GB2193045A - An armature for an electric motor - Google Patents

Abstract

The armature has windings (28) supported by a core (23) mounted on a shaft (25). At least a part of the armature, and preferably the windings (28) and any wires connecting the windings (28) to a commutator (24) are encapsulated in an insert moulded body (30) of plastics material, typically nylon. <IMAGE>

Description

SPECIFICATION An armature for an electric motor This invention relates to an armature for an electric motor and to an electric motor provided with such an armature.

A conventional armature for an electric motor essentially comprises a core, usually in the form of a lamination stack, a commutator or slip rings at one end of the core, a spacer at the other end of the core to prevent the windings rubbing against the inside of the motor frame and to fix the length of the motor to give a predetermined end play between supporting bearings, and windings supported by the core and connected by lengths of wire to the commutator or slip rings. The core, commutator or slip rings, and the spacer are individually mounted as interference fits on the motor shaft commonly with spaces therebetween. Such an armature suffers several drawbacks. Rigidity along the length of the armature is sometimes small thus allowing the shaft to flex when radial loads are applied to that portion of the shaft outside the motor frame and also should the armature be out of balance.This flexing reduces the performance of the motor and can lead to complete failure if excessive. Under heavy axial loads movement of, for example, the commutator or spacer along the shaft can cause end play to become excessive. Moreover, torsional strain on the shaft when the armature is used in applications requiring rapid acceleration and deceleration can establish torsional resonances on the armature creating problems of control.

In a conventional armature the forces of acceleration and deceleration when the motor is started or stopped combined with thermal expansion and contraction of wires due to selfheating, centrifugal forces of rotation, and any external vibrations cause wires lying next to one another to rub together and wear away their insulation layers, thus causing shorting of turns and subsequent loss of performance of the motor. Moreover, the lengths of wire which connect the winding coils to the commutator or slip rings are suspended in air and having been soldered or welded to the commutator or slip rings have usually experienced some embrittlement. The forces of acceleration and deceleration, centrifugal force, and external vibrations on these free lengths of wire can cause them to fracture and disconnect the motor.

Moreover, it is not uncommon for localised heating to take place in the windings of con ventional armatures causing high temperature rises and failure to occur due to breakdown of insulation.

In a conventional armature, particularly one having an odd number of poles, it is always difficult to ensure an even distribution of the amount of copper in each slot and/or the position of copper in each slot relative to the axis of rotation of the armature. This creates a degree of imbalance resulting in vibration.

It is known to add elements for, inter alia, suppression, which elements are located at or adjacent to the commutator and connected to the commutator or to the wires leading from the winding coils to the commutator. These elements may be fragile and they and the connections to them may suffer serious damage due to the forces of vibration.

Furthermore, in a conventional armature in which joints between the winding wire and the commutator (or slip rings) or the aforesaid elements are used some solder flux becomes deposited on surrounding areas. At some time later during operation of the motor this flux may vapourise and recondense on the surface of the commutator (or slip rings) or on brush gear causing failure of the motor.

Finally, windage losses caused by the drag effect of the uneven slotted surfaces of conventional armature cores and by cavities in the slots themselves can represent a power loss in the motor.

The present invention seeks to mitigate at least some of the aforesaid drawbacks and/or to improve the armature in other respects.

According to the invention there is provided an armature for an electric motor, at least a part of which has been encapsulated in a body of plastics material by insert moulding.

Preferred and/or optional features of the invention are set forth in the subsidiary claims.

In a preferred embodiment the windings including end turns thereof are entirely encapsulated in the body of plastics material, typically of nylon. Advantageously, free lengths of wire which connect the windings to the commutator and where possible the actual connections themselves are also encapsulated in the body of plastics material.

