GB2372156A

GB2372156A - Rotor with reduced cogging torque

Info

Publication number: GB2372156A
Application number: GB0103189A
Authority: GB
Inventors: Georg Strobl
Original assignee: Johnson Electric SA
Current assignee: Johnson Electric SA
Priority date: 2001-02-09
Filing date: 2001-02-09
Publication date: 2002-08-14
Also published as: GB0103189D0

Abstract

A wound rotor 24 for a miniature electric motor 10 has a laminated rotor core 28. Each lamination has a slot cut into the pole face creating a slit running axially along each pole face of the rotor core. This significantly reduces the variation in the radial magnetic force as the rotor rotates within the stator. It also significantly reduces the flux leakage across the pole face as the pole passes from one stator magnet to the other. After winding, the pole faces are deformed by radially pressing a double tapered wedge into the slits 54 causing the slits 54 to be widened between the ends of the rotor core. This widening causes the winding gap 48 between adjacent pole faces to be correspondingly narrowed. The winding wires are then captured within the winding tunnels without the use of slot sticks.

Description

Rotor

Field of the Invention This invention relates to electric motors and in particular to a wound rotor for such a motor.

Background of the Invention Electric motors generally have a permanent magnet or an electric field stator. A wound rotor is rotatably mounted so as to confront the stator across a small air gap. The rotor has a salient pole laminated core with each salient pole at its distal end having lateral extensions or wings giving the poles a T-shaped creating an enlarged pole face. Thus between adjacent poles is a tunnel for receiving rotor windings. A winding gap exists between adjacent wings to enable the rotor windings to be laid in the winding tunnels.

Recently there has been a desire to reduce cogging torque on the rotor. One way this has been achieved is by reducing the width of the winding gap which also prevents wires from being flung out of the winding tunnel during use without the need for slot sticks. See for example, EP 1024586 A2 and EP 1047177 A2. However, for a quiet motor, the rotor design needs to consider as well as cogging torque, variations in the radial force exerted on the rotor core by the magnets as the rotor rotates, and the variation in torque caused by flux leakage as a pole face passes from one stator magnet to another.

Summary of the Invention Accordingly, the present invention provides a rotor for an electric motor comprising: a shaft; a laminated rotor core mounted on the shaft and having a plurality of salient poles ; a commutator mounted on the shaft adjacent the motor core; rotor windings wound around the salient poles of the rotor core and terminated on the commutator; wherein the salient poles have a radially extending stem portion and a pole face portion formed by integral wings extending circumferentially from the distal end of the stem portion; and wherein a slit extends axially along the pole face dividing the pole face into two portions.

According to a second aspect, the present invention provides a method for manufacturing a rotor for an electric motor comprising the steps of : stamping laminations from a silicon steel sheet, the laminations having a plurality of salient poles with a slot in a pole face of each salient pole; stacking and aligning a plurality

of the laminations to form a rotor core with axially extending slits in each pole face of the rotor core ; mounting the rotor core onto a motor shaft ; mounting a commutator onto the motor shaft; winding motor windings about the poles of the rotor core and terminating the windings on the commutator.

The present invention also provides a rotating electric machine having a wound rotor as described above.

Brief Description of the Drawings One preferred embodiment of the invention will now be described, by way of example only, in which: Figure 1 is a partially sectioned miniature motor according to the preferred embodiment; Figure 2 is a view of a lamination forming a part of the rotor core of Figure 1; and Figure 3 is an illustrative diagram of a rotor core of the motor of Figure 1 with windings removed.

Detailed Description of the Preferred Embodiment As an illustrative example, the invention will be described with reference to a miniature, permanent magnet d. c. electric motor.

The miniature electric motor of Figure 1 has a deep drawn metallic housing 12 having one closed end supporting a bearing 14 and an open end closed by a molded resin end cap 16. The end cap 16 supports motor terminals 18, brushes 20 and another bearing 14. The housing 12 accommodates two permanent magnets 22 forming the stator. A wound rotor 24 is joumalled in the bearings and confronts the stator across an air gap.

The rotor has a shaft 26, a rotor core 28, a commutator 30 and windings 32 as is usual.

An oil slinger 34 is fitted to the shaft next to the commutator 30.

The rotor core 28 is a salient pole rotor core made by stacking together a plurality of rotor laminations stamped from magnetic steel sheet. Each lamination 36 as shown in Figure 2 has a number of (in this case, five) salient poles 38. Each pole is T-shaped with a stem 40 extending radially from a central portion of the lamination. The distal end of each stem has lateral extensions or wings 42 which with the end of the stem form the pole face 44.

Each pole face 44 has a central slot 46 extending radially to a depth approximately equal to the radial thickness of the wings 42 where they join the stem 40. Figure 2 also shows the winding gap 48 formed between adjacent wing tips 50 of the pole faces 44 and the space between adjacent poles defined by the stems 40 and adjacent wings 42 which form the winding tunnels 52 in which the rotor windings are laid.

