GB2353902A

GB2353902A - Canned magnet structure for motor

Info

Publication number: GB2353902A
Application number: GB9920321A
Authority: GB
Inventors: Xuefeng Wu; Tetsumi Watanabe
Original assignee: Aisin Seiki Co Ltd
Current assignee: Aisin Corp
Priority date: 1999-08-28
Filing date: 1999-08-28
Publication date: 2001-03-07
Also published as: JP2001069699A; GB9920321D0

Abstract

An array of permanent magnets 13 is retained on a cylindrical rotor body 11 by a carbon fibre can assembly. Each magnet has a depression 14 at the centre section such that differential interference is established between the magnets and the can assembly, significant high pressure zones being established between the can assembly and the magnet at the axial ends thereof and a significant low pressure zone is established at the axial central portion. The depressions may be shaped so as to provide a continuous depressed band about the rotor and the transition 17 is designed with a sufficient length to avoid stress. The can may comprise two sections 19,20.

Description

3r, TITLE Rotor f or permanent magnet motor

DESCRIPTION

Field of the Invention The invention relates to an improvement in the structure for attaching permanent magnets of a rotor for a motor.

Description of the prior art

A conventional rotor for a permanent magnet motor is disclosed in USA-4,433,261. Referring to Figure 5,a rotor 61 is constructed in such a manner that permanent magnets 62 are adhered around a rotor shaft 63 with a adhesive (not shown) and are secured by a can 64 around the outer periphery thereof as well as by end plates 65 and 66 at the axial ends thereof. The end plates 65 and 66 play a very important role to hold the permanent magnets 62 as well as the can 64 and prevent the can 64 from moving outwards. Any component which moves will cause loss of the dynamic balance of the rotor and will thus cause a large vibration. Because of the necessity of having two end plates, there are the following disadvantages:

(a) the end plates increase the weight and the axial length of the motor rotor. They are not good f or rotor dynamics; (b) it is expensive to manufacture the rotor in order to control the correct interference between the shafts and the end plates, to eliminate the gaps between the end plates and the cans; (c) eddy current losses are expected in the end plates, reducing the efficiency of the motor.

Summary of the Invention

It is an object of the invention to omit any end plate from a rotor of a permanent magnet type motor.

The invention provides a rotor for a permanent magnet motor, comprising: a cylindrical rotor body; permanent magnets located around the periphery of the cylindrical rotor body, each magnet having a depression at an intermediate axial zone along its outer surface; and a can or can assembly surrounding the magnets to retain them on the rotor body, and establis hing different interference fits with the depressions in the magnets and the non-depression end portions of the magnets axially outwardly of the depressions.

Description of the Drawings

Figure 1 is an axial section through a rotor for a permanent magnet motor according to an embodiment of the invention; Figure 2 is an enlarged overview of a magnet and can assembly of the rotor of Figure 1; Figure 3 is a left half section of the magnet of Figure 1 and a graph of a pressure variation applied to the magnet at each axial position; Figure 4 is a section of the magnet when the magnet is deformed under the motor operation; and Figure 5 is a similar view to Figure 1, but showing the prior art rotor of USA-4433261.

Detailed Description of the Invention

Figure 1 shows an embodiment of a rotor of a permanent magnet motor. A cylindrical rotor body 11 is fixed to a rotor shaft (not shown) in such a manner that the rotor shaft is tightly inserted into a through hole 12 of the cylindrical rotor body 11. Permanent magnets 13, each of which is a sector of a cylinder are assembled together as a complete cylinder located around the outer periphery of the cylindrical rotor body 11. The number of sectors of the permanent magnets 13 depends on the number of poles of the motor.

