GB2392315A

GB2392315A - Rotor and stator for a permanent magnet motor

Info

Publication number: GB2392315A
Application number: GB0210745A
Authority: GB
Inventors: Leonard Dovey; Nabeel Shirazee; Praveen Choudhary
Original assignee: DELTA INVEST Ltd
Current assignee: DELTA INVEST Ltd
Priority date: 2002-05-10
Filing date: 2002-05-10
Publication date: 2004-02-25
Also published as: GB0210745D0

Abstract

A rotor B of a permanent magnet motor is formed externally of a stator A. The stator comprises soft magnetic material laminations 3 on which are wound coils 4. The rotor incorporates permanent magnets of a hard magnetic material held in place by pole pieces 7 of soft magnetic material. These pole pieces 7 have inwardly extending lower portions 9 which lock below the magnets 1 and also increase the surface area of the rotor B with respect to the stator A. The parts of the rotor B are held in place by an external holding tube 11 secured to the pole pieces 7 by screws 12. A shaft in the central zone can be keyed to the rotor B at both ends by cast non-magnetic flanges 13, having bearings to ensure that the rotor is always centralised. Also disclosed is a rotor within a stator, as in Figure 1.

Description

"Improvements Relating to Permanent Magnet Motors" A conventional

permanent magnet motor incorporates cylindrical bodies, one within the other, one of which comprises a stator provided with coils wound onto soft magnetic material, whilst the other comprises a rotor incorporating 5 permanent magnets. It is an object of this invention to produce such a motor which is of compact design.

According to one aspect of the invention there is provided a cylindrical rotor for location around a stator assembly of a permanent magnet motor, the rotor having a series of hard magnetic bodies with their poles facing 10 circumferentially around the rotor and being separated by soft magnetic material pole pieces, the continuous array of hard magnetic bodies and pole pieces being held in place by an external holding tube of non-magnetic material fixed to the pole pieces.

With this arrangement the hard magnetic bodies are held in the desired 15 locations by the interposed pole pieces which wedge between the hard magnetic bodies to prevent inward movement. Outward movement is prevented by the surrounding holding tube. The holding tube could be fixed to the pole pieces by screws or some form of adhesion or locking device.

Ideally the pole pieces will extend partially below the adjacent hard 20 magnetic body, but leaving a gap below the hard magnetic body and between the inwardly extending ends of the pole pieces, which gap is filled by air or a non-magnetic material. Thus, the pole pieces overlap the hard magnetic bodies and will increase the surface area contact between the pole pieces and the hard magnetic bodies and result in a better usage of the magnetic poles. Preferably 25 the lower portions of the inwardly extending ends of the pole pieces extend closer to one another than the parts nearest to the hard magnetic body, so as to nnaximise the rotor surface area adjacent to the stator.

From a further aspect the invention provides a rotor of circular cross section for location within a stator assembly of a permanent magnet motor, the rotor having an internal keeper body of a soft magnetic material and a series of hard magnetic material bodies of generally rectangular shape in a continuous 5 array around the keeper body, with their poles facing inwardly and outwardly, with the essentially triangular gaps between the ends of adjacent hard magnetic bodies being filled by wedges of nonmagnetic material which hold the hard magnetic bodies in place and which are keyed into the internal keeper body.

The key feature of this design comprises the wedges which hold the hard 10 magnetic bodies in place. In the preferred design, outer pole pieces of a soft magnetic material are located outside each of the hard magnetic bodies and between and held by the wedges, to define the outer cylindrical surface of the rotor. For this purpose, of course, the outer pole pieces will be narrower in width at their outer edges than at their inner edges, so that the wedges lock the pole 15 pieces into place. With a design using the hard magnetic bodies only the side walls of the hard magnetic bodies would have to be tapered down from the inner to the outer surfaces so that the wedges will key them into place. Furthermore, it would be necessary to form the hard magnetic bodies with a curved outer face.

Currently available highly magnetic materials (such as rare-earth magnets) are 20 often difficult to shape and are expensive so that it is undesirable to shave parts off and thus loose the material. The use of outer pole pieces therefore has the particular advantage that these pole pieces can readily be shaped to the desired form both to lock between the wedges and provide the necessary outer curved surface. 25 The wedges themselves could be held in place by screws passing through the wedges and into the internal keeper body. It is preferred that the centre of the keeper body should be hollow. This hollow region could

incorporate fins which will act to fan cooling air through the motor as the rotor rotates. In both designs it is desirable that all comers of the soft magnetic parts should be chamfered to minimise the tendency for intermittent rotational 5 movement during operation.

The invention may be performed in various ways and a preferred example thereof will now be described, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a sectional view through one design of a permanent magnet 10 motor according to the present invention; Figure 2 is a detail of part of the arrangement shown in Figure 1; Figure 3 is a sectional view through a quadrant of an alternative version of a permanent magnet motor in accordance with the invention; and Figure 4 illustrates a method of interconnecting the stator and rotor of the 15 arrangement of Figure 3.