Encapsulation prevents movement between individual turns and wires and thus prevents rubbing or fracture. The body of plastics material also serves to dissipate heat from hot spots in the armature resulting in a more balanced thermal system. It will also improve the mechanical balance and rigidity of the armature The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an exploded perspective view of a known electric motor, with part of the casing cut away; Figure 2 is a side view of one embodiment of an armature according to the invention, which can be substituted for the armature shown in Fig. 1, Figure 3 is a section taken along line Ill-Ill of Fig. 2, and Figure 4 is a longitudinal section taken along line IV-IV of Fig. 3.

Referring firstly to Fig. 1 of the drawings, the motor shown therein is a fractional horse power permanent magnet dirept current motor comprising a cylindrical can-like casing 10 which is closed at one end and a plastic end cap 11, typically of nylon, which is fitted in the other end of the casing. Two stator mag nets 12 and 13 are fixed within the casing in conventional manner and an armature 14 is supported for rotation within the casing and between the stator magnets by bearings 16 and 17 mounted, respectively, in the end cap 11 and in the closed end of the casing 10.

The end cap 11 supports two brush leaves 18 and 19, each carrying a carbon brush 20.

The brush leaves 18 and 19 are connected to terminals 21 and 22, respectively, which ex tend through apertures in the end cap 11.

The armature 14 comprises a laminated core 23 and a commutator 24 both of which are mounted as an interference fit on a shaft 25. The core 23 has five (in this example) pole pieces 26 of mushroom-shaped cross section and is spaced from the commutator 24 along the shaft 25.

Slots 27 between the pole pieces 26 acco modate armature windings 28 which are con nected to the segments of the commutator 24.

A spacer 9 is also mounted on the shaft 25 to prevent end turns of the windings 28 from rubbing against the closed end of the casing 10.

A suppression element in the form of a ring-shaped varistor 29 is soldered to each commutator segment.

The construction thus far described is well known in the art and embodied in a small p.m.d.c. motor manufactured and sold by the applicants under catalogue No. HC315.

The armature shown in Figs. 2 and 3 differs from that shown in Fig. 1 in that the windings 28 are encapsulated in a body 30 of plastics material, such as nylon. The encapsulation process is effected by insert moulding, which term is to include compression or injection moulding, and the outer surface of the body 30 takes the shape of the mould rather than that of the windings. As shown in Fig. 3, the plastics material fills any unoccupied space in the slots 27 and as shown in Fig. 2 not only encapsulates the windings 28, including end -turns thereof, but also encapsulates the varistor 29 and wires which connect the winding coils to the segments of the commutator 24.

Indeed, the connections themselves between the wires and the commutator segments are also protected.

As shown, the body 30 has smooth end surfaces which are rotationally symmetrical.

However, these end surfaces may be grooved or otherwise made uneven to-create air turbu lence during rotation of the armature to thereby enhance cooling. Indeed, the body 30 could be shaped so as to create a fan for moving air or pump blades for moving liquids.

Each and every one of the wires are adequately located and all movement thereof prevented by the plastics body 30. Shorted or grounded turns do not there of occur, nor does fracture of wires connecting the winding coils to the commutator segments.

The body 30 provides an additional heat sink to inhibit the development of hot spots in the windings and furthermore to change the thermal time constant of the armature in a beneficial way.

As shown in Fig. 3 the distribution of wires in the slots 27 differs from slot to slot. This creates a degree of imbalance resulting in vibration as described previously. Plastics material has a higher density than air and plastics material which fills the slots 27, whilst not entirely eliminating any such imbalance, improves the out of balance forces of rotation.

The body 30 also improves the overall rigidity of the armature by integrating the windings 28, the core 23, the commutator 24, and the spacer 9. As a result it may be possible to use a thinner shaft 25. In any event the additional support provided between on the one hand the core 23 and commutator 24 and on the other hand the core 23 and the spacer 9 will prevent axial movement of the commutator 24 and the spacer 9 towards the core 23 if the motor is used under heavy axial loads,e.g. with a worm drive.

The body 30 gives mechanical strength to the suppression element, in this example varistor 29, or indeed any other element encapsulated therein, and encases the soldering flux to prevent vapourisation thereof and subsequent recondensation on the commutator or brush gear.

