GB2402556A

GB2402556A - Commutator including moulded varistor

Info

Publication number: GB2402556A
Application number: GB0312167A
Authority: GB
Inventors: Lucux Law
Original assignee: Johnson Electric SA
Current assignee: Johnson Electric SA
Priority date: 2003-05-28
Filing date: 2003-05-28
Publication date: 2004-12-08
Also published as: GB0312167D0

Abstract

In a miniature PMDC motor, a noise suppression varistor 42 is moulded as the crown of the commutator 20 eliminating assembly steps and misalignment of the varistor 5 which results in an unbalanced rotor. An insulating sleeve may be provided between the crown and the shaft 16. Alternatively the moulded varistor may comprise the base 22 or a ring there on.

Description

1 2402556 Varistor Commutator This invention relates to electric motors

and has a particular application to miniature DC electric permanent magnet motors incorporating a noise suppression component.

Ceramic varistors are a commonly used electrical noise suppression component for miniature electric motors due to its excellent noise suppressing characteristics.

However, to be effective, the varistor should be located close to the source of the noise, i.e., it should be mounted on the rotor and electrically connected to the commutator segments.

The ceramic varistor is an annular disc which is soldered directly to the tangs of the commutator segments. Being ceramic, it is brittle and cannot be pressed onto the commutator. To take into account manufacturing tolerances, there is always a gap between the commutator O.D. and the varistor I.D. which leads to the varistor being mounted off centre. Also, being ceramic, the varistor is sensitive to heat and pressure as is applied during the soldering process. In addition, due to environmental concerns, solder is no longer a preferred manufacturing substance.

Accordingly, it is now desirable to have a varistor which does not require soldering, does not affect the balance of the rotor and is easy to assemble.

This is achieved in the present invention by making a part of the commutator out of a mouldable varistor material.

Accordingly, the present invention provides a miniature electric motor having a stator and a wound rotor; the rotor including a commutator; wherein the commutator has at least one part which is in electrical contact with each segment and is formed of a mouldable varistor material.

Preferred or optional features are defined in the dependent claims.

One preferred embodiment will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a typical electric motor to which this invention is applicable; and Figure 2 is a sectional view of a two part commutator in accordance with the preferred embodiment of the present invention.

Figure 1 illustrates a small PMDC motor 10 as used, for example, in an accessory for a vehicle. In such an application, it may be necessary for the motor to be suppressed to reduce the level of electromagnetic interference generated by the motor. The motor has a cup shaped housing 12 with an open end closed by an end cap 14. A shaft 16 extends through the housing 12 and the end cap 14. Inside the housing 12 is a permanent magnet stator comprising two arcuate ceramic magnets. A rotor is fitted to shaft 16 and is rotatably supported within the housing in confrontation with the stator.

The rotor has a rotor core supporting rotor windings terminated on a commutator.

Thus far described, the motor is of known construction.

The preferred commutator is shown in detail in Figure 2.

The commutator 20 is a two part mechanical connection type commutator having a crown 42 and a base portion 22. The base portion 22 has a sleeve 24 of heat resistant electrically insulating resin material with a plurality of copper commutator segments 26 mounted thereon. Each segment has a terminal 28 and a brush contact portion 30.

The terminal 28 is rectangular having a central slot 32 which in use, straddles and grips a lead wire 34 from the rotor windings. The opposite end of the segment 26 is bent to form a re-entrant finger 36 which locates within a groove or recess 38 in an axial end face of the sleeve. A barb 40 formed on the finger 36 retains the finger 36 within the slot 38.

The crown 42 has a number of housings 44 extending radially from a boss 46 which acts as a spacer for the commutator 20. Each housing 44 has a central, axially extending projection 48 which supports the lead wires during insertion of the terminals.

The segments 26 are arranged so that as the crown 42 and base portion 22 are slid together, the terminals 28 engage respective housings 44 formed in the crown 42.

Lead wires 34 of the rotor windings are drawn across the housings 44 during winding of the rotor where upon entry of the terminals 28 into the housings 44, makes an electrical connection between the segments 26 and the rotor windings.

The crown 42 is a moulded part formed from a mouldable varistor material and hence is the varistor. Thus, the varistor is an integral part of the commutator, is formed as at.

part of the commutator and does not cause any additional out of balance of the rotor nor requires any soldering or additional assembly steps.

The embodiment described above 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 as defined in the appended claims. For example, the varistor could be a ring fitted to the commutator base in direct contact with the commutator segments or the base could be moulded from a heat resistant mouldable varistor material. Where necessary, the varistor crown could be moulded to or otherwise fitted with a sleeve of insulating material to electrically isolate the crown from the shad.

Claims

Claims 1. A miniature electric motor having a stator and a wound rotor;

the rotor including a commutator; wherein the commutator has at least one part which is in electrical contact with each segment and is formed of a mouldable varistor material.
2. A motor according to claim 1, wherein the commutator is a two-part commutator.
3. A motor according to claim I or 2, wherein the commutator is a mechanical connection type commutator with a plurality of segments mounted on a base, each segment having a mechanical connection type terminal and a crown which is fitted to the base and has housings accommodating the terminals, the crown being said one part of mouldable varistor material.
4. A motor according to claim 3, wherein the housings in the crown support lead wires which connect rotor windings to the commutator terminals, the terminals have a slot which straddles and grips the lead wires upon insertion of the terminal into a respective housing in the crown and makes electrical connection between the terminal and the lead wire.
5. A motor according to claim 4, wherein each terminal has a barb, which upon insertion, engages the housing to prevent removal.
6. A miniature PMDC motor substantially as hereinbefore described with reference to the accompanying drawings. l