GB2213653A

GB2213653A - A permanent magnet D.C. motor with flux concentrator

Info

Publication number: GB2213653A
Application number: GB8729127A
Authority: GB
Inventors: Roger Frederick Baines
Original assignee: Johnson Electric Industrial Manufactory Ltd
Current assignee: Johnson Electric Industrial Manufactory Ltd
Priority date: 1987-12-14
Filing date: 1987-12-14
Publication date: 1989-08-16
Also published as: GB8729127D0

Abstract

A low reluctance flux concentrator (24) is provided on the side of the winding 13 remote from the permanent magnet rotor 11 to bend and hence concentrate the flux in the effective parts of the winding i.e. in those parts of the winding which have a radial component to the direction in which they extend. In an alternative embodiment, a commutator motor is disclosed. <IMAGE>

Description

An electric motor This invention relates to a permanent magnet direct current electric motor and in particular to an axial airgap motor.

It is common in small axial airgap motors to employ a plurality of discrete winding coils, each coil being energised at the appropriate time and with the right polarity to create a continuity of torque produced.

The permanent magnet or magnets of the motor are usually arranged to fully overlie the winding coils to obtain as much flux as possible and it is common practice to provide a low reluctance return path on the side of the winding coils remote from the magnet(s). In known motors this return path extends at least to the boundaries of the magnets as viewed in the axial direction of the motor.

However in such motors the effective parts of the winding are only those sections of the winding wire which run in and out as radials and which therefore cut flux at the appropriate angle and consequently flux passing through purely circumferentially extending parts of the coils is wasted.

The present invention seeks to mitigate this drawback.

According to one aspect of the present invention, there is provided a permanent magnet direct current electric motor comprising a motor winding and permanent magnet means mounted for relative rotation about an axis of the motor and having an axial airgap therebetween, wherein low reluctance flux concentrating means is provided on the side of the winding remote from the permanent magnet means to concentrate the flux in the effective parts of the winding.

By "effective parts" we mean the parts of the coils which do not extend purely circumferentially, i.e.

parts which have a radial component to the direction in which they extend.

By concentrating flux in the effective parts of the coils flux wastage is avoided.

The invention further provides a permanent magnet direct current electric motor comprising a motor winding and permanent magnet means mounted for relative rotation about an axis of the motor and having an axial airgap therebetween, wherein the motor winding comprises one or more winding coils and wherein low reluctance flux concentrating means is provided on the side of the winding coil(s) remote from the permanent magnet means, the flux concentrating means being substantially annular and having a substantially uniform radial dimension which is less than the radial dimension of the permanent magnet means.

The invention yet further provides a permanent magnet direct current electric motor comprising a motor winding and permanent magnet means mounted for relative rotation about an axis of the motor and having an axial airgap therebetween, wherein the motor winding comprises a plurality of discrete winding coils angularly spaced apart about the axis of the motor and wherein low reluctance flux concentrating means is provided on the side of the winding coils remote from the permanent magnet means, wherein the flux concentrating means has a radial dimension which is less than the radial extent of each of the discrete winding coils.

The invention still further provides a permanent magnet direct current electric motor comprising a motor winding and permanent magnet means mounted for relative rotation about an axis of the motor and having an axial airgap therebetween, wherein low reluctance flux concentrating means is provided on the side of the winding remote from the permanent magnet means, the flux concentrating means overlying or underlying at most effective parts of the motor winding.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an exploded perspective view of one embodiment of an electric motor according to the invention, and Figure 2 is a section taken through the axis of the motor of Figure 1.

Referring to the drawings, the motor shown therein is a brushless motor having a stator 10 and a rotor 11.

The stator 10 comprises a disc-like winding support 12, typically of plastics material, and four discrete winding coils 13 fixed to the support, such as by glue, in equi-angularly spaced relationship. The support 12 has a central hole 14 in which a self-aligning journal bearing 15 is mounted. A positional detector and switching circuit 16 is mounted on the support 12 between two of the winding coils 13.

The rotor 11 comprises a shaft 17 mounted for rotation in the bearing 15, and a disc-like metal plate 18 supporting four segmental permanent magnets 19. The magnets 19 are glued to the plate 18 and adjacent magnets are magnetised in opposite axial directions.

The stator 10 and rotor 11 are mounted in a motor frame comprising a drawn shallow metal can 20 closed at one end by an integral end plate 21 which provides a thrust face for a ball end 22 of the rotor shaft 17. The can is closed at its other end by a metal end cap 23 having an integral apertured boss 26 for seating a further self-aligning bearing 27 through which the shaft 17 extends.

Diametrically opposite winding coils 13 are connected electrically in series and the positional detector and switching circuit 16 includes a Hall effect device which detects the position of the rotor relative to the stator and two electronic switches which operate in response to the Hall effect device to alternately energise the two pairs of series connected winding coils 13, as the magnets 19 come into register with the coils 13.

An annular, low reluctance flux concentrator 24, conveniently of mild steel, of uniform cross-section is mounted in a groove 25 in the underside of winding support 12.

