GB2201843A

GB2201843A - Magnetically biased bearing for an electric motor

Info

Publication number: GB2201843A
Application number: GB8720741A
Authority: GB
Inventors: Roger Frederick Baines
Original assignee: Johnson Electric Industrial Manufactory Ltd
Current assignee: Johnson Electric Industrial Manufactory Ltd
Priority date: 1987-03-02
Filing date: 1987-09-03
Publication date: 1988-09-07
Anticipated expiration: 2007-09-03
Also published as: GB2201843B; GB8704827D0; GB2201841A; GB8720741D0

Abstract

A motor comprising a motor frame (10, 11, 12), a moving coil armature (18, 19, 22) having a shaft (18) which is supported for rotation in the motor frame, and a thrust face (17) on one side of the armature, first and second magnets (33, 34) are mounted with like poles adjacent on the other side of the armature, magnet (33) with respect 10 the shaft and magnet (34) with respect to the motor frame so that repulsion forces between the magnets urge an end of the shaft 18a into contact with the thrust face 17. <IMAGE>

Description

1 An electric motor 2201843 This invention relates to an electric motor

and particularly but not exclusively to a moving coil disc motor suitable for driving audio equipment.

It is a known advantage to create an end thrust in small p.m.d.c. motors in order to force a shaft with a part spherical end against a flat thrust face or to force a flat ended shaft against a thrust ball. This action helps to keep brushgear on a constant track at the commutator and provides a lower friction than if washers on the shaft are to run against the face of a bearing.

It is common to accomplish this end force in a conventional laminated armature by offsetting the centreline of the lamination stack axially with respect to the centreline of the permanent magnet in the housing.

However in moving coil motors there is no iron to be offset and this arrangement cannot work.

2 It is known from JP 57-160346 to use the attraction forces between two magnets to draw the ball end of a motor shaft into contact with a thrust face. However. this requires the magnets to be located at the non-drive end of the motor and often space at the non-drive end is limi ted. Moreover. the attraction forces are at a maximum when the ball end of the shaft is in contact with the thrust face and these forces may create undesirable friction between the shaft and the thrust face.

According to the present invention there is provided an electric motor comprising a motor frame, a moving coil armature having a shaft which is supported for rotation in the motor frame. a thrust face provided by or mounted with respect to the motor frame on one side of the armature. and first and second magnets mounted with Like poles adjacent on the other side of the armature one with respect to the shaft and the other with respect to the motor frame so that repulsion forces between the magnets urge an end of the shaft into contact with the thrust face.

With this arrangement the two magnets are located at the drive end of the motor where it is often easier - 3 to make space available for the magnets. Moreover, a repulsion system has the advantage over an attraction system that the forces between the mag. nets are weakest when the shaft is in contact with the thrust face. Hencer there need only be a gentle force between the magnets when the end of the shaft is in contact with the thrust face. minimising friction between the-shaft and the thrust face. If the end of the shaft tries to move away from the thrust face the force between the magnets will increase to urge the shaft end back.

Preferred and/or optional features of the invention are set forth in claims 2 to 7, inclusive.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawings. in which:

Figure 1 is a sectional view of one embodiment of an electric motor according to the present invention, the section through the armature being taken along line I-I of Figure 2.

- 4 Figure 2 is of Figure 1, an underneath plan view of the armature Figure 3 is a top plan view of the armature of Figure ir Figure 4 is a sectional view of another embodiment of an electric motor according to the invention.

Figure 5 is a top plan view of the armature of Figure 4 with the film bearing the commutator omitted, and Figure 6 is a section taken along line VI-VI of Figure 4, with the electronic components omitted from the printed circuit film.

Referring first to Figures 1 to 3 of the drawings, the motor shown therein has a motor frame comprising a drawn shallow metal can 10 closed by a metal end cap 11.

A plate 12 is provided within the can 10 and forms part of the motor frame. The plate 12 is located axially against a plastics sleeve 13 disposed within the can 10 and the plate 12 is secured in position by i dimpling the can from the outside to pinch and thus secure the plate 12 in position.

The inner surface of the base 14 of the can 10 supports four segmental permanent- magnets 15. The magnets 15 are glued to the base 14 and adjacent magnets are magnetised in opposite axial directions. The magnets 15 are stepped at their outer circumferentially extending edges and these stepped edges are engaged by an annular. stepped, radially inwardly extending portion of the sleeve 13 to provide additional support for the magnets. The magnets could be replaced by a single annulus appropriately magnetised.

A radial bearing 16 is fixed in the base 14 of the can 10 and a thrust face 17 is provided on the inside of the plate 12.

A motor armature comprises a shaft 18 which is journalled in the radial bearing 16 and which has a part spherical non-driving end 18a bearing against the thrust face 17. The armature also comprises a plastics winding support 19 mounted fast on the shaft 18. The support 19 comprises a hub 20 and six radially extending spokes 21. Six discrete winding coils 22 are fixed between the spokes 21 of the sup port 19, such as by glue, and are equi- angularly spaced apart.

The hub 20 of the winding support 19 also defines a base for a commutator, conveniently a face plate commutator 23. The commutator 23 and the connections between the commutator 23 and the winding coils 22 are printed on a film 24 which is secured to the winding support 19, such as by glue. The film 24 is.

conveniently, an epoxy resin or polyester based film clad with copper, etched. and subsequently masked in areas to which connections are not made. The commutator comprises six segments 25 arranged in a common plane and if desired these segments may be plated with noble metal. Typically, this would be achieved by plating the copper with nickel and then plating the.nickel with noble metal. One end of each winding 22 is connected to a star point provided by conducting track 26 and the other end of each winding 22 is connected to a respective commutator segment 25 by respective conducting tracks 27. Diametrically opposed commutator segments 25 are electrically connected together by respective conducting tracks 28.

