GB2205692A

GB2205692A - A permanent magnet electric motor having rotor mounted brush gear

Info

Publication number: GB2205692A
Application number: GB08713079A
Authority: GB
Inventors: Roger Frederick Baines
Original assignee: Johnson Electric Industrial Manufactory Ltd
Current assignee: Johnson Electric Industrial Manufactory Ltd
Priority date: 1987-06-04
Filing date: 1987-06-04
Publication date: 1988-12-14
Also published as: GB8713079D0

Abstract

The motor comprises a wound stator (16), a permanent magnet rotor (25), a commutator comprising a plurality of commutator segments (20) connected to winding coils (19) of the stator, two electrically conductive rings (22, 23) for connection respectively to a positive and negative electricity supply, and brush gear on the rotor. The brush gear comprises two brushes (30), one of which connects one of the rings to the commutator segments and the other of which connects the other ring to the commutator segments. <IMAGE>

Description

AN ELECTRIC MOTOR This invention relates to a direct current electric motor.

It is well known in the art to make brushless motors in which the windings become the stator and the magnet is made to rotate. Various means of detecting the magnet rotor position so that the appropriate stator coils can be energised are well known.

Occasionally the cost of a brushless system with its associated electronic circuitry is considered too high yet the advantage of having a high inertia moving magnet rotor is desirable.

According to the present invention there is provided a direct current electric motor comprising a wound stator, a permanent magnet rotor, a commutator comprising a plurality of commutator segments connected to winding coils of the stator, two electrically conductive rings for connection respectively to a positive and negative electricity supply, and brush gear on the rotor, the brush gear comprising two brushes, one of which connects one of said conductive rings to the commutator segments and the other of which connects the other of said conductive rings to the commutator segments'.

Conveniently, the conductive rings and commutator segments are arranged in a common plane perpendicular to the axis of the motor.

Conveniently, the motor is an axial air gap motor.

Preferably, the rotor includes two disc members mounted, respectively, on opposite sides of the stator for rotation in unison, at least one of said disc members supporting permanent magnets. In this case the other disc member may provide a return path for the magnetic field to minimise eddy current losses.

The invention will now be more partiuclarly described, by way of example, with reference to the accompanying drawing which is an exploded perspective view of one embodiment of a motor according to the invention.

Referring to the drawing, the motor shown'therein is an axial air gap fractional horsepower electric motor comprises a motor frame in the form of a drawn metal can 10 closed by a metal end cap 11. Journal bearings 12 and 13 are provided in the base of the can 10 and the end cap 11, respectively.

The can 10 has a circumferentially extending ledge 14 intermediate its ends. This ledge 14 is provided by a metal ring 15 disposed in the inner end of the can 10. However, the ledge 14 could be replaced by a plurality of tongues punched out and bent inwards from the circumferential wall of the can. A stator 16 is located axially against the ledge 14 and is fixed in position by dimples (not shown) pressed in the can to interfere with the outer circumferential edge of the stator 16.

The stator 16 comprises a disc-like.plastics support 17 have a central through hole 18. Three discrete coiled windings 19 are fixed to one side of the support 17, such as by glue, and are equi-angularly spaced apart. A face plate commutator, comprising three commutator segments 20, is mounted on a raised central portion 21 of the support 17. Two concentric slip rings 22 and 23 are also mounted on the central portion. The commutator segments 20 and slip rings 22 and 23 are arranged in a common plane with the commutator segments 20 interposed between the inner and outer slip rings 22 and 23, respectively.The commutator segments 20 are connected to respective windings 19 and the slip rings 22 and 23 are connected to power supply wires (not shown) which are secured to the under side of the support 17 and which are led out of the can 10 through an aperture 24 for connection to an electricity supply, e.g. a battery.

A rotor 25 comprises a shaft 26 and two steel disc-like plates 27 and 28 which are disposed on opposite sides of the stator 16. The inner surface of the plate 28 supports two segmental permanent magnets 29. The magnets 29 are glued to the plate 28 and are magnetised in opposite axial directions.

The plate 27 is fixed to the shaft 26, such as by shrink fitting it on the shaft, and the plate 28 is made an interference fit on a stepped end portion of the shaft 26, after the shaft 26 has been inserted in bearing 12 and the stator 16 has been located against the ledge 14.

The plate 27 serves as a low reluctance return path for the magnet field and minimises eddy current losses. The magnets 29 could be attached to the plate 27 instead of the plate 28, or indeed, magnets could be attached to both plates 27 and 28.

The inner surface of the plate 28 also supports two resilient brush leaves 30 supporting brushes proper. The brushes make contact with the commutator at positions which are spaced apart by 1800 and which are radially offset from one another. One of the brushes connects the inner slip ring 22 to the commutator by bridging the gap between the slip ring 22 and the commutator and the other brush connects the outer slip ring 23 to the commutator by bridging the gap between the slip ring 23 and the commutator. Therefore, the brushes respectively connect the commutator to a positive and negative electricity supply.

The motor described above has a high inertia moving magnet rotor consistent with known brushless motors, but avoids the cost of electronic circuitry conventially associated with brushless motors.

The above embodiment is 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 direct current electric motor comprising a wound stator, a permanent magnet rotor, a commutator comprising a plurality of commutator segments connected to winding coils of the stator, two electrically conductive rings for connection respectively to a positive and negative electricity supply, and brush gear on the rotor, the brush gear comprising two brushes, one of which connects one of said conductive rings to the commutator segments and the other of which connects the other of said conductive rings to the commutator segments.

2. An electric motor as claimed in claim 1, wherein the conductive rings. and commutator segments are arranged in a common plane perpendicular to the axis of the motor.

3. An electric motor as claimed in claim 1 or claim 2, wherein the motor is an axial air gap motor.

4. An electric motor as claimed in claim 3, wherein the stator has a through bore receiving a rotor shaft.

5. An electric motor as claimed in claim 4, wherein the rotor includes two disc members mounted, respectively, on opposite sides of the stator for rotation in unison, at least one of said disc members supporting permanent magnets.

6. An electric motor as claimed in claim 5, wherein said other disc member provides a return path for the magnetic field.

7. A direct current electric motor substantially as hereinbefore described with reference to the accompanying drawing.