GB2393587A

GB2393587A - Damping stator vibration in brushless dc motor

Info

Publication number: GB2393587A
Application number: GB0222441A
Authority: GB
Inventors: Michael John Werson
Original assignee: Automotive Motion Technology Ltd
Current assignee: Automotive Motion Technology Ltd
Priority date: 2002-09-27
Filing date: 2002-09-27
Publication date: 2004-03-31
Also published as: GB0222441D0

Abstract

An electric motor including a rotor (10) and a stator (12), the rotor (10) being adapted to rotate around the stator (12), the stator (12) being mounted on a mounting member (14) between a base (14b) and a retaining member (20) which act to limit axial movement of the stator (12) with respect to the mounting member (14), the motor further including a spacer (16, 22) which is located between either the base (14b) and the stator (12) or the stator (12) and the retaining member (20), the spacer (16, 22) including two outer layers separated by a layer of visco-elastic material, the outer layers each comprising at least one generally rigid element.

Description

PATENTS ACT 1977

A106 13GB-DJL/ACL

Title: Electric Motor Description of Invention

The present invention relates to an electric motor, and more particularly to an electric motor in which a stator of an electric motor is mounted on a generally cylindrical spigot, the spigot extending through a generally cylindrical aperture which extends axially through the stator.

In order to retain the stator on the spigot, it is known to provide a steel spring clip which is placed around the spigot to engage with the top of the stator, and which engages with the spigot, substantially to prevent axial movement of the stator with respect to the spigot. A disadvantage of such a system is that as the motor increases in temperature during use, the length of the spigot increases slightly, and as a result, the spring clip no longer engages with the top of the stator, and some axial movement of the stator with respect to the spigot may occur.

This is a particular problem in electrically commutated motors such as brushless DC or switched reluctance motors. In such motors, the torque exerted by coils in the stator on the rotor pulses as the rotor rotates. Thus, the equal and opposite reaction force exerted on the stator also pulses, and as a result, if some movement of the stator is permitted, the stator vibrates with vibration pulses being set up. These vibration pulses are transmitted through the spigot to the motor casing, and this increases the noise generated by the motor to an undesirable level. The problem is exacerbated if the motor is adapted such that the rotor rotates at high speeds, for example, where the motor is used in an application such as driving a steering pump in an automotive vehicle.

According to the invention, we provide an electric motor including a rotor and a stator, the rotor being adapted to rotate around the stator, the stator being mounted on a mounting member between a base and a retaining member

which act to limit axial movement of the stator with respect to the mounting member, the motor farther including a spacer which is located between either the base and the stator or the stator and the retaining member, the spacer including two outer layers separated by a layer of visco-elastic material, the outer layers each comprising at least one generally rigid element.

The presence of such a spacer reduces transmission of any vibration pulses from the stator to the stator mounting assembly and the motor housing, and hence reduces the level of noise generated by the motor in use. Moreover, the visco-elastic material may be slightly compressed when mounting the stator and retaining member on the spigot such that, when the length of the mounting member increases as the temperature of the motor increases, the spacer may expand to fill the resulting gap. Thus, the presence of such a spacer may also reduce vibration of the stator.

Preferably the motor includes two spacers one of which is located between the base arid the stator and the other between the stator and the retaining device, the spacers each including two outer layers separated by a layer of visco-elastic material, the outer layers each including at least one generally rigid element.

The presence of a second spacer further reduces vibration of the stator and transmission of vibration pulses from the stator to the stator mounting assembly and motor housing, and hence further reduces the level of noise generated by the motor in use.

Preferably the motor is an electrically commutated motor.

The mounting member may be a generally cylindrical spigot. In which case, preferably the or each spacer is generally annular.

Embodiments to the invention will now be described, by way of example only, with reference to the accompanying drawings, of which, FIGURE 1 is an illustration of a longitudinal cross-section through an electric motor according to the invention,

( FIGURE 2 is an illustration of a transverse cross-section of the stator shown in Figure 1.

Referring now to the Figures, there is shown an electric motor with a rotor 10 mounted for rotation around a stator 12. The motor is an electrically commutated motor, such as a brushless DC motor or a switched reluctance motor. The stator 12 is elongate with a generally annular transverse cross-

section, and is mounted on a mounting member 14, which is this example is a generally cylindrical spigot 14, the spigot 14 extending generally centrally through the stator 12.

The spigot 14 is generally cylindrical with a longitudinal axis A, which encloses a motor shaft 11, and is rigidly connected to or integral with a base 14b which extends radially outwardly from a lower end of the spigot 14. The base 14b may be integral with a housing of the motor, or may be connected to a motor housing.

The rotor 10 is mounted on the motor shaft 11 for rotation around the stator 12, and the motor shaft 11 extends along axis A and is mounted in a bearing 1 la for rotation about its longitudinal axis A. Rotation of the stator 12 with respect to the spigot 14 is prevented by means of a retaining member, which in this example is an axially extending roll-pin 18. An exterior wall 14a of the spigot 14 includes a generally semi-

circular groove 14', the groove 14' extending generally parallel to the longitudinal axis A of the spigot 14. A corresponding generally semicircular groove 12' is also provided in an inside surface of the stator 12, and when the stator 12 is mounted on the spigot 14, the groove 12' also extends generally parallel to the longitudinal axis A ofthe spigot 14.

