing 111 predicted by this analysis. As is obvious from Fig. 5, when the width (W) of the groove 121a is wider than the width (W1) of the base of the stator pole portion, the radial movement of the point C of the housing 111 is distributed and reduced by the groove 121a. Furthermore, when the relationship between the width (W) of the groove 121a and the width (W1) of the base of the stator pole portion 113a is W=1.5W1, 14% of the radial movement of the point C of the housing 111 is eliminated. With respect to the depth of the groove, the analysis by the inventors made it clear that there is no particular effect where the depth is deeper than 1 mm.

As mentioned above, according to this embodiment, the stress applied to the housing 11 by the sudden release of the strain of the stator 12 is distributed by the grooves 21a, 21b, 22a, 22b, 23a, 23b and thereby the strain of the housing 11 is reduced. Accordingly, the vibration of the housing 11 is reduced and the objectionable acoustic noise is reduced.

Fig. 6 shows a housing 211, a stator 212 and a rotor 217 of a second embodiment of a switched reluctance motor in accordance with the present invention. In this embodiment, grooves 221a, 221b, 222a, 222b, 223a, 223b extending in the axial direction are formed on an inner circumferential surface of the inner bore of the housing 211 opposite areas of the stator 12 where the stator pole portions 213a, 213b; 214a, 214b; 215a, 215b are formed.

Namely, each of the grooves 22 > 221b, 222a, 222b, 223a, 223b is formed so as to be aligned radially with each of the stator pole portions 213a, 213b; 214a, 214b; 215a, 215b, respectively. Further, the width of the grooves is wider than the width of the bases of the stator pole portions in the circumferential direction. This second embodiment is able to obtain the same effects as the above first embodiment.

In the above mentioned two 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, the stress applied to the housing by the sudden release of the strain of the stator is distributed by the grooves and thereby the strain of the housing is reduced.

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

Furthermore, according to the present invention, it is possible to effectively cool the housing and the stator heated by magnetization, if a heat-exchanger fluid such as air, oil or water is supplied to the grooves.

Claims

CLAIMS:

1. A switched reluctance motor comprising: a housing having an inner bore extended in the axial direction, a stator fixed in said inner bore of said 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 said 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 coils wound on said pairs of stator pole portions, and a plurality of gaps extending in the axial direction, one of said gaps being formed between said housing and an outer circumferential surface of said stator at each part of said stator where one of said stator pole portions is formed.

2. A switched reluctance motor as recited in claim 1, wherein each of said gaps is a groove extending in the axial direction and formed on the outer circumferential surface of said stator.

3. A switched reluctance motor as recited in claim 1, wherein each of said gaps is a groove extending in the axial direction and formed on an inner circumferential surface of said inner bore of said housing.

4. A switched reluctance motor as recited in claim 2 or claim 3, wherein the width of each groove is wider than the width of each of said stator pole portions in the circumferential direction.

5. A switched reluctance motor substantially as described herein with reference to Figs. 1 to 5 or Fig.6.