GB2389241A

GB2389241A - Stator assembly for electric motor

Info

Publication number: GB2389241A
Application number: GB0212364A
Authority: GB
Inventors: Peter Maybury
Original assignee: EUROP ELECTRICAL LAMINATIONS L; European Electrical Laminations Ltd
Current assignee: EUROP ELECTRICAL LAMINATIONS L; European Electrical Laminations Ltd
Priority date: 2002-05-29
Filing date: 2002-05-29
Publication date: 2003-12-03
Also published as: GB0212364D0

Abstract

A stator assembly for an electric motor comprises stack (fig. 10) of stator elements 52 formed integrally as pressings from a steel strip 82. Adjacent stator elements 52 comprise pole portions 56 which are mutually offset with respect to a centre line of the body portion 54. (figs. 4a, 8a, 9a.) Each stator element 52 is produced by the use of a die assembly to punch the element from the steel strip 82. Part at least of the body portion 54 of the stator element is produced by a first die member 46, and part at least of the pole portion 56 is produced by a second die member 48. Relative rotation is effected between the die members between successive punching operations, the second die member 46 being rotatable around pivot point 76.

Description

( PATENTS ACT 1977

Title: Improvements Relating To Electric Motors Description of Invention

This invention is concerned with improvements relating to electric motors, which term is to be deemed to include generators, and is particularly concerned with motors of the type comprising a stator assembly comprising a plurality of stator members defining a generally hollow cylindrical chamber, and a rotor comprising a shaft, adapted to be mounted within the stator, the rotor extending radially of the shaft. Such an electric motor is hereinafter referred to as being ofthe kind specified.

A conventional stator member for a motor of the kind specified is constructed from a plurality of stator elements, whilst a conventional rotor for a motor of the kind specified is constructed from a plurality of rotor elements.

The stator elements and rotor elements are conveniently in the form of steel pressings, which lie parallel to a plane extending at right angles to the axis of the motor. The stator elements and rotor elements are collectively referred to as pole elements.

Conventionally each stator element comprises a body portion, and, positioned radially inwardly of the body portion, a pole portion having an extended inner surface. Each rotor element comprises a generally circular pressing with a plurality (e.g. four) radially-extending slots, the portions of the rotor elements between the slots forming poles. The radius of the rotor elements is marginally smaller than the radius of the pole portions of the stator elements, there being a small air gap between the outer surface of the rotor elements and the inner surface of the stator elements. In order to keep this gap constant, and, importantly, maintain a unifonn flux over the pole elements it is highly desirable that the pole elements are manufactured in one piece.

( A problem commonly encountered in electric motors is that the rotor relies on its momentum to continue to rotate as magnetic poles are reversed on the stator plates. There are, therefore, "dead points" in the revolution of the rotor when Me rotor may be slowed or even stalled. One way of overcoming this problem is to offset the rotor elements slightly so that each pole portion passes through the dead point at a different point in time. Preferably the pole portions of the rotor are offset in a manner such as to present, in front view, the appearance of a chevron.

One problem, however, with having offset rotor elements is that it makes efficient automated winding of the rotor difficult to achieve. Since it is difficult to wind the rotor once the rotor elements have been skewed, the rotor is generally wound before the elements are offset, which means that the rotor must be wound loosely to allow the elements to be moved. If wound too tightly the windings will stretch, insufficient skew will be achieved, and the windings will tend to pull the rotor elements back into a non-skewed position. If wound too loosely, on the other hand, the contact between the windings and the rotor will be compromised.

According to a first aspect of this invention there is provided a stator for an electric motor of the kind specified wherein adjacent stator elements comprise pole portions which are mutually offset.

According to a second aspect of this invention there is provided a stator adapted to be mounted around a rotor comprising a plurality of stator members each stator member comprising a plurality of stator elements each comprising a body portion and a pole portion, wherein the pole portions of adjacent stator elements are offset.

This invention enables the rotor to be wound much more easily and consequently eases assembly of the electric motor, whilst achieving the advantages of an offset pole portion design.

( Preferably each stator element is formed as an integral pressing, in which, conveniently, the pole portion of each stator element is offset in a transverse direction from the body portion by a distance which varies in a direction parallel to the axis of the shaft.