The motor design shown in Figure 1 incorporates a stator A surrounding a central rotor B. The stator A is built up from a number of parts 3 formed from soft magnetic material laminations onto which are wound coils 4 in a conventional manner.

20 The rotor is built up on a central core 5 of soft magnetic material. Around this are distributed a continuous array of permanent magnets 1 of hard magnetic material. On the outer faces of the magnets 1 (whose poles face inwardly and outwardly of the rotor structure) are provided outer pole pieces 7 of soft magnetic material which are shaped on their outer faces to create the outer 25 circumference of the rotor B. AS can be seen particular from Figure 2, the pole pieces 7 (and thus the magnets 1) are held in place by wedges 6 fixed by screws 8 into the central core 5. The pole pieces 7 are suitably tapered so that

they are locked into place by the wedges 6. By this means, the magnets 1 can be constructed as parallel-sided bodies. This is particularly important when using modem hard magnetic materials (such as those of the rare-earth type) which are difficult to shape as they can be of a very hard and/or brittle nature.

5 The central core 5 (acting as an internal keeper body) is formed with an internal cylindrical cavity 2. Straight or angled fins can be formed to project into the hollow interior of the central core 5 which will then act to fan cooling air through the motor as the rotor rotates.

In the arrangement shown in Figure 3 the rotor B is formed externally of 10 the stator A. The stator comprises soft magnetic material laminations 3 on which are wound coils 4 in a conventional manner. The central region 2 of the stator is either empty or comprises non-magnetic material and can incorporate cooling fins.

The rotor incorporates permanent magnets of a hard magnetic material 15 held in place by pole pieces 7 of soft magnetic material. These pole pieces 7 have inwardly extending lower portions 9 which lock below the magnets 1 and also increase the surface area of the rotor B with respect to the stator A. Nevertheless, a gap 10 is provided below the magnet 1 and between the portions 9 of the pole pieces 7 either as air or in the form of a non-magnetic 20 material to prevent short-circuiting of the pole pieces.

The parts of the rotor B are held in place by an external holding tube 11 secured to the pole pieces 7 by screws 12. The method of assembly would be to locate the rotor parts 1 and 7 about a drum (with magnets in the drum to hold the magnets and pole pieces in place). The pre-formed stator A would then be 25 inserted from one end to push out the central drum. At the same time, the outer holding tube 11 would be slid into place outside the rotor B. Finally, the screws 12 would be inserted to hold the tube 1 1 to the pole pieces 7. A shaft 12 (Figure

4) positioned in the central zone can then be keyed to the rotor B at both ends by cast non-magnetic flanges 13 having bearings 14, to ensure that the rotor is always centralised. A fan carried by the flange 13 could act to blow air over the cooling fins in the cavity 2.

5 In both of the arrangements shown in Figures 1 and 3 the total design achieves a large flux with relatively small magnets because both pole pieces are used to maximise effect within the rotor body. In the external rotor design both poles of the magnet (North and South) are used to provide useful flux in the rotor air-gap.

Claims

1. A cylindrical rotor for location around a stator assembly of a permanent 5 magnet motor, the rotor having a series of hard magnetic bodies with their poles facing circumferentially around the rotor and being separated by soft magnetic material pole pieces, the continuous array of hard magnetic bodies and pole pieces being held in place by an external holding tube of non-magnetic material fixed to the pole pieces.

10 2. A rotor according to claim 1, wherein the pole pieces extend partially below the adjacent hard magnetic body, but leaving a gap below the hard magnetic body and between the inwardly extending ends of the pole pieces, which gap is filled by air or a non-magnetic material.

3. A rotor according to claim 2, wherein the lower portions of the inwardly 15 extending ends of the pole pieces extend closer to one another than the parts nearest to the hard magnetic body.

4. A rotor according to any one of claims 1 to 3, wherein the holding tube is fixed to the pole pieces by screws or some form of adhesion or locking device.

5. A rotor of circular cross-section for location within a stator assembly of a 20 permanent magnet motor, the rotor having an internal keeper body of a soft magnetic material and a series of hard magnetic material bodies of generally rectangular shape in a continuous array around the keeper body, with their poles facing inwardly and outwardly, with the essentially triangular gaps between the ends of adjacent hard magnetic bodies being filled by wedges of non-magnetic 25 material which hold the hard magnetic bodies in place and which are keyed into the internal keeper body.

6. A rotor according to claim 5, wherein outer pole pieces of a soft magnetic

material are located outside each of the hard magnetic bodies and between and held by the wedges, to define the outer cylindrical surface of the rotor.

7. A rotor according to claim 5 or claim 6, wherein the wedges are held in place by screws passing through the wedges and into the internal keeper body.

5 8. A rotor according to any one of claims 5 to 7, wherein the centre of the internal keeper body is hollow.

9. A rotor according to claim 8, wherein the hollow region incorporates fins.

10. A rotor of the form substantially as herein described with reference to Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.

10 11. Any novel combination of features of a rotor of the form as described herein and/or as illustrated in the accompanying drawings.