ARMATURE IN AN ELECTRIC MOTOR

Abstract: An armature for a fractional horsepower motor is provided with spiders (10A) having spokes (20) and arms (24). The spiders (10A) fit at at least an end of laminated armature stacks around which the armature windings are wrapped. The spokes (20) are profiled to provide upstanding rounded surfaces (23) for the windings and to reduce drag. Reduction in drag and the profiled shape of the spokes (20) improves air flow through the motor and reduces the tendency of build up of carbon dust in the region of the extremities (A) of arm of the laminated stack as well as reduce motor noise in use.

Description: ARMATURE IN AN ELECTRIC MOTOR The present invention relates to an electric motor. More particularly, the invention relates to a wound armature for an electric motor having a laminated stack and to an electric motor incorporating such an armature. Such armature are conventionally made from a stack of steel laminations which stack comprise a plurality of mushroom or T-shaped laminations with their stems extending radially out from a central hub which fits on a motor shaft. Each stems has arms which extend circumferentially of the rotational axis of the motor on each side of the stem. Wire is wrapped tightly around the stems of the stack to form armature windings. The wire used usually has a polymer coating to provide insulation between each of the wire turns and between the wire and stack. It is necessary to wind the wire under a tension in order to ensure that the windings are tightly packed and avoid chaffing between the wire and stack. It has been found that when the wire is wound onto the stems, cuts may be formed into the layer of insulation coating the wire due to the tension under which the wire is wound. A simple "spider" comprising a spoked plastic disc may be placed against the outer surface of each end lamination to prevent damage to the wound wires of each coil. According to the invention .there is provided an armature, for an electric motor, including a rotary shaft, the armature being supported on the shaft and having a lamination stack, the stack comprising a plurality of T- shaped steel laminations stacked one against another, each lamination having radially extending stems with arms which extend circumferentially with respect to the rotational axis of the shaft each side of the stems, armature windings wrapped around the stems of the stack, a spider at one end of the stack positioned inside the armature windings and formed of electrically insulating material and having a plurality of profiled spokes, the spokes each have one surface which fits against and covers a respective stem of the stack and an opposing arcuate surface to form a central upstanding profile extending to adjacent a radially remote end of the respective spoke, and arms extending circumferentially each side of the spoke at the end of the spoke to partly cover respective arms of the stack. The spider may have a central boss which extends axially beyond a surface of each spoke and snugly fits over the shaft. The spider are preferably profiled over the whole length of their spokes to provide a smooth curved base against which the wires of the armature windings are urged at each end of the stack when the windings are formed. The invention will be further described by way of example with reference to the accompanyingd drawings, in whic :- Figure 1 is a plan view of a fractional horsepower universal motor embodying the invention; Figure 2 is an elevation of a spider for an armature of the motor showing, in part, armature windings and one end of an armature stack. Figure 3 is a part-sectional side view of the spider; Figure 4 is a sectional view along IV-IV of Figure 2; and Figure 5 is an enlarged isometric view of a radially remote end of one spoke of the spider. Referring to the drawings in Figure 1 , a universal motor for a food mixer comprises a pressed steel frame in two parts 1, 2. The frame parts 1, 2 are joined to opposite sides of a U-shaped laminated stator 3. A rotor 4 of the motor comprises a shaft 5 having a worm 6 at one end and carrying an armature 7, with armature windings 8, and a commutator 9. The armature is formed as a laminated stack of a plurality of T-shaped laminations and spiders 10A and 10B are mounted at each respective end of the stack. Brushgear 11 is mounted on the frame part 2 and supplies current to the commutator 9. The frame part 1 carries two implement holders 11, shown in dotted outline, for holding food beaters or the like, which mesh with the worm 6. The frame part 1 comprises a bearing support part 12 for a bearing 13. The frame part 2 comprises two arms 14 which extend on opposite sides of the commutator 9 and are joined at their first ends by a bearing support or rear wall 15 and are mounted at their second ends 16 on the stator 3. Rear wall 15 is dished to receive a bearing 17 for the motor shaft 5. The frame parts 1 and 2 are held on the stack 3 by means of two screws 18. The frame part 2 carries brushgear 19, which comprises a plastics brush frame which straddles the commutator 9. To assemble the armature, the spiders 10A and 10B are placed on the shaft at either end of the laminated stack. The spiders are formed of non-conducting material and have almost the same cross-section in elevation as the steel laminations (see below). The spiders 10A and 10B are slidingly fitted on the shaft 19 to line up against the stack. The commutator 9 is then fitted on the shaft 5 and the armature is then wound with wire in the normal way to form the windings 8. The spider 10A, at least, is formed and shaped in a particular manner in accordance with the present invention. The spider 10A is better seen in Figures 2 to 6. Importantly, it will be seen that the spider 10A has a plurality of profiled spokes 20 (see especially Figure 4). One surface of each spoke comprises a mating planar surface 21 which fits snugly against a respective end face of the armature stack. An opposing upstanding profiled surface of each spoke consists of an arcuate surface against part of which wires of the armature windings 8 bears. In Figure 2, the armature windings are shown for illustration purposes on only three of the spokes 20. The spider 10A protects the wire.: from bearing directly against sharp corner edges of stems of the laminations and supports the hires as they are turned around the ends of the armatur* stack during winding. Exposed parts 23 of the surfaces 22, that is those parts not covered with wires of the armature windings, present an arcuate surface to any cooling medium, usually air, which in use is forced axially through the motor past the spider 10A. Even where no forced cooling is applied the exposed and profiled surfaces 23 generate less turbulent disturbance of surrounding air than a non-profiled surface, so that the described motor, which is designed to run at high rotational speeds, produces less noise. The spokes 20 are formed integrally with circumferentially extending arms 24, which are preferably thinner in an axial direction than the axial depth of the spokes 20, at each side at their ends to partly cover respective arms of the laminations. It will be seen at A that the lamination arms extend somewhat beyond the ends of the arms 24. In other words, the spokes 20 of the spider 10A have generally the same cross-section as the stems of the lamination stack but the arms 24 do not completely cover the arms of the laminations. It will also be noted (see Figure 5) that the transitions between the arms and the spokes is smoothly curved to streamline the transitions in the direction of rotational movement of the spider. The spider 10A serves a further purpose. As the motor brushes wear in use, carbon dust is released and tends to be blown axially through the motor and along the inside of the armature 7. Some of the dust normally tends to collect on one end of the armature and can build up to create electrical shorts between the armature wires and the exposed edges A of the end lamination and even short to the motor casing. Because the stems of the laminations, at least at the one end next to the brush gear, are covered by the spokes 20 and the arms of the laminations are partly covered by the arms 24, carbon dust no longer tend to build up against the end of the lamination stack. This is because the spokes 20 are profiled and shaped as described and shown so that drag in the region of the end laminations is reduced and air currents more readily carry the carbon dust through the motor and away from the end lamination. The reduction of drag in any event encourages better air flow patterns so reducing the motor temperature in use and so improve the motor efficiency. It will be noted as stated that the arms 24 do not completely cover the respective arms of the end lamination. There is in practice little tendency where the spider 10A is provided and shaped as described for dust to remain on the small uncovered areas A due to the profiling of the spokes. Nor does dust accumulate at the small step-downs where the arms 24 terminate against the surfaces of the arms of the laminations. As the arms 24 have little inherent strength, they are deliberately made somewhat shorter than the arms of the laminations 24 so as not to be subjected to undue strain when the armature wires are being wound onto the lamination stack. Further, the described arms 24 do not interfere in any way with the feeding of armature wires on to the lamination stack as the armature is wound, or become damaged during winding. As mentioned earlier, simple spiders may be normally provided primarily to prevent scuffing of armature winding wires at the ends of a laminated stack. However, the spiders 10A are especially profiled normally along the full length of each spoke 20, but importantly the profile extends or is present in the region not covered by the armature windings and extends to the radially remote end of each spoke. This enhances the aerodynamic performance of the armature to reduce noise, reduce the build up of carbon dust as described, which may result in shorting, and reduces drag. In other words, armatures with spiders 10A, according to the invention and fitted at least at one end of the armature stack, enable more reliable and more efficient motors to be manfuactured. The spider 10B may be for example a "simple spider". The spiders 10A are readily formed by plastics moulding techniques and can be "profiled" to further suit different shaped armature laminations and/or different applications. A compromise in the profiling may be chosen for example to particularly reduce noise in situations where the build-up of carbon dust is expected to be less of a problem. The exposed surfaces 23 of the spokes are then less rounded or flatter in profile than say the non-exposed parts of the spokes. In any event, suitable profiles can be developed and easily reproduced which provide optimum characteristics for removal of carbon dust or reduction of drag, and/or reduction of noise. Various modifications may be made to the described embodiment and it is desired to include all such modifications as fall within the scope of the accompanying claims.

