GB2090478A

GB2090478A - Permanent magnet electromagnetic machines

Info

Publication number: GB2090478A
Application number: GB8120831A
Authority: GB
Original assignee: PA Management Consultants Ltd
Current assignee: PA Management Consultants Ltd
Priority date: 1977-10-06
Filing date: 1978-10-03
Publication date: 1982-07-07
Also published as: GB2024529A; DE7829681U1; GB2024529B; GB2090478B

Abstract

An electromagnetic machine (particularly an electric motor) is based on the design described in British Patent Application No. 48794/74 (Serial Number 1 541 211). Eddy current losses are reduced by forming the backing plates 1, 2 from ferrite composite material (16, 18) which may be sintered and mounting an array of discrete magnets (20, 20a, 22, 22a) thereon (Fig. 2). Interpolar flux leakage may be reduced by radially staggering the U-shaped pole pieces around the stator coil (Fig. 3 not shown). Differently shaped pole pieces may be employed, alternatively, around the stator assembly (Fig. 4 not shown). <IMAGE>

Description

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SPECIFICATION

Improvements to electromagnetic machines

5 Field of invention

This invention concerns electromagnetic machines such as motors and generators.

Background to the invention 10 There have been many developments in the field of electromagnetic machines one of which has been to develop an electromagnetic machine typically embodied as a motor which incorporates a circular array of permanent 15 magnets and a similarly arranged plurality of pole pieces around a stator which are energised by a coil so as to produce alternate North and South poles therearound. The rotating field is obtained by commutating the 20 current flowing in the coil so as to obtain interaction between the pole pieces and the permanent magnets. This design allows a very thin motor or other machine to be produced and has the advantage that the power avail-25 able can be increased by simply mounting additional units on a single drive shaft in the form of a stack.

Prior art ,

30 One form of electromagnetic machine constructed in accordance with this general principle is known from British Patent Application 48794/74 in which the stator comprises an annular array of U-shaped pole pieces ar-35 ranged around an annular coil so as to form alternate North and South poles therearound. The rotor comprises one (or two) discs mounted on one side (or both sides) of the stator and is attached to a shaft which passes 40 through an aperture in the stator. Usually two such rotor discs are employed. On each disc are circularly arranged the magnets which present alternately North and South poles to the stator.

45 Fig. 1 of the accompanying drawings illustrates diagrammatically one embodiment of such a machine. Rotation of the rotor assembly is achieved by supplying electric current to the stator coil either through a commutator so 50 as to produce magnetic pole reversal of the stator U-shaped pole pieces as the rotor rotates through the angular distance between the magnets on the rotor disc (or discs), or by supplying alternating electric current to the 55 coil around the stator.

It will be seen that the magnetic flux in the rotor assembly will fluctuate at twice the frequency of the alternation of the current flowing through the stator coil (whether due to 60 commutation or because it is alternating current that is supplied to the coil).

This results in high iron losses in the rotor assembly.

The power output of this type of machine 65 when operating as a motor is commonly found to ben restricted by the inductance of the armature coil.

Object of the invention 70 It is an object of the present invention to provide alternative forms of construction for this type of machine so as to reduce the high iron losses which otherwise arise.

75 The invention

According to the invention the rotor is formed from a ferrite composite material which serves as a backing for a plurality of separate magnets which are arranged in a 80 circular array to present alternatively North and South poles, or as a backing for a complete ring of permanent backing material which is magnetised to present alternate South and North poles around its circular 85 faces.

In an arrangement in accordance with this aspect of the invention the material forming the ring is preferably sintered.

Preferably, the U-shaped pole pieces which 90 are arranged around the annular coil to form the stator assembly are alternately radially displaced so that the interpolar flux paths between the bridging section of one pole piece and the separate poles of an adjoining 95 pole piece are increased so as to reduce interpolar flux leakage.

In the drawings Figure 1 illustrates by way of an exploded 100 perspective view one embodiment of the known form of machine.

Figure 2 illustrates part of an electric motor assembly constructed in accordance with the invention and also illustrates the magnetic 105 path through the armature and rotor assembly.

Figure 3 illustrates the principle of staggering the pole pieces so as to reduce interpolar flux leakage between adjoining poles in the 110 stator assembly, and

Figure 4 is a view of another alternative arrangement for reducing interpolar flux leakage.

115 Detailed description of the drawings

Fig. 1 corresponds to Fig. 1 of the drawings accompanying British Patent Application No. 48794/74 and reference is made to that application for detail of the construction and 120 operation of the motor illustrated in Fig. 1. Fig. 2 illustrates an embodiment of the present invention in which the two rotors (normally formed by steel backing plates 2 and 4 with a plurality of magnets 1, 1 a, 3, 3a 125 mounted thereon as in Fig. 1) are replaced by two rings 16 and 18 of ferrite composite material which may be sintered on which are mounted two circular arrays of permanent magnets 20, 20a and 22, 22a. The perma-130 nent magnets are arranged so as to present

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alternate North and South poles to the sandwiched stator assembly. This asembly comprises a circular coil 7 straddled by U-shaped pole pieces 6 and 6a. The pole pieces are 5 arranged around the coil so that the bridging section of each U-shaped pole piece is alternately radially outside and radially inside the coil so that a current flowing through the coil will produce alternate North and South poles 10 at the open ends of the U-shaped pole pieces.

