GB2151413A - Electric motor - Google Patents

Abstract

A miniature electric motor has a generally cylindrical motor case 7 integrally formed with a first closed axial end provided centrally with a first bearing 9 for rotatably supporting the motor shaft 11. The other axial end of the motor case is generally closed by a motor case cover 16 formed of synthetic resin material and provided centrally with a second bearing 10 for rotatably supporting the motor shaft. The motor case cover is adapted for the support of brush gear 12 for the motor and terminals 14 for connection of the motor in an external electric circuit. A thin metal film 17 is present on one or both axial surfaces of the motor case cover 16 to provide at least partial shielding axially of the motor for electromagnetic noise generated within the motor in use. <IMAGE>

Description

SPECIFICATION Electric motor This invention relates to electric motors.

Miniature electric motors are widely employed in industry. They usually have a generally cylindrical motor case and a motor case cover or end cap for the motor case and made of a synthetic resin material. The use of such material is advantageous since it allows relatively complicated parts such as brush mounts and terminal fixing means to be integrally moulded on and to be part of the motor case cover. A second advantage is that of electrical insulation for the various electrically conductive components such as brushes and terminals which can conveniently be mounted on and passed through the motor case cover.

However, in such commutator type motors, radio frequency interference and other forms of electro-magnetic noise result for example from sparks generated between the commutator and the brush gear. A motor case cover made of synthetic resin material has little effect in shielding against such electromagnetic noises.

Such noise is very undesirable when the miniature electric motors are to be used for example in apparatus including a magnetic tape device. In an effort to prevent the radiation of electromagnetic noise, one previous approach has been to enclose the motor with a shield case made of silicon steel and other materials.

The present invention is directed to a wholly different and substantially less expensive approach to the problem.

In accordance with the present invention, we provide an electric motor, having a generally cylindrical motor case integrally formed with a first closed axial and centrally provided with a first bearing for rotatably supporting the motor shaft, the other axial end of the motor case being generally closed by a motor case cover or end cap formed of synthetic resin material and centrally provided with a second bearing for rotatably supporting the motor shaft, the motor case cover or end cap being adapted for the support of brush gear for the motor and terminals for connection of the motor in an external electric circuit, and a thin metal film on one or both axial surfaces of the motor case cover or end cap to provide at least partial shielding axially of the motor for electromagnetic noise generated within the motor in use.

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 shielding arrangement for miniature electric motors; Figure 1B is an end view of the arrangement shown in Fig. 1 as seen from the right in Fig. 1; Figure 2A is a part-sectional side elevational view of an embodiment of electric motor in accordance with the present invention; Figure 2B is an end plan view of the motor of Fig. 2A as seen from the right in Fig. 2A; Figure 3A is a part-sectional side elevation of the motor case cover of the embodiment of Figs. 2A and 2B indicating in phantom lines and in stipple the location of the thin metal film; Figure 3B is a view generally similar to Fig.

2B indicating by means of phantom lines and stippling the location of the thin metal film; and Figures 4A to 4E show graphs illustrating experimental data for electric motors having various shielding arrangements.

In the arrangements of Figs. 1A and B, a miniature electric motor 1 is mounted within a shield casing comprising a shield case 2 proper and a shield case cover 3. For proper location of the motor within the shield case and for the avoidance of vibrations, the motor is mounted against the shield case cover through the means of elastic mats 4 and 5 at opposite axial ends of the shield casing. A governor support plate is provided at 6.

As will be appreciated, in this arrangement the radiation of electromagnetic noise in the radial direction of the motor is double shielded by means of firstly the motor case and secondly the shield case, both made of metallic materials. For additional radial shielding a ferromagnetic metallic sheet may be wrapped around the motor case between the motor case and the shield casing. On the other hand, noise radiated in the axial direction of the motor and specifically in the direction of the motor case cover cannot satisfactorily be shielded with a shield case cover alone.

It should also be noted that the provision of the shield casing adds significant expense to the cost of the motor overall.

The miniature electric motor shown in Figs.

2 A to 3B has a generally cylindrical motor case 7 integrally formed with a first closed axial end. The motor stator is provided by permanent magnets mounted on the interior of the cylindrical wall of the case 7. The rotor 8 is mounted for rotation within the case 7. A first bearing is centrally provided at the closed axial end of the motor case. A second bearing 10 is centrally provided on a motor case cover or end cap formed of synthetic resin material and closing the other end of the motor case.