Preferably the offset varies from a maximum in one direction, diminishing through zero to a maximu n in the opposite direction, and increasing through zero to a maximum again in said one direction. However, the offset may increase continuously from one side of the stator to the other.

According to a third aspect of this invention there is provided a method of manufacture of pole elements involving the use of a die assembly to punch the pole element from strip material, part at least of the body portion being produced by a first die member and part at least of the pole portion being produced by a second die member, characterized by the steps of effecting relative rotation between the first and second die members between successive punching operations.

The method set out in the paragraph above is potentially suitable for use in the manufacture of stator elements, but may be used in the manufacture of rotor elements.

Preferably the method involves the steps of stamping a plurality of pole elements, wherein an incremental adjustment of the position of the first die member relative to the second die member is produced after each punching operation. In this way pole elements may be produced which have pole portions which are offset by different distances relative to a centre line of the body portion. Preferably the part of the pole portion which is produced by the second die member is that part of the pole portion which is common to all pole elements throughout the range.

The second die member may define a curved inner surface of the pole portion. According to a fourth aspect of this invention there is provided a machine for the manufacture of pole elements comprising a die assembly comprising a first die member for the punching of part at least of the body portion of the pole element, and a second die member for the punching a part at least of a pole portion of the pole element, the first and second die members being mounted for relative rotation to produce pole elements the pole portion of which may be offset by different distances from a centre line of their associated body portions.

Each die member has associated therewith a punch, and preferably the second member is adapted to produce a curved inner surface of the pole portions of the pole elements.

Preferably machine comprising means to cause all the die members to perform a punching operation simultaneously.

According to a fifth aspect of this invention there is provided a method of manufacture of stator elements involving the use of a die assembly to punch the stator element from strip material, part at least of the body portion being produced by a first die member, and part at least of the pole portion being produced by a second die member, characterised by the steps of effecting relative movement between the first and second die members between successive punching operations.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: FIGURE 1 shows schematically a cross section through part of a motor of the kind specified; FIGURE 2 shows schematically a front view of a prior art rotor

assembly;

FIGURE 3 shows schematically a perspective view of part of a machine which is used for producing stator elements in accordance with the invention; FIGURE 4_ is a view of a stator element produced by the machine shown in Figure 3; FIGI IRE 4b shows schematically a plan view of the machine shown in Figure 3; FIGURE 5 is a side view of the machine shown in Figure 3b; FIGURE 6 is a front section of the machine shown in Figure 3b along the line B-B; FIGURE 7 shows schematically a plan view of the machine of Figure 2 showing its range of movement; FIGURE 8_ shows schematically a view of a stator element having a pole portion offset in one direction; FIGURE 8b is a plan view of the machine of Figure 2 when rotated to one extreme of its range of movement; FIGURE 9a shows schematically a view of a stator element having a pole portion offset in the opposite direction to that shown in Figure 8_; and FIGURE 9b is a plan view of the machine of Figure 2 when rotated to the other extreme of its range of movement; FIGURE 10 shows a set of stator elements stacked to form a chevron, and FIGURE 11 shows schematically a motor having a stator assembly according to the invention.

A section through part of a motor of the kind specified, in this example a four pole motor, is shown in Figure l The motor 10 comprises a stator 12 and a rotor 14. The rotor 14 comprises four rotor pole elements 16 which extend from a central shaft 18, and are separated by slots 19. The outermost surface 20 of each pole element 16 comprises part of the circumference of a circle, giving the rotor 14 a generally cylindrical form.

( The stator 12 comprises four separate arrays of stator pole elements 22.

These stator elements 22 are generally in the form of a "Y", having an innermost surface 24 which comprises an arc having a circumference a little larger than that of the outer surface 20 of the corresponding rotor elements 16.

Each stator element has a body portion 26 and a pole portion 28.

In order to magnetise the stator 12 and the rotor 14 windings 30, 32 are wrapped around the pole elements 16, 22 after the elements have been stacked on top of each other. The rotor windings 32 pass up one slot and down a different slot, making the rotor relatively difficult to wind. The stator windings 30 are wrapped around the body portions 26 of the pole elements, and are thus relatively easy to wind.

The difficulty of winding the rotor is increased when, as shown in Figure 2 slots 34 in rotor elements 36 are offset, in this example to form a chevron.