Although not shown commutating means may be provided to reverse the current flowing through the coil 7 so as to produce a rotating magnetic field around the stator or 15 alternating current may be supplied to the coil 7 so as to produce the same effect.

The use of sintered ferrite composite material in the rotor reduces the eddy current losses to a small amount.

20 The same result can be achieved if the magnets 20, 20a, 22, 22a are replaced by a ring of permanent magnetic material magnetised with alternate North and South poles.

In addition to the magnetic flux paths be-25 tween axially spaced faces of magnets and pole pieces such as 9, 10 and 11 which are shown in Fig. 1, interpolar flux paths exist between the bridging section of each pole piece and the adjoining poles of adjacent pole 30 pieces. These interpolar flux paths are denoted by the dotted lines denoted by reference numerals 13 and 15 and it is possible to reduce the flux leakage along these interpolar flux paths by use of the invention subject of 35 our copending Patent Application No. 7921402.

This proposes radially displacing adjoining U-shaped pole pieces 6 and 6a, as shown in Fig. 3.

40 In Fig. 3 the pole pieces 6a in which the bridging section is radially outermost are displaced in a radially outward direction.

The other pole pieces 6 in which the bridging section is located radially inboard of the 45 coil 7 are not displaced but are left in their original position similar to that shown in Fig.

1.

In this way the North and South poles of the U-shaped poles pieces 6a are moved away 50 from the inboard bridging sections of the poles 6 and the result is a reduction in the interpolar flux leakage between adjoining U-shaped pole pieces.

Although not shown the same result can be 55 obtained by radially displacing the U-shaped poles pieces 6a by half the amount shown in Fig. 3 and inwardly radially displacing the pole pieces 6 by a corresponding distance.

The U-shaped pole pieces 6a (i.e. those in 60 which the bridging section is radially outermost) need not be of the same form as the U-shaped pole pieces 6 (i.e. those in which the bridging section is radially inmost) and as shown in Fig. 4 a reduction in interpolar flux 65 leakage can be obtained by forming the U-

shaped pole pieces 6a as radially short and stubby members but compensated by a greater width in the circular direction relative to the pole pieces 6a which are formed of greater radial length but are narrow in the circular direction.

A typical modified pole piece 6a is shown at 6a' and a typical modified pole piece 6 is shown at 6' in Fig. 4.

All aspects of the invention may be applied to linear motors and radial flux machines operating on the same principle as that embodied in the motor shown in Fig. 1.

Claims

1. An electromagnetic machine of the type comprising a first array of magnets, a magnetic path between pairs of the magnets in the first array, a second array of magnets arranged parallel to the first array, a magnetic path between pairs of the magnets in the second array, an array of pole pieces arranged between the first and second arrays of magnets, the first and second arrays of magnets and the array of pole pieces being movable relative to one another, and coil means arranged to magnetise the pole pieces, the arrangement being such that a magnetic path can be provided between a pair of magnets in the first array and a pair of magnets in the second array via a pair of the pole pieces characterized in that the first and second arrays of magnets are formed by one or more members of permanent magnetic'material presenting alternate North and South poles around two circular members (16, 18) on which said permanent magnet member or members are mounted, and in which the circular members (16, 18) are formed from ferrite composite material.

2. An electromagnetic machine according to claim 1, wherein a plurality of separate permanent magnets (20, 20a, 22, 22a) are mounted on said circular members.

3. An electromagnetic machine according to claim 1 or claim 2, wherein said ferrite composite material is a sintered material.

4. An electromagnetic machine according to claim 1 or claim 2 or claim 3, wherein adjacent pole pieces (6, 6a) are in staggered relationship in the radial direction.

CLAIMS (15 Dec 1981)

1. An electromagnetic machine of the type comprising a first array of magnets, a magnetic path between pairs of the magnets in the first array, a second array of magnets arranged parallel to the first array, a magnetic path between pairs of the magnets in the second array, an array of pole pieces arranged between the first and second arrays of magnets, the first and second arrays of magnets and the array of pole pieces being movable relative to one another, and coil means arranged to magnetise the pole pieces, the

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arrangement being such that a magnetic path can be provided between a pair of magnets in the first array and a pair of magnets in the second array via a pair of the pole pieces 5 characterized in that the first and second arrays of magnets are formed by one or more members of permanent magnetic material presenting a large plurality of alternate North and South poles around two circular members 10 (16, 18) on which said permanent magnet member or members are mounted, there being a corresponding large plurality of pole pieces around a circular magnetising coil between the first and second arrays of magnets, 15 and in which the circular member (16, 18) are formed from ferrite composite material.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd.—1982.

Published at The Patent Office, 25 Southampton Buildings,

London, WC2A 1AY, from which copies may be obtained.