The motor shaft 11 is rotatably supported by means of the bearings 9 and 10. The motor brush gear is supported by the motor case cover and comprises respective brushes 1 2 mounted for electrical contact with a commutator 13. The brushes 12 are integrally formed with terminal portions 14 which extend through terminal holes 1 5 in the motor case cover 16.

The motor case cover 16 is suitably moulded from an ABS resin, polyacetal resin, nylon resin or any other readily mouldable synthetic resin material. By the use of such materials, supports for or mounting means for the brushes 12, the terminals 14 and the bearing 10 can readily be provided on the inner side of the motor case cover so that all that can be seen from the exterior are the three openings 11' and 14', 14' for the motor shaft and terminals.

A thin metal film 1 7 is provided on the outer axial surface of the motor case cover 1 6 in the localities illustrated in Figs. 3A and 3B by means of phantom chain lines and by stippling. Suitable metals include copper or aluminium. Suitable techniques for the deposit of such metals on the motor case cover 1 6 as a thin film include chemical or electrolytic plating, vacuum deposition, or a combination of chemical and electrolytic plating techniques or vacuum and electrlytic plating techniques. The selection of the most appropriate deposition technique will depend upon the material of the synthetic resin motor case cover 16.

The thin film 1 7 may be provided as shown in Figs. 3A and 3B on the outer axial surface of the motor case cover 1 6, or alternatively on the inner axial surface of the motor case cover 16, or on both the inner and outer axial surfaces of the motor case cover. Care must, however, be taken, in depositing the thin metal film on the motor case cover to ensure that electrical insulation is maintained between the brushes 1 2 and between the motor terminals 14. As best shown in Fig. 3B, the metal film 1 7 is omitted from a region 1 8 surrounding the motor terminal openings 14'.

The thickness of the thin metal film may readily be determined experimentally based on the type of motor and the service conditions to be encountered. The particular embodiment of Figs. 2A to 3B has a motor case cover with a thin metal film 17 of 20 microns thickness present on the outer axial surface only of the case cover, and this has been found to provide a sufficient electro-magnetic shielding effect.

Referring now to Figs. 4A to E, in each of these graphs, the detected noise level in dB axially of the motor (ordinate) is plotted with variation in angle (abscissa) of the rotor.

The graph of Fig. 4A illustrates the shielding effect of a case cover made of iron. Fig.

4B illustrates the absence of any significant shielding effect in a motor substantially similar to that of Figs. 2A to 3B but without any thin metal film. Fig. 4C shows the corresponding effect in the same motor when a 1.0 micron thick metal film is applied to the outer axial surface of the motor case cover. Fig. 4D illustrates the substantially greater shielding effect when a 20 micron metal film is present on the outer axial surface of the motor case cover. Fig. 4E shows almost complete shielding with a motor case cover having a 40 micron metal film on the axial outer surface thereof. Thus, it will be seen that the provision of a thin metal film can provide an effective electromagnetic shielding effect against the radiation of electromagnetic noises in the axial direction of the motor.

The external appearance of the motor can be enhanced by the manner in which the thin metal film is applied or by a decorative effect provided on the thin metal film.

As will be clear from comparing either Fig.

4D or 4E with Fig. 4A, the mere provision of a thin metal film can provide a shielding effect comparable with that which would be provided by a metallic case cover.

Claims (5)

1. An electric motor, having a generally cylindrical motor case integrally formed with a first closed axial end centrally provided with a first bearing for rotatably supporting the motor shaft, the other axial end of the motor case being generally closed by a motor case cover or end cap formed of synthetic resin material and centrally provided with a second bearing for rotatably supporting the motor shaft, the motor case cover or end cap being adapted for the support of brush gear for the motor and terminals for connection of the motor in an external electric circuit, and a thin metal film on one or both axial surfaces of the motor case cover or end cap to provide at least partial shielding axially of the motor for electromagnetic noise generated within the motor in use.

2. An electric motor according to Claim 1, wherein the motor case cover or end cap is provided with respective terminal holes through which terminals extend externally of the motor for connection of the motor in an electric circuit, and wherein there is a region surrounding each said terminal hole where the said thin metal film is omitted.

3. A motor according to Claim 1 or Claim 2, wherein said thin metal film is formed of copper or aluminium.

4. A motor according to any preceding Claim, wherein said thin metal film has a thickness of not less than 20 microns.

5. An electric motor substantially as here inbefore described with reference to and/or as shown in Fig.s 2A to 3B and 4C to 4E of the accompanying drawings.