This type of shape is very difficult to wind, so the elements 36 are generally wound with the slots 34 aligned and the slots 34 are then offset to form the chevron. The problem with offsetting the slots 34 after winding the rotor is that some slack must be present in windings 38 to enable the slots to be offset. The amount must be precisely judged since excess slack (as shown in Figure 2) will greatly diminish the amount of magnetism in the motor whilst less slaclc will tend to pull the slots back to their aligned position and/or stretch the windings 38. By offsetting the pole portions of the stator elements rather than the rotor elements a similar effect of pulling the motor through dead spots may be achieved, whilst the motor remains relatively easy to wind.

Figure 3 shows schematically part of a machine suitable for the manufacture of stator elements having offset pole potions. Importantly, this machine is able to manufacture stator elements as integral pressings, thus maximising the flux in the pole portions. A lower part of the machine, a die assembly 40 is shown in Figure 3. The die assembly comprises a base plate 42,

a feed rail 44, a moveable die member 46 and a stationary die member 48. A strip of material 50 is shown and the way that the material 50 takes shape as it passes the die members can be seen from the Figure.

One of the stator elements 52 produced by the machine is shown in Figure 4a. The stator element 52 comprises a body portion 54 and a pole portion 56. Rivet holes 55 are used to assemble a stack of stator elements together. The pole portion 56 has a curved inner surface 60, which has a radius of curvature positioned generally on line A-A, the centreline of the body portion 54 of the stator element 52. This stator element 52 has pole portions which are generally symmetrical around the centreline A-A.

A plan view of the machine of Figure 3 is shown in Figure 4b, the moveable die member 46 being shown in its central position, which will produce stator elements such as that shown in Figure 4a having a pole portion which is generally symmetrical about the centre line of the body portion.

The means 62 for rotating the moveable die member 46 is also shown. It comprises a lead screw 64 attached to one side of the moveable die member 46.

The lead screw 64 may be driven by a motor (not shown) to rotate the die member, or it may be manually moved by rotation of a knurled knob 66.

The details of the machine are shown in Figure 4b, in combination with Figures 5 and 6. The feed rail 44 comprises a parallel sided channel along which a strip of sheet material may pass, and overhead guide means 45 comprising a plurality of rods 65.

The moveable die member 46 is positioned between the feed rail 44 and the stationary die member 48. It comprises a die plate 68 which is shaped to form two die apertures 68a, 68b, separated by a support 70, and two punches 72, shaped to correspond with the die apertures 68a, 68_, and extending from a punch plate 74. The entire assembly can be rotated around a pivot point 76 by rotation of knurled knob 66.

The stationary die member 48 is adjacent to the moveable die member 46. It comprises a die plate 78 shaped to form a die aperture 78a, and a correspondingly shaped punch 80.

The operation of the machine will now be described. A strip 82 of sheet material, for example steel, is fed into the channel of the feed rail 44. The overhead guide means 45 is lowered until a piercing rod 65 pierces a guide hole 58 either side of the strip of material. The overhead guide means 45 is then moved to the right, transporting the strip material 82 through the machine. A second rod 65 enters the guide holes 58 one movement step to the right of the initial rod which ensures the strip 82 is maintained in parallel alignment.

When the strip material 82 has been advanced, the punches 72 and 80 are lowered and stamp out portions of the strip material. The moveable punch 72 stamps out side surfaces of the body portion 54 and the underside of the pole portion 56 of the stator element 52. It also punches further guide holes 58 into the centre of the strip so that the strip can continue to be moved through the machine. The stationary punch 80 stamps out the periphery of the pole portion 56 and the inner surface 60 of the stator element 52. It simultaneously severs a stator element from the strip material.

Between each movement step the moveable die member is rotated one increment. This has the effect of changing the position of the parts of the stator element removed by the moveable punch members 72 relative to the parts removed by the stationary punch member 80. Essentially the body portion 54 is moved along the pole portion 56, which, when the body portions of the stator elements are aligned, has the effect of offsetting the pole portions of the stator elements. The offset is thus incrementally moved from a maximum in one direction to a maximum in the other direction. In this way the stator elements which are produced by the machine will be "stacked" in the correct sequence

( Figure 7 shows the range of rotation of the moveable die assembly 46, whilst Figures 8b and 9b shows the die assembly at either end of its range of movement and Figures 8_ and 9a show the stator elements produced when the die assembly is in these positions, illustrating the offset which can be produced.