As better seen from Figure 2, the magnets 19 fully overlie the winding coils 13 and therefore have a radial dimension which is at least as great as the maximum radial extent of the winding coils 13. This is to obtain as much flux as possible.

However, the radial dimension of the flux concentrator 24 is less than the radial dimension of each of the magnets and also less than the radial extent of the winding coils 13. Indeed it is preferable that the flux concentrator 24 underlies at most the effective parts of the winding coils 13 i.e.

radial parts 13a and not the circumferentially extending parts 13b. The flux concentrator 24 will cause the flux between the magnets 19 and flux concentrator to bend inwards as shown in broken lines in Figure 2 thereby concentrating the flux in the effective parts of the winding coils 13 and avoiding flux wastage.

The internal arrangement of the above motor could, of course, be arranged with the winding support 12 attached to the end cap 23 in which case the magnets would underlie the winding coils and the flux concentrator would overlie the winding coils.

The coils 13 need not be symmetrically arranged and indeed the motor could have a single bifilar winding.

The flux concentrator 24 need not be of uniform cross-section as small deviations of cross-section could be used, for example, to create a pole couple for starting purposes.

The flux concentrator 24 need not be annular; indeed it could comprise a plurality of segmental parts to prevent eddy current circulation.

The invention may be equally applied to a conventional commutator motor having an armature including a plurality of discrete winding coils mounted for rotation about the axis of the motor and connected to a commutator, brush gear for the motor and permanent magnet means fixed to a motor frame on one side of the armature and flux concentrating means fixed to the motor frame on the other side of the armature. If the motor frame is in the form of a drawn shallow can-like casing having an end cap as described in connection with the embodiment shown in the drawing, the permanent magnet means which could be in the form of a plurality of segmental permanent magnets or an annular magnet appropriately magnetised could be attached to the base of the can-like casing and the flux concentrating means and brush gear could be supported by the end cap.

The above embodiments are given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the invention defined by the appended claims.

Claims

1. A permanent magnet direct current electric motor comprising a motor winding and permanent magnet means mounted for relative rotation about an axis of the motor and having an axial airgap therebetween, wherein low reluctance flux concentrating means is provided on the side of the winding remote from the permanent magnet means to concentrate flux in the effective parts of the winding.

2. An electric motor as claimed in claim 1, wherein the flux concentrating means bends the flux at the radially outer edge of the permanent magnet means radially inwards of said outer edge and the flux at the radially inner edge of the permanent magnet means radially outwards of said inner edge.

3. An electric motor as claimed in claim 1 or claim 2, wherein the motor winding comprises one or more discrete winding coils and the flux concentrating means has a radial dimension which is less than the radial extent of the or each discrete winding coil.

4. An electric motor as claimed in any one of the preceding claims, wherein the flux concentrating means overlies or underlies at most effective parts of the winding.

5. An electric motor as claimed in any one of the preceding claims, wherein the motor winding comprises a plurality of discrete winding coils angularly spaced apart about the axis of the motor, the permanent magnet means comprises an annular magnet or a plurality of segmental magnets, and the flux concentrating means comprises an annular flux concentrator or a plurality of segmental flux concentrators, the mean radius of the winding coils, the permanent magnet means and the flux concentrating means being substantially equal and the radial dimension of the flux concentrating means being less than the radial dimension of the permanent magnet means.

6. An electric motor as claimed in any one of the preceding claims, wherein the motor is a brushless motor having a permanent magnet rotor, the motor winding and the flux concentrating means being fixed with respect to the motor frame.

7. An electric motor as claimed in any one of claims 1 to 5, wherein the permanent magnet means and the flux concentrating means are supported by a motor frame and the motor winding forms part of an armature of disc form supported for rotation in the motor frame between the permanent magnet means and the flux concentrating means.

8. An electric motor as claimed in claim 6 or claim 7, wherein the motor frame comprises a drawn shallow metal can-like casing and an end cover closing the open end of the casing.

9. A permanent magnet direct current electric motor comprising a motor winding and permanent magnet means mounted for relative rotation about an axis of the motor and having an axial airgap therebetween, wherein the motor winding comprises one or more winding coils and wherein low reluctance flux concentrating means is provided on the side of the winding coil(s) remote from the permanent magnet means, the flux concentrating means being substantially annular and having a substantially uniform radial dimension which is less than the radial dimension of the permanent magnet means.

10. A permanent magnet direct current electric motor comprising a motor winding and permanent magnet means mounted for relative rotation about an axis of the motor and having an axial airgap therebetween, wherein the motor winding comprises a plurality of discrete winding coils angularly spaced apart about the axis of the motor and wherein low reluctance flux concentrating means is provided on the side of the winding coils remote from the permanent magnet means, wherein the flux concentrating means has a radial dimension which is less than the radial extent of each of the discrete winding coils.

11. A permanent magnet direct current eletric motor comprising a motor winding and permanent magnet means mounted for relative rotation about an axis of the motor and having an axial air gap therebetween, wherein low reluctance flux concentrating means is provided on the side of the winding remote from the permanent magnet means, the flux concentrating means overlying or underlying at most effective parts of the motor winding.

12. A permanent magnet direct current electric motor substantially as hereinbefore described with reference to the accompanying drawings.