Referring to Figure 2, a shallow depression 14 is formed around the outer periphery of the permanent magnets 13 at a central axial zone between the opposite axial ends. In Figure 2 the depressions of the different magnets link together so that in the assembled rotor the depressions form a continuous shallow annular depression around the entire magnet assembly. Alternatively, however, the depressions may be discrete indentations or depressions in the outer surfaces of the individual magnets so that when assembled into the complete rotor assembly there is provided a circular array of shallow dimple-like depressions around the magnet assembly. The full diameter portions of the magnets 13 at the opposite axial ends of each depression 14 will be referred to as non-depression portions 15 and 16. The depression 14 links smoothly to the non-depression portions 15 and 16 by transition slopes 17 and 18. A can assembly comprising two carbon-fibre can portions 19 and 20 is tightly assembled with a differential interference fit on the magnets 13 so as to retain the magnets 13 around the cylindrical rotor body 11. The interference fit is a differential interference, with the can portions 19 and 20 generally following the contour of the outer periphery of the magnets to establish different interference fits with the non-depression portions 15 and 16 and the depression 14. Theref ore a higher radial interference D1 is established between the can portions 19 and 20 and the magnet 13 at both the non-depression portions 15 and 16 at the axial end sections of the magnets 13, while a lower radial interference D2 is established between the can portions 19 and 20 and the magnets 13 at the depression 14, at the axial centre section of the magnets 13. Because of the -4 differential interference, a characteristic pressure variation at the inner diameter of the magnets 13, where they contact the cylindrical rotor body 11, is established axially along each magnet 13. As shown in Figure 3, high pressure bands are established corresponding to the non- depression portions 15 and 16 next to the transition slopes 17 and 18 at opposite axial ends of the magnet 13 and significant low pressure bands are established corresponding to the axially outer portions of the depression 14 next to the transition slopes 17 and 18 at the centre of the magnet 13. Figure 3 also shows the much smoother pressure variation a xially along the magnets of the prior art which establishes a more uniform interference between the magnets and the can or can assembly.

Because of the high pressure bands the following advantages are established.

(a) The outward radial bending or deformation of the magnets 13 at their axial ends is reduced as shown in Figure 4. This is a 129,.- reduction in comparison with the prior art. This outward radial bending is unfavourable in rotor dynamics, so that the reduced bending is advantageous.

(b) The stress distribution exerted on the can portions 16 and 17 is changed relative to the prior art so that the can portions 16 and 17 are prevented from moving axially outwards. Thus a self-locking mechanism for the can portions 16 and 17 is established, sufficient to permit the end plates of the prior art to be omitted according to the invention.

The maximum equivalent stress occury at the inner diameter of the can portions 16 and 17 and the maximum equivalent stress is reduced because of the differential interference, so that the fatigue life of the rotor is increased. Furthermore, the maximum stress point occurs at two zones each about one quarter of the total axial magnet length from the ends. In contrast, in the prior art the maximum stress point occurs at the centre of the total axial magnet length.

The axial length of each transition slope is preferably larger than a predetermined value, since a smaller axial length of slope means a steeper slope and thus an undesirable stress concentration at the inner diameter of the can or can assembly.

Since the above rotor uses no end plates, the weight and the axial length of the rotor are reduced, so that the manufacturing cost is significantly reduced. The above rotor is applicable for a high speed turbo alternator/motor shown in GB-A-2324913, for example. Such an alternator/motor could be driven at 120,000 rpm.

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Claims

1 A rotor for a permanent magnet motor, comprising: a cylindrical rotor body; permanent magnets located around the periphery of the cylindrical rotor body, each magnet having a depression at an intermediate axial zone along its outer surface; and a can or can assembly surrounding the magnets to retain them on the rotor body, and establishing different interference fits with both the depressions in the magnets and the non-depression end portions of the magnets axially outwardly of the depressions.

2 A rotor according to claim 1, wherein the depression of each magnet links smoothly to non-depression end portions of the magnets by transition slopes.

3 A rotor according to claim 1, wherein the magnets are surrounded by a can assembly axially divided into two can portions.

4 A rotor according to any preceding claim, wherein the depressions of the different magnets link together so that in the assembled rotor the depressions form a continuous shallow annular depression around the entire magnet assembly.

A rotor for a permanent magnet motor, substantially as described herein with reference to Figures 1 to 4.