The distance marked as C on Figure 4b is from the centre of the pivot point to what will become the inner surface of the stator element positioned over die assembly 46. In order to ensure that the body portions are aligned when the stator elements are stacked together, this distance must be substantially the same as the radius of curvature of the inner surface of the stator elements. This ensures that the radius of curvature of the inner surface of each stator element remains aligned with the centre line of its body portion.

Figures 10 and 1 1 illustrate how the chevron formation may be achieved with a stack of such stator elements.

The stator assembly is then assembled so that the body portions of the stator elements are aligned, whilst the pole portions are offset. Since the body portions are aligned the stator elements may still be wound very easily, whilst, since it is not necessary to offset the rotor elements, the rotor may be wound more easily also. In this way the advantages of having offset poles may be achieved without the disadvantages of the difficulty of winding the rotor assembly. Although described in relation to stator elements, the method and machine for manufacturing the pressings may be applied to pole elements generally, particularly to rotor elements.

In the present specification "comprises" means "includes or consists of''

and "comprising" means "including or consisting of".

The features disclosed in the foregoing description, or the following

claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any

( combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1 A stator for an electric motor of the kind specified wherein adjacent stator elements comprise pole portions which are mutually offset.

2 A stator adapted to be mounted around a rotor, the stator comprising a plurality of stator members, each stator member comprising a plurality of stator elements, each comprising a body portion and a pole portion wherein the pole portions of adjacent stator elements are offset.

3 A stator according to Claim 1 or Claim 2 wherein the stator elements are

formed as integral pressings.

4 A stator according to any preceding claim wherein the pole portion of each stator element is offset in a transverse direction from the body portion by a distance which varies in a direction parallel to the axis of the shaft.

5 A stator according to Claim 4 wherein the offset varies from a maxirrum in one direction, diminishing through zero to a maximum in the opposite direction, and increasing through zero to a maximum again in said one direction.

6 A method of manufacture of pole elements involving the use of a die assembly to punch the pole element from strip material, part at least of the body portion being produced by a first die member and part at least of the pole portion being produced by a second die member, characterized by the steps of effecting relative rotation between the first and second die members between successive punching operations.

(l

7 A method according to Claim 6 involving the steps of stamping a plurality of pole elements, wherein an incremental adjustment of the position of the first die member relative to the second die member is produced after each punching operation.

8 A method according to one of Claims 6 and 7 wherein the part of the pole portion which is produced by the second die member is that part of the pole portion which is common to all pole elements throughout the range.

9 A method according to Claim 8 wherein the second die member defines a curved inner surface of the pole portion.

10 A method according to any of Claims 7 to 9 wherein the first die member is rotated after each punching operation.

11 A machine for the manufacture of pole elements comprising a die assembly comprising a first die member for the punching of part at least of the body portion of the pole element, and a second die member for the punching of part at least of a pole portion of the pole element, the first and second die members being mounted for relative rotation to produce pole elements the pole portion of which may be offset by different distances from a centre line of their associated body portions.

12 A machine according to Claim 11 in which the first die member is mounted for rotation.

f

13 A machine according to Claim 11 or 12 wherein each die member has associated therewith a punch, and the second die member is adapted to produce a curved inner surface of the pole portions of the pole elements.

14 A machine according to any one of Claims 1 1 to 13 comprising means to cause all the die members to perform a punching operation simultaneously.

15 A method for the manufacture of stator elements involving the use of a die assembly to punch the stator element from strip material, part at least of the body portion being produced by a first member, and part at least of the pole portion being produced by a second die member, characterized by the steps of effecting relative movement between the first and second die members between successive punching operations.

16 A stator for an electric motor of the kind specified, comprising stator elements constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

17 A stator assembly constructed, arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.

18 A method of manufacture of pole elements, when carried out substantially as hereinbefore described with reference to the accompanying drawings.

19 A machine for the manufacture of stator elements, constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.

20 Any novel feature or novel combination of features hereinbefore described andlor shown in the accompanying drawings.