Because all cavities in the armature are filled with plastics material, the surface of the armature will be smoother than that of conventional armatures. The surface of the armature will thus offer less drag resistance with the result that windage losses will be reduced.

If the motor is used for a purpose in which a fluid (i.e. a liquid or gas) is caused to flow through the motor there will be less torque losses in the motor.

The smoother contour presented to the gas or liquid creates a lower surface drag and eliminates much of the turbulence normally caused by slotting in the armature core 23.

Moreover, there will be less restriction to fluid as compared with a conventional armature.

The body 30 may include a pigment to impart a matt black colour thereto. In this case, the large surface area of the body 30 will increase the thermal emissivity of the armature.

The body 30 may include one or more of the following additional or alternative additives.

(a) an additive, such as iron powder, to change the permeability of the armature and reduce the reluctance of the motor's magnetic circuit.

(b) a conductive powder, e.g. copper, to provide a resistive bridge between the segments of the commutator 24. This resistive bridge can be used as a discharge element to release inductive energies created in the winding coils.

(c) an additive, e.g. metallic powder, which could be magnetic or non-magnetic, to improve the thermal conductivity of the plastics body.

The above embodiment is given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the invention. For example, the invention is not limited to an armature for a p.m.d.c. motor nor indeed to an armature having a commutator. It is equally applicable to an armature provided with slip rings to which the windings are connected.

Moreover, the body 30 need not encapsulate the armature to the extent described above. It could instead simply encapsulate a part thereof such as any one or more of the part of the shaft between the spacer and the core, the part of the shaft between the commutator or slip rings and the core, the free lengths of wire connecting the winding coils to the commutator, the connections themselves between the aforesaid free lengths of wire and the commutator, the suppression or other elements, and the windings alone.

Claims (18)

1. An armature for an electric motor, in which at least a part of the armature has been encapsulated in a body of plastics material by insert moulding.

2. The armature of claim 1, having windings which comprise portions supported in slots in a core, any unoccupied space in the slots being filled or substantially filled with the plastics material.

3. The armature of claim 1 or claim 2, having windings which comprise portions supported in slots in a core and end turns at opposite axial ends of the core. the or at least a part of the end turns being encapsulated in the plastics material.

4. The armature of anyone of the preceding claims, including a core, which supports windings of the armature, and a commutator mounted on a shaft in spaced relationship, the part of the shaft between the commutator and the core being encapsulated in the plastics material.

5. The armature of anyone of the preceding claims, including a core, which supports windings of the armature, and a spacer mounted on a shaft in spaced relationship, the part of the shaft between the core and the spacer being encapsulated in the plastics material.

6. The armature of anyone of the preceding claims, including a core, which supports windings of the armature, and a commutator to which the windings are connected, wires connecting the windings to the commutator and/or connections between the wires and the commutator being encapsulated in the plastics material.

7. The armature of anyone of the preceding claims, including one or more suppression elements encapsulated within the plastics material.

8. The armature of anyone of the preceding claims, wherein the plastics material has an outer surface the shape of which is defined by the shape of the mould used during the insert moulding process.

9. The armature of the preceding claims, wherein the plastics material includes an additive to change the permeability of the armature.

10. The armature of anyone of the preceding claims, wherein the plastics material includes an additive to impart a black colour thereto.

11. The armature of anyone of the preceding claims, wherein the plastics material includes an additive to improve the thermal conductivity of the body.

12. The armature of anyone of the preceding claims, wherein the plastics material includes an additive which has an ohmic effect.

13. The armature of anyone of the preceding claims, wherein the plastics material is nylon.

14. The armature of anyone of the preceding claims, wherein at least part of the outer surface of the body of plastics material is uneven to create air turbulence as the armature in use rotates.

15. The armature of anyone of the preceding claims, wherein fan or pump blades are integrally formed with the plastics body.

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

17. An electric motor having an armature according to anyone of the preceding claims.

18. A fractional horsepower permanent magnet direct current electric motor having an armature according to anyone of claims 1 to 16.