After the laminations are stacked and mounted on the motor shaft, the rotor can be wound wherein a magnet wire is wound through the winding gaps, around individual poles or groups of poles as desired and terminated on the commutator fitted to the shaft adjacent one end of the rotor core. Before being wound, the rotor core may be coated with an insulating material such as an epoxy resin, or at least that part of the rotor core which would otherwise be in direct contact with the windings.

Once stacked together, the slots in the pole faces of the laminations form axially extending slits in the pole faces of the rotor core. The effect of this is to double the resonance frequency, thereby significantly reducing the variation in the radial magnetic force as the rotor rotates within the stator. It also significantly reduces the flux leakage across the pole face as the pole passes from one stator magnet to the other.

While this would produce a motor with a significantly improved rotor, the preferred embodiment goes one step further as will be explained with particular reference to Figure 3.

Figure 3 is an exemplary view of the rotor core 28 showing further modification. The pole faces 44 are deformed by widening the slits 54 progressively from the ends of the rotor core towards the centre. This can be achieved, for example, by forcefully inserting a double tapered wedged shaped tool into the slits 54. As the slits 54 are widened, the adjacent winding gaps 48 are correspondingly narrowed. The width of the winding gaps 48 can be narrowed to substantially close the winding gap 48, if desired.

For applications where it is desired to narrow the air gap to be less than the wire diameter of the winding plus a suitable allowance for clearance, it would be necessary to make this further modification after the rotor was fully wound. For a rotor which

has been wound, the ends of the slits are not widened to avoid stretching or stressing the turns of the windings laying next to the wings. Expanding the slit away from the ends of the core will not unduly stress the windings.

By reducing the air gap to a width less than the diameter of the winding wire, the winding wires are captured within the winding tunnels without the use of slot sticks.

Also, cogging is reduced.

While the invention has been described in connection with the preferred embodiment of a miniature permanent magnet d. c. motor, the invention is not so limited and it is intended that the patent cover all variations and modifications which fall within the spirit of the invention and the following claims. In particular, the rotor may be used in any rotating electric machine requiring a wound rotor and the stator field may be generated by electromagnets or field coils instead of permanent magnets.

Claims

1. A rotor for a miniature motor comprising : a shaft ; a laminated rotor core mounted on the shaft and having a plurality of salient poles ; a commutator mounted on the shaft adjacent the motor core; rotor windings wound around the salient poles of the rotor core and terminated on the commutator; wherein the salient poles have a radially extending stem portion and a pole face portion formed by integral wings extending circumferentially from the distal end of the stem portion; and wherein a slit extends axially along the pole face dividing the pole face into two portions.

2. A rotor according to Claim 1, wherein each slit extends from one axial end of the rotor core to the other.

3. A rotor according to Claim 1 or 2, wherein the slits have a depth approximately equal to a radial root dimension of the wings.

4. A rotor according to any one of Claims 1 to 3, wherein the slit is worked whereby the slit is wider at the middle of the rotor core than at the ends.

5. A rotor according to Claim 4, wherein tips of adjacent wings are separated by a winding gap and the size of the winding gaps vary along the length of the rotor core complementing the variation in the width of the slits.

6. A rotor according to Claim 5, wherein the working of the slits narrows the winding gaps to less than a wire diameter of the winding.

7. A rotor for a miniature electric motor substantially as hereinbefore described with reference to the accompanying drawings.

8. A miniature electric motor comprising a permanent magnet stator ; a wound rotor; at least one motor terminal, and brush gear for transferring electrical power to the rotor, wherein the rotor is as defined in any one of the preceding claims.

9. A method of producing a rotor for a miniature electric motor comprising the steps of : stamping laminations from a silicon steel sheet, the laminations having a plurality of salient poles with a slot in a pole face of each salient pole; stacking and aligning a plurality of the laminations to form a rotor core with axially extending slits in each pole face of the rotor core; mounting the rotor core onto a motor shaft; mounting a commutator onto the motor shaft; winding motor windings about the poles of the rotor core and terminating the windings on the commutator.

10. A method according to Claim 9, wherein the rotor core is covered with an insulating material before the rotor windings are wound, at least in the area of the rotor core in contact with the motor windings.

11. A method according to Claim 9 or 10, wherein the poles are deformed after winding to widen the slits and to reduce the gap between the tips of the wings of adjacent poles.

12. A method according to any one of Claims 9 to 11, wherein the slits are widened gradually from the ends to the middle.

13. A method according to any one of Claims 9 to 12, wherein the pole face is deformed by inserting a double tapered wedge into the slit.

14. A method of producing a rotor for a miniature electric motor substantially as hereinbefore described with reference to the accompanying drawings.