GB2222317A

GB2222317A - Armature insulation in an electric motor

Info

Publication number: GB2222317A
Application number: GB8820089A
Authority: GB
Inventors: Strobl Georg
Original assignee: Johnson Electric Industrial Manufactory Ltd
Current assignee: Johnson Electric Industrial Manufactory Ltd
Priority date: 1988-08-24
Filing date: 1988-08-24
Publication date: 1990-02-28
Also published as: GB8820089D0; DE8910158U1; JPH0260475U

Abstract

An armature 21 in an electric motor comprises a stack 22 of steel laminations mounted on a shaft 19. End laminations 23 of electrically non-conducting material are fitted on the stack 22 and then the assembled laminations are spray coated with an epoxy resin paint prior to winding the armature. It is known to use such an epoxy coating on a lamination stack but with heavy gauge armature the wire may bite into the coating and contact the end laminations at the edges of the arms. By adding suitable non-conducting end laminations before coating the user can still achieve the benefits of an epoxy coated armature.

Description

ARMATURE IN AN ELECTRIC MOTOR The present invention relates to an armature in an electric motor. More particularly, the invention relates to a wound armature having a laminated stack and to an electric motor, particularly a fractional horsepower PMDC motor, incorporating such an armature.

Such armatures are made from a stack of steel laminations which each comprise a plurality of mushroom shaped arms extending radially out from a central hub which fits on the motor shaft. Wire is wound tightly around the stem of the mushroom shaped arms of the stack, to form the armature coils. The wire used usually has a polymer coating to provide insulation between the wire turns and the wire and stack. However, it is necessary to provide a further layer of insulation between the wire and the stack to minimise the risk of shorting between the wire and stack.

There are various reasons for this: as the wire is wound onto the stack the polymer coating may be damaged; the wound wire, and stack, is subject to high centrifugal forces during use because of the very high r.p.m. at which small PMDC motors are run causing the wire to rub against the stack; and the motor may reach very high temperatures, particularly under stall conditions, giving rise to thermal stresses and relative movement of the wire and stack.

It is known to coat the armature stack with a polymer paint, particularly an epoxy based material, at high temperature to bake the "paint onto the stack and form a hard insulating coating which serves also to hold the laminations together. Also, non-conductive end laminations of plastic film or fibre filled phenolic material may be used. The end laminations may be allowed to overlap the stems of the mushroom shaped arms so as to protect the wire from the edges of the steel end laminations.

When heavy gauge wire is wound on the armature it is necessary to wind the wire under a high tension in order to ensure that the wire coils are tightly packed and avoid chaffing between the wire and stack. It has been found that when the wire is wound on epoxy coated stacks it may cut into the layer of epoxy coating at the edges of the end laminations due to the tension under which the wire is wound. Some users require an epoxy coated stack and so the option of using and laminations of insulating material has not heretofore been open to them.

The present invention provides an armature for an electric motor, the armature having a lamination stack carrying a wire winding, the stack comprising a plurality of steel laminations stacked one against another and end laminations of electrically insulating material, the stack being coated with a high temperature resilient polymeric coating. One particular such coating is epoxy resin paint.

The end laninations of electrically insulating material ensure that any wire which cuts through the polymeric coating at the edges on the end of the stack does not contact the steel laminations.

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 perspective view of a fractional horsepower permanent magnet direct current motor embodying the invention; and Figure 2 is an exploded perspective view showing a lamination stack of the motor of Figure 1.

In Figure 1, a PMDC motor comprises a deep drawn can-like steel casing 10 closed at one end 11 which carries a bearing 12. Two permanent magnets 13 are located in the casing 10. An end cap 14 of plastics material is adapted to close the casing 10 and carries a bearing 15, two brushleaves 16 which carry brushes 17, and two terminals 18 (one is shown) electrically connected to the brushleaves 15. The motor rotor comprises a shaft 19 carrying a commutator 20 and armature 21 both mounted fast on the shaft 19 for rotation therewith. The shaft 19 runs in the bearings 12, 15.

To assemble the motor, the rotor is installed in the casing with one end the shaft 19 in bearing 12. The end cap 14 is slid over the other end of the shaft to close the casing, the brushes 17 bearing on the commutator 20. The construction thus for described is well known in the art.

Referring to Figure 2, the armature 21 comprises a plurality of steel laminations 22 which are stacked together on the shaft 19. The laminations are a tight fit on the shaft 19. Two end laminations 23 are then placed on the shaft at either end of the stack 22. The laminations 23 are of non-conducting material, and in the example shown are of a high temperature plastics such as fibre-filled phenolic material. The thickness of the laminations 23 is exaggerated in the drawing.

Typically they will be less than lmm. thick. The laminations 23 match the dimensions of the steel laminations, particularly where the wire contacts the end laminations. It is preferred, for example, that the width W of the mushroom shaped arms of the end laminations be equal to and no greater than that of the steel laminations in order to avoid any tendency of the end laminations 23 to buckle when the armature is wound. The end laminations 23 are pushed on the shaft 19 to line up against the stack 22. The assembly is then heated, typically to 2250C and the laminations spray coated with an epoxy resin paint of a type typically used in the art. This serves to provide an insulating coating over the armature stack and also to hold the body of laminations together.

The commutator 20 is then fitted on the shaft 19 and the armature is then wound with wire 24 in the usual way.

Various modifications will be apparent to those skilled in the art and it is desired to include all such modifications as fall within the scope of the accompanying claims.

Claims

1. An armature for an electric motor, the armature having a lamination stack carrying a wire winding, the stack comprising a plurality of steel laminations stacked one against another and end laminations of electrically insulating material, the stack being coated with a high temperature resistant polymeric coating.

2. An armature as claimed in claim 1, wherein the coating is an epoxy resin paint.

3. An armature as claimed in claim 1 or 2, wherein the end laminations are of high temperature resistant plastics material.

4. An armature as claimed in claim 3, wherein the end laminations are of fibre filled phenolic material.

5. An armature as claimed in any one of claims 1 to 4, wherein the outer diameter of the end laminations is less than the outer diameter of the steel laminations.

6. An armature as claimed in any one of claims 1 to 5, wherein the laminations each comprises mushroom-shaped cross-section arms extending outward from a central hub and the width of stems of the arms of the end laminations when viewed along the axis of the stack is equal to the width of the stems of the arms of the steel laminations.

7. An armature for an electric motor, substantially as hereinbefore described with reference to the accompanying drawings.

8. A fractional horsepower PMDC motor, comprising a deep drawn can-like steel casing carrying permanent magnets, an end cap carrying brushgear and closing the casing, a rotor comprising a shaft mounted in bearings in the end cap and casing, an armature and a commutator mounted fast on the shaft, wherein the armature has a lamination stack comprising a plurality of steel laminations stacked one against another and end laminations of electrically insulating material, the stack being coated with a high temperature resistant polymeric coating.

9. A fractional horsepower PMDC motor substantially as hereinbefore described with reference to the accompanying drawings.