The two grooves 14', 12' are aligned such that together they form a passage between the stator 12 and spigot 14 with a generally circular cross-

section, and the roll-pin 18 is located in this passage. The roll-pin 18 has an

appropriate diameter substantially to fill the passage, and thus prevents rotation of the stator 12 with respect to the spigot 14.

Whilst in this example the retaining member is a roll-pin 18, and thus the passage between the stator 12 and the spigot 14 has a generally circular cross-

section, it should be appreciated that this need not be the case, and the passage and retaining member 16 may have a triangular, square or other appropriately shaped cross-section. Alternatively, any other antirotation feature may be used. Axial movement ofthe stator 12 with respect to the spigot 14, i.e. sliding of the stator 12 up and down on the spigot 14, is substantially prevented by means of a retaining device 20, which in this example is a generally annular steel spring clip 20, which is located around the spigot 14. A circlip or star washer or any other similar retaining device may be used in place of the spring clip 20. The spring clip 20 engages with the cylindrical wall 14a of the spigot 14 such that movement of the spring clip 20 relative to the spigot 14 is substantially prevented.

The motor 10 further includes a first generally annular spacer 16 which is mounted around the cylindrical spigot 14, and which is located between the stator 12 and the base 14b such that the stator 12 rests on the first spacer 16.

In this example, the motor also includes a second spacer 22 which, like the first spacer 16, is mounted around the cylindrical spigot 14, and which is located between the spring clip 20 and the stator 12.

Both spacers 16, 22 have a layered structure and include two outer layers separated by a layer of visco-elastic material, the outer layers each being a metallic sheet. The layers extend generally normal to axis A such that for the first spacer 16, the stator 12 rests on one outer layer, and the other outer layer rests on the base 14b, and for the second spacer 22, one outer layer rests on the stator 12, and the other engages with the spring clip 20. The spacers 16, 22 may, for example be made from BONDAL.

: When the motor is used, the torque exerted by coils in the stator 12 on the rotor pulses. Thus the equal and opposite torque exerted by the rotor on the stator 12 also pulses, and this can cause the stator to vibrate, and vibration pulses to be set up.

The layered spacers 16, 22, in particular the lower spacer 16, have been found to assist in damping such vibrations of the stator 12, and to reduce transmission of the vibrations from the stator 12 to the motor housing.

Moreover, the spring clip 20 is mounted on the spigot 14 such that at room temperature, the visco-elastic material in the spacers 16, 22 is compressed. If the temperature of the motor increases during use, some increase in the separation between the spring clip 20 and the base 14b due to thermal expansion of the spigot 14 can be accommodated by expansion of the visco-

elastic material in the spacers 16, 22, without the formation of a gap between the second spacer 22 and the spring clip 20. Thus, the stator 12 is generally held firmly between the spring clip 20 and the base 14b even whilst the motor is in use.

Thus an electric motor according to the invention is less noisy than a prior art arrangement.

Various modifications may be made to the motor within the scope of the invention. For example, the mounting member 14 need not be a generally cylindrical spigot. It may instead be an elongate element with any other shaped transverse cross-section. For example, the mounting member 14 may have a generally square transverse cross-section, in which case the aperture in the stator may have a corresponding square cross-section, thus making it unnecessary to provide an anti-rotation device such as a roll-pin to prevent rotation of the stator 12 about the mounting member 14.

It is not necessary to provide two spacers 16, 20. A single spacer 22 located between the spring clip 20 and the stator 12 would assist in reducing the

( possibility of the stator vibrating in use, and a single spacer 16 located between the base 14b and the stator 12 would assist in reducing the possibility of the stator vibrating in use and in reducing transmission of any vibrations from the stator to the motor housing.

The spacers 16, 22 need not be made from a material such as BONDAL(D and the outer layers need not be made from metal sheet. The outer layers may, for example, be made from or include a generally rigid plastic sheet, or may include a plurality of generally rigid elements.

The motor may be a non-electrically commutated motor such as a brushed DC motor.

The features disclosed in the foregoing description, or the following

claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

( CLAIMS

1. An electric motor including a rotor and a stator, the rotor being adapted to rotate around the stator, the stator being mounted on a mounting member between a base and a retaining member which act to limit axial movement of the stator with respect to the mounting member, the motor further including a spacer which is located between either the base and the stator or the stator and the retaining member, the spacer including two outer layers separated by a layer of visco-elastic material, the outer layers each comprising at least one generally rigid element.

2. A motor according to claim 1 wherein the motor includes two spacers one of which is located between the base and the stator and the other between the stator and the retaining device, the spacers each including two outer layers separated by a layer of visco-elastic material, the outer layers each including at least one generally rigid element.

3. A motor according to claim 1 or 2 wherein the motor is an electrically commutated motor.

4. A motor according to any preceding claim wherein the mounting member is a generally cylindrical spigot.

5. A motor according to claim 4 wherein the or each spacer is generally annular.

6. A motor substantially as hereinbefore described with reference to and/or as shown in the accompanying drawings.

(

7. Any novel feature or novel combination of features hereinbefore described or shown in the accompanying drawings.