GB2256538A

GB2256538A - Reduced diameter commutator arrangement

Info

Publication number: GB2256538A
Application number: GB9112369A
Authority: GB
Inventors: Jean-Pierre Steiner
Original assignee: Johnson Electric SA
Current assignee: Johnson Electric SA
Priority date: 1991-06-08
Filing date: 1991-06-08
Publication date: 1992-12-09
Also published as: GB9112369D0

Abstract

An electric motor has a separate body 22 supporting commutator segments which fits between one end of a rotor shaft 13 and an end bearing 19. The outside diameter around the commutator segments 23 is approximately the same as the outside diameter of the shaft 13 which is significantly less than in prior art motors, leading to less wear of brushes 20 and the commutator segments 23. <IMAGE>

Description

Electric~Motor The invention relates to electric motors.

The invention relates more particularly to fractional horsepower motors having finger leaf brushes.

In small motors, which also run at very low voltages, the contact resistance between the brush and the.

commutator becomes a very important factor, and it is desirable to reduce this resistance as much as possible. The usual way to reduce this contact resistance is to use a commutator plated with a precious metal, such as silver, and metal finger leaves. The metal finger leaves usually have an inlay of silver, or an alloy of silver, to further reduce resistance. The resistance from the external power source through the motor terminals, brush anci commutator to the coils of the armature in this way can be reduced.

This method does produce the desired effect of reducing the contact resistance. The problem with these metal finger leaf brushes is that they tend to wear rapidly.

The objective Or this invention is to reduce the wear of metal finger leaf brushes and precious metal commutator.

According to the invention there is provided a fractional horsepower direct current electric motor having a stator, a casing supporting two end bearings, a rotor mounted on a shaft for rotation between the two end bearings, and a commutator assembly arranged to one side of the rotor and rotatable concentrically with the shaft, in which the commutator assembly comprises a separate body which fits between one end of the shaft and a respective one of the end bearings.

The commutator assembly may include commutator segments extending along and spaced around the assembly, in which the mean outside diameter of the assembly about the segments is substantially the same or less than the diameter of the shaft.

The commutator assembly may comprise an integral body of ceramic material formed at one with a bore which push fits over the end of the shaft and at the other end with a journal which fits into the respective one of the end bearings.

An fractional horsepower direct current electric motor according to the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows part sectioned a side view of the motor; Figure 2 shows in section part of a commutator assembly; and Figure 3 shows a top view of the commutator assembly.

Referring to the drawings. a rotor 10 has a laminated armature 11 with windings 12 supported on a shaft 13 supported at one end in a bearing 14 in an end cap 15 of a casing 16. Two magnets 17 are fixed in the casing 16. Another end cap 18 of the casing 16 supports a second bearing 19 and sets of finger leaf brushes 20.

A commutator assembly 21 comprises a central body 22 on which are mounted three commutator segments 23 having tangs 24 which in use are connected to the respective armature windings 12. The segments 23 are held on the body 22 by fitting into slots 25 and by a ring clamp 26.

The body 22, as best seen in Figure 2, is formed integrally of ceramic material and provides a bore 27 at one end which fits securely over an end of the shaft 13 and at the other end with a journal 28 which fits into the bearing 19. The bore 27 is a push fit on the end of the shaft 13 but the bore may be keyed and/or secured by glue if desired to prevent relative rotation between the body 22 and the shaft 13. The segments 23 fit against the body surface at A.

Whereas in the described motor, the shaft 13 is supported between the end bearings 14 and 19, directly by the bearing 14 and indirectly, via the body 22, in the bearing 19, in presently used motor the shaft extends the whole distance between such bearings. As such, in presently used motors the commutator segments are carried by a supporting piece, for the segments, mounted on the shaft. The mean external diameter of commutator segments must therefore always be somewhat larger than the outside diameter of the shaft. The provision, as described in this application, of having a separate body 22 bridging between the shaft 13 and the bearing 19 enables the outside diameter of the commutator segments to be chosen independently of the diameter of the shaft 13, and in particular makes it possible to reduce this diameter considerably as compared with prior art motors.Importantly, by reducing the diameter, the distance the brushes travel in contact against the segments is reduced so that less wear inherently takes place and the effective working life of the brushes and of each the commutator segment is I ncrease In a typical . presently used fractional horsepower motor, the shaft has a diameter of 1.5 mm and the smallest mean outside diameter of the commutator segments is 2.6 mm. With the described arrangement the outside diameter can for example be 1.5 mm or less, to reduce the commutator circumference, and thus the distance 'travelled' by the brush leaves by approximately 50% compared to presently used motors.

Various ways are already known for fixing commutator segments to supporting bodies and it will be appreciated that embodiments of the invention can include such various ways. Additionally, because the body 22 is provided separately, the body and the segments are readily made up as a sub-assembly for mounting on on the end of the shaft. Alternatively, the segments may be connected to the armature windings and then fitted later to the body 22 after the windings have been wound on the armature laminations. In all embodiments, the provision of the separate body enables the overall diameter the commutator segments to be almost totally independent of the diameter of the shaft and in particular much smaller than before, so as to reduce the wear of the brush leaves and the commutator segments.

Claims

1. .A fractional horsepower direct current electric motor having a stator, a casing supporting two end bearings, a rotor mounted on a shaft for rotation between the two end bearings, and a commutator assembly arranged to one side of the rotor and rotatable concentrically with the shaft, in which the commutator assembly comprises a separate body which fits between one end of the shaft and a respective one of the end bearings.

2. An electric motor according to claim 1, in which the commutator assembly includes commutator segments extending along and spaced around the assembly, in which the mean outside diameter of the assembly about the segments is substantially the same or less than the diameter of the shaft.

3. An electric motor according to claim 1 or 2, in which the commutator assembly comprises an integral body of ceramic material formed at one with a bore which push fits over the end of the shaft and at the other end with a journal which fits into the respective one of the end bearings.

4., An electric motor substantially as herein described with. reference to Figures 1 to 3 of the accompanying drawings.