GB2159668A - Electric motor - Google Patents

Abstract

A rotary electric motor has a generally cylindrical motor case open at one end at which is a motor case end cover 2 at least the external surface of which is metallic. The rotor shaft is supported in respective bearings mounted on the motor case and on the motor case end cover. The rotor shaft supports a commutator. The motor case cover has terminal holes through which extend terminals 4 for coupling the commutator brush gear to an external electric circuit. An electrically insulating film 13 is provided on the external surface of the motor case cover at least in the vicinity of and surrounding the terminal holes. The film is suitably formed by painting on solder- resistant ink. <IMAGE>

Description

SPECIFICATION Electric motor This invention relates to electric motors.

In one common construction for miniature electric motors, the motor has a generally cylindrical motor case open at one end and having permanent magnets mounted on its interior cylindrical surface and serving as the stator. The motor case cover is formed of a synthetic resin material and mounts the brush gear which co-operates with a commutator mounted on the rotor shaft, terminals extending through terminal holes in the motor case cover for connection in an external electric circuit. Since the material of the motor case cover is electrically insulating, there are no electrical insulation problems with this arrangement. However, dimensional accuracy is difficult to achieve and to maintain because of the problems of deformation, plasticity and post-mould shrinkage of the synthetic resin.

In an attempt to overcome this problem, it has been suggested to form the motor case cover in a composite fashion, that is of a metallic material such as iron (which additionally provides magnetic shielding) on its external surface, and an insulator material such as synthetic resin on its internal surface and adapted for supporting the brush gear. The terminals must extend through terminal holes in the motor case cover in electrically insulating relation with respect to the metallic surfaces. As we shall explain below with particular reference to Figs. 1 A to 1 C, this arrangement has not proved entirely satisfactory and may result in unacceptable occurrence of lowered electrical insulation and short circuiting in mass production of miniature electric motors unless the fabrication techniques are carried out extremely carefully.The present invention has arisen from our work seeking to overcome these difficulties and to reduce or eliminate the occurence of lowered electrical insulation and short circuiting while maintaining or improving the level of productivity in mass production of miniature electric motors.

In accordance with the present invention, there is provided a rotary electric motor having a generally cylindrical motor case open at one end and having a motor case cover at least the external surface of which is metallic; the rotor shaft being supported in respective bearings mounted on the motor case and on the motor case cover; the rotor shaft supporting a commutator; and the motor case cover having terminal holes through which extend terminals for coupling the commutator brush gear to an external electric circuit, and an electrically insulating film on said external surface at least in the vicinity of and surrounding said terminal holes.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings in which: Figure 1A is a part sectional side elevational view of a previously proposed embodiment of miniature electric motor; Figure 1 B is a perspective view of the motor of Fig. 1A; Figure 1C is a side elevational of the motor of Figs. 1 A and 1 B showing how the insulation problem may arise; Figure 2 is an axial end view of an embodiment of motor in accordance with the present invention illustrating the external surface of the motor case cover thereof; and Figure 3 is a view similar to Fig. 2 showing an alternative embodiment of electric motor in accordance with the present invention.

Figs. 1A to 1C show an embodiment of miniature electric motor having a generally cylindrical motor case 1 and a motor case cover comprising an outer metallic member 2 on which is mounted an insulator 3, part of which 3' extends through terminal holes provided in the metal member 2 in effect to provide insulating bosses for terminals 4 of the motor brush gear, the terminals 4 extending to the exterior for connection in an external electric circuit. In the arrangement illustrated, the terminals 4 are integrally formed with brushes 5 which co-operate with a commutator 6. A permanent magnet 7 is mounted within the motor case 1 and serves as the stator. The rotor windings 8 are mounted on the rotor shaft 9 which is supported in respective bearings 10 mounted on the motor case and on the motor case cover.

To connect the terminals 4 with an external electric circuit, lead wires 11 will generally be coupled to the respective terminals 4 by means of solder 12, as shown in Fig. 1 B.

There will be a tendency, whenever soldering, for the solder to splash or run, possibly as shown in Fig. 1C. If splashing and running of the solder is to be avoided, operators must work very slowly and very carefully leading to significantly lowered work efficiency, deteriorating productivity and increased manufacturing cost. If soldering is conducted on a mass production basis, as is necessary to gain proper advantage of good work efficiency and productivity and to maintain the manufacturing cost of the miniature electric motors at a low level, the tendency for splashing and running of solder will be considerably exacerbated. We have found that this manner of fabrication produces an unacceptably large proportion of miniature motors with reduced electrical insulation or short circuiting in extreme cases between the terminal 4 and the metallic member 2 of the motor case cover.

Referring now to Figs. 2 and 3 which illustrate embodiments of electric motor construction in accordance with the present invention, it should be understood that apart from the exterior face of the motor case cover 2 illustrated in these figures, the construction of the motor of Fig. 2 or of Fig. 3 may be identical to the construction of the motor of Figs. 1A to 1C.

The embodiments of Figs. 2 and 3 have an insulating film 1 3 formed by applying solderresist ink onto the exterior surface of the metallic portion of the motor case cover 2 in the vicinity of the terminal 4. The solder-resist ink used to form the insulating film 1 3 is of liquid form, does not evaporate during application and curing and shows excellent workability during application. The insulating film 1 3 formed with the solder-resist ink has superior properties, including heat resistance, electric insulation and adhesion. Consequently, when the lead wire 11 (shown in Fig. 1) is connected to the terminal 4 by soldering, no poor insulation is caused between the terminal 4 and the metallic portion of motor case cover 2 due to the splashing and running of solder.

The operation of forming the insulating film 1 3 may be incorporated into the mass production line for the miniature motors. In mass production of miniature motors, insulating film 1 3 can be provided in advance, for example, by applying the solder-resist ink over a predetermined area of a blank for the metallic portion of motor case cover 2 using a paint roller, a spray or other means, before stamping. The solder-resist ink may also be applied over a predetermined area using a paint brush or other means.

In the embodiments shown in Figs. 2 and 3, the insulating film 13 is formed only over an area near the respective terminals 4, but this invention is not limited to this arrangement. Needless to say, the insulating film 1 3 may be formed over the entire external surface of the metallic motor case cover 2.

Claims (4)

1. A rotary electric motor having a generally cylindrical motor case open at one end and having a motor case cover at least the external surface of which is metallic; the rotor shaft being supported in respective bearings mounted on the motor case and on the motor case cover; the rotor shaft supporting a commutator; and the motor case cover having terminal holes through which extend terminals for coupling the commutator brush gear to an external electric circuit and an electrically insulating film on said external surface at least in the vicinity of and surrounding said terminal holes.

2. A motor according to Claim 1, wherein said electrically insulating film extends over the whole of the said external surface.

3. A motor according to Claim 1 or Claim 2, wherein the said electrically insulating film is formed by painting on solder-resistant ink.

4. A rotary electric motor substantially as herein before described with reference to and as shown in Figs. 2 and 3 of the accompanying drawings or Figs. 1A to 1 C of the accompanying drawings modified as shown in Fig. 2 or in Fig. 3 of the accompanying drawings.