Claims: Claims 1. An armature, for an electric motor, including a rotary shaft, the armature being supported on the shaft and having a lamination stack, the stack comprising a plurality of T- shaped steel laminations stacked one against another, each lamination having radially extending stems with arms which extend circumferentially with respect to the rotational axis of the shaft each side of the stems, armature windings wrapped around the stems of the stack, a spider at one end of the stack positioned inside the armature windings and formed of electrically insulating material and having a plurality of profiled spokes, the spokes each have one surface which fits against and covers a respective stem of the stack and an opposing arcuate surface to form a central upstanding profile extending to adjacent a radially remote end of the respective spoke, and arms extending circumferentially each side of the spoke at the end of the spoke to partly cover respective arms of the stack. 2. An armature according to claim 1, in which the arms are thinner in the axial direction than the axial thickness of the upstanding profile. 3. An armature according to claim 2, in which the transition between the ends of the spokes and the arms are smoothly curved to be streamlined in the direction of rotational movement of the spider. 4. An armature adcording to any one of claims 1 to 3, in which the spider has a central boss which extends axially beyond a surface of each spoke and snugly fits over the shaft. 5. A universal motor having an armature according to any one of claims 1 to 4.

Inventors: MOK KAM-SHING, WONG CHI-NUNG, MOK, KAM-SHING, WONG, CHI-NUNG
Applicants: JOHNSON ELECTRIC SA, JOHNSON ELECTRIC S.A.
Application year: 1991
Main classification: H02K3
IPC codes: H02K3