7 A speed regulating circuit 29 provided on a printed circuit film 30 is attached to the outer surface of the plate 12. Electrical terminations of the motor are printed on a tab 31 integral with the film 30 and are led out of the can 10 through an opening in the side wall thereof.

Two resilient precious metal brush leaves 32 forming brush gear of the motor are soldered directly to the printed circuit film 30 and extend through holes 33 in the plate 12. The free end of each brush leaf 32 is forked and these ends of the brush leaves define brushes proper which make contact with the commutator at positions which are spaced apart geometrically by 90 degrees and, as considered electrically, at positions appropriate for optimum commutation. A small elastomeric pad 37 is interposed between each leaf 32 and the film 30,at a position close to the soldered connection to dampen vibration.

In the motor described above each winding coil is open circuited for a period as it passes across a pole face and during this period experiences no change in flux energy. This is particularly advantageous if the flux density across the pole face is substantially trapezoidal, which is likely if high energy magnets, such as neodymium iron boron magnets, are used.

In contrast, if the magnets are of a low energy content then the flux _dens i ty across the pole faces will be sinusoidal and in that case it may be advantageous to connect the windings to form a parallel pair of delta windings so that each coil plays a continuous part in developing torque.

Coaxial ring magnets 33 and 34 are fitted on the hub 20 and on the radial bearing 16, respectively. The magnets 33 and 34 have their magnetic fields axially oriented with like poles adjacent so that the repulsion forces between the magnets urge the shaft 18 into contact with the thrust face 17.

the can 10 by splayed The end cap 11 is secured t'o'.:-. lugs on the end cap 11 engaged in notches in the end of the can 10.

Referring now to Figures 4 to 6. the motor shown therein has a motor frame comprising a drawn shallow metal can 110 closed by a metal cover plate 111.

- 9 A mild steel keeper ring 112 is glued to the base 114 of the can 110.

A radial bearing 116 is fixed in the base 114 and a thrust face 117 is provided on the inside of the cover plate 111.

A motor armature comprises -a shaft 118 which is journalled in the radial bearing 116 and which has a part spherical non-driving end 118a bearing against the thrust face 117. The armature also comprises a plastics winding support 119 mounted fast on the shaft 118. The support 119 comprises a hub 120, a thin annular -base 113 and six triangular.

equi-angularly spaced bosses 121 upstanding from the base 113. Six dis.crete winding coils 122 are fixed about respective bosses 121, such as by glue.

A film 124, substantially identical to film 24 (Figure 3), has a face plate commutator 123 and connections 127 between the winding coils 122 printed thereon. The film 124, which may be self adhesive, is secured to the upper surface of each boss 121 and to a shoulder 120a provided on the hub 120 of the winding support 119. Connections between the winding coils 122 and the segments of the commutator 123 are the same as described previously in connection with the embodiment shown in Figures 1 to 3.

---- A speed regulating circuit 129 provided on a printed circuit film 130 is attached to the inner surface of the cover plate 111. Electrical terminations of the motor are printed on a tab 131 integral with the film and are led out of an opening in the side wall thereof or through the cover plate 111.

An annular magnet 115 appropriately magnetised to define a four pole magnetic field is secured to the printed circuit film 130, which may be self adhesive.

Cut outs 115a and 115b are provided in the side of the magnet 115 remote from the film 130 in order to accommodate two resilient precious metal brush leaves 132 which are soldered directly to the printed circuit film 130. The free end of each brush leaf 132 is forked and these ends of the brush leaves define brushes proper which -make contact with the commutator at positions which are spaced apart geometrically by 90 degrees and, as considered electrically, at positions appropriate for optimum z c X,k commutation. A small elastomeric pad may be interposed between each leaf 132 and the magnet 115 at a position close to the soldered connection to dampen vibration.

G - Coaxial ring magnets 133 and 134 are fitted on the hub 120 and on the radial bearing 116. respectively. The magnets have their magnetic fields axially oriented with like poles adjacent so that the repulsion forces between the magnets urge the shaft

118 into contact with the thrust face 117.

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 inventiofi defined by the appended claims.

Claims

1. An electric motor comprising a motor frame. a moving coil armature having a shaft which is supported for rotation in the motor frame. a thrust face provided by ormounted with respect to the motor frame on one side of the armature. and first and second magnets mounted with like poles adjacent on the other side of the armature one with respect to the shaft and the other with respect to the motor frame so that repulsion forces between the magnets urge an end of the shaft into contact with the thrust face.

2. An electric motor as claimed in claim 1, wherein the magnets are ring magnets having their magnetic fields axially oriented.

3. An electric motor as claimed in claim 1. or claim 2. wherein the moving coil armature is of disc form.

4. An electric motor as claimed in claim 3, wherein the armature comprises a winding support and a plurality of winding coils mounted on the support.

1 X J

5. An electric motor as claimed in claim 4, wherein said one magnet is fixed to the winding support.

6. An electric motor as claimed in any one of the preceding claimsi wherein the said other magnet is fixed to a radial bearing supported by the motor frame.

7. An electric motor as claimed in any one of the preceding claims, wherein the said end of the shaft is part spherical.

8. An electric motor as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

Published 1988 at The Patent OMce, State House, 88/71 High Holborn, London WClR 4TP. Further copies maybe obtained from The Patent OfIlce, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1187.