GB2209880A

GB2209880A - Magnet retainer in an electric motor

Info

Publication number: GB2209880A
Application number: GB8721804A
Authority: GB
Inventors: To-Fan Wong
Original assignee: Johnson Electric Industrial Manufactory Ltd
Current assignee: Johnson Electric Industrial Manufactory Ltd
Priority date: 1987-09-16
Filing date: 1987-09-16
Publication date: 1989-05-24
Anticipated expiration: 2007-09-16
Also published as: JPH0171971U; GB2209880B; DE8811786U1; HK131994A; GB8721804D0

Abstract

To retain magnets 15 in a fractional horsepower PMDC motor, a U-shaped spring 23 urges the magnets 15 against axial stops 18 and circumferential stops 22. The spring 23 is hooked over a tang 24 pressed from the motor casing wall. <IMAGE>

Description

MAGNET RETAINER IN AN ELECTRIC MOTOR The present invention relates to a fractional horsepower PMDC motor, and in particular to the location of magnets in such motors.

GB2182208 addresses the problem of securing magnets in wminiaturew PMDC motors and in particular the need to compensate for variations in the length of bosses which are provided on the motor end cap to locate the magnets in the axial direction. If the bosses are too short the magnet is not adequately secured and if the bosses are too long the end cap cannot be properly fitted to the motor casing. GB2182208 proposes to provide bosses which are a press fit in the end cap, the bosses being positioned during assembly to abut the magnets. One drawback of this proposal is that such motors often have to operate in environments where there is substantial vibration.

Under these conditions the pins will creep through the end cap, away from the magnets, leaving the magnets free to move axially.

Also the pins use up useful space in the motor, space which could be used by a cooling fan, mounted on the rotor, for example, the blades of the fan extending out to the motor casing.

The present invention provides a fractional horsepower permanent magnet direct current motor comprising a tubular casing carrying a permanent magnet, wherein said magnet is located in the casing against movement in an axial direction by means of a spring.

More particularly the invention provides a fractional horsepower permanent magnet direct current motor comprising a tubular casing carrying at least two permanent magnets, wherein said magnets are located in the casing against axial and circumferential movement by means of a spring, said spring being positioned between adjacent axial edges of said magnets and having a generally U-shaped configuration, said spring being at the base of the U-shape hooked over a stop in the casing, each arm of said U-shape bearing against an axial edge of a respective magnet to urge it circumferentially, and ends of said arms being hooked over axial ends of the respective magnets to urge the magnets in the axial direction against an abutment.

By locating the magnets in the axial direction by means of a spring the need for bosses on the end cap is avoided, thus providing more room inside the motor for a fan etc.; and also the spring is able to take up automatically tolerances in the magnet length and hold the magnets securely in position in environments subject to vibration.

Other preferred features and advantages of the invention will be apparent from the following description and the accompanying claims.

The invention will be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is an exploded view of a fractional horsepower PMDC motor embodying the invention; Figure 2 is a cross-sectional part view along the line II-II of Figure 1; and Figure 3 is a cross-sectional view along line III-III of Figure 1.

In Figure 1, a PMDC motor comprises an end cap 2 carrying brushgear 3, a rotor 4 and a stator assembly The end cap 2 is moulded from plastics material and carries a bearing 6 for a shaft of the rotor 4.

Brushgear 3 in the end cap comprises a pair of brushes 7 each carried by a brushleaf 8 mounted in the end cap and having a terminal 9 extending through the end cap for connection to a power supply. The end cap 2 has a stepped outer peripheral surface, the smaller diameter portion 10 being received in the end of a casing of the stator assembly.

The rotor 4 comprises a shaft 11 carrying a wound armature 12 and a commutator 13.

The stator assembly 5 comprises a deep drawn can-like steel casing 14 carrying two permanent magnets 15 and a bearing 16 on an end wall 17 of the casing for the shaft 11.

The present invention resides in the manner of holding the magnets 15 in the casing 14.

Four inwardly directed protrusions 18 are stamped in the casing sidewall 19, the protrusions being spaced apart in the circumferential direction. Each magnet 15 abuts at its lower axial end 20 against a pair of the protrusion 18. First adjacent axial edges 21a, 21b of the magnets 15 abut butterfly wings 22 which are stamped in the casing sidewall 19 (Figure 3).

The magnets are held against the protrusions 18 and wings 22 by a spring 23, as seen in Figure 2. Spring 23 is generally U-shaped and the base of the U-shape is hooked under a tang 24 pressed out from the casing sidewall 9, diametrically opposite the wings 22.

Arms 25 of the U-shape are bowed outwardly and abut axial edges 26a, 26b of the magnets 15. When the spring 23 is installed in the motor, the arms 25 are compressed against each other, and hence exert force on the magnets 15, urging them against the wings 22.

The ends 27 of the spring arms are bent outwardly and back towards the base of the U-shape. The ends 27 clip over the axial ends 28 of the magnets 15. The ends 27 are deformed when clipped over the magnet ends 28 and so urge the magnets back against the stops 18.

To assemble the stator 2, the casing 14 is drawn and the protrusions 18 and wings 22 stamped in the casing wall. The magnets are positioned against the protrusions and wings and the spring 23 then pushed between the magnet edges 26 and hooked over the tang 24. The rotor is then inserted in the stator and the end cap 2 fitted to the casing 14.

Various modifications may be made to the described embodiment. For example, a second spring 23 may be used in place of the wings 22. The invention is equally applicable to motors having, for example, four magnets, a spring 23 being located between pairs of adjacent magnets. The springs may be urged against the end wall 17 rather than protrusions 18.

It is desired to include all such modifications as fall within the scope of the accompanying claims.

Claims

1. A fractional horsepower permanent magnet direct current motor comprising a tubular casing carrying a permanent magnet, wherein said magnet is located in the casing against movement in an axial direction by means of a spring.

2. A motor as claimed in claim 1, wherein the spring urges the magnet in the axial direction against an abutment in the casing.

3. A motor as claimed in claim 1 or 2, wherein the spring is hooked around a stop in the casing to locate the spring in the casing.

4. A motor as claimed in claim 3, wherein the spring extends from said stop along an axially extending edge of the magnet and is clipped over an axial end of the magnet to urge the magnet in the axial direction.

5. A motor as claimed in any one of claims 1 to 4, having two magnets.

6. A motor as claimed in claim 5, wherein a common spring urges two said magnets in the axial direction.

7. A fractional horsepower permanent magnet direct current motor comprising a tubular casing carrying at least two permanent magnets, wherein said magnets are located in the casing against axial and circumferential movement by means of a spring, said spring being positioned between adjacent axial edges of said magnets and having a generally U-shaped configuration, said spring being at the base of the U-shape hooked over a stop in the casing, each arm of said U-shape bearing against an axial edge of a respective magnet to urge it circumferentially, and ends of said arms being hooked over axial ends of the respective magnets to urge the magnets in the axial direction against an abutment.

8. A motor as claimed in claim 7, whrein the stop is formed by a tang pressed from the casing wall.

9. A motor as claimed in claim 8, wherein the magnets are urged circumferentially against an abutment or abutments in the casing.

10. A motor as claimed in claim 7 or 8, wherein a magnet is positioned in the circumferential direction between two said springs.

11. A fractional horsepower permanent magnet direct current motor substantially as hereinbefore described with reference to the accompanying drawings.