WO2002049188A2

WO2002049188A2 - Permanent magnet rotor with high robustness and method of manufacturing the same

Info

Publication number: WO2002049188A2
Application number: PCT/US2001/046599
Authority: WO
Inventors: Jennifer M. Ambrose
Original assignee: Crumax Magnetics, Inc.
Priority date: 2000-12-11
Filing date: 2001-12-04
Publication date: 2002-06-20
Also published as: AU2002225928A1; WO2002049188A3

Abstract

a rotor (10) having magnet segments (14) injection-moulded onto the exterior (18) of the rotor hub (12) with expansion joints (24) between the magnet segments and keyways (22) located along the rotor hub for securing the magnet segments about the rotor hub. A molten solution of magnet powder and plastic powder fills the keyways and hardens, thereby forming a mechanical lock between the resulting magnet segments and the rotor hub. The expansion joints between the magnet segments accommodate expansion and contraction of the rotor hub, thereby avoiding magnet cracking.

Description

Permanent Magnet Motor Rotor Assembly and Method of Manufacturing of

Same for Robustness

Field of the Invention This invention relates to the manufacturing of a permanent magnet motor rotor, resulting in improved mechanical magnet retention for use in a motor.

Background of the Invention

In a brushless motor, the magnets necessary to generate the magnetic rotor field that interacts with the magnetic stator field to produce torque and thereby power the motor are affixed to the outside of the rotor. The rotor typically consists of a metal rotor hub. Magnets are affixed to the outside of the hub. In the past, this was done by gluing discrete fully-dense magnet segments onto the outside surface of the hub. When the rotor rotated at high revolutions per minute (rpm), however, the glue bond oftentimes proved insufficient to hold the segments onto the rotor hub. Manufacturers, therefore, developed and employed several alternative mechanical retention means to ensure that the magnet segments remained secured to the rotor hub during operation. Manufacturers began doing one of three things: 1) moulding a polymer such as plastic over the magnets on the rotor; 2) placing a sheet of metal over the magnets and rotor hub; or 3) affixing metal or steel bands around the magnets and rotor hub. In addition to proving time-consuming, these additional securing means significantly inflated the manufacturing costs of the rotors.

To reduce costs, manufacturers began gluing a continuous magnet ring onto the exterior of the rotor hubs. This configuration proved problematic. During operation of the rotor, the metal rotor hub would heat and expand at a faster rate than the magnet. This resulted in magnet cracking.

It is a feature of the present invention to improve the integrity of the rotor by integrally-forming the magnets with the rotor hub.

It is another feature of the present invention to improve the integrity of the rotor by reducing magnet cracking.

It is yet another feature of the present invention to improve the integrity of the rotor by providing means integral to the rotor hub for retaining the magnets. It is still another feature of the present invention to obviate the need for and expense of additional mechanical retention means.

Prior Art Document GB-A-2 308 017 discloses an alternating current generator for a vehicle having a holding member (12) comprising an annular steel ring portion (25a) positioned between a field winding (13) and field cores (12a) to serve as holding member base material. Resin permanent magnet portions (25b) are formed by moulding so as to surround the ring portion (25a) and are placed so as to fill the gaps between the filed cores (12a). Fig. 3 shows these gaps to have a radially outwardly tapering cross-section.

Summary of the Invention

According to the present invention, a rotor comprises a rotor hub having a keyway and a magnet located at least partially about the rotor hub and within the key ay.

Also according to the present invention, a method of manufacturing a rotor comprises locating a magnet at least partially about a rotor hub and within a hub keyway. The rotor of the present invention addresses these retention and cracking problems by providing a rotor having arcuate magnet segments moulded to the exterior of the rotor hub and means integral to the rotor hub for securing the magnet segments onto the rotor hub. Expansion joints located between the magnet segments accommodate expansion and contraction of the rotor hub, thereby avoiding magnet cracking. In the past, however, the use of magnet segments necessitated additional retention means for securing the segments onto the hub. The present invention obviates the need for such additional retention means by providing keyways along the side of the rotor hub. The magnet segments are injection-moulded onto the side of the rotor hub. The molten solution of magnet powder and plastic powder fills the keyways and hardens, thereby forming a mechanical lock between the resulting magnet segments and the rotor hub. The keyways prevent the magnet segments from disengaging during operation of the rotor. Moreover, because additional securing means as have been used in the past are therefore unnecessary, the manufacturing costs of the rotor are reduced.

Further features of the present invention are claimed in the appended claims or are disclosed in the drawings.

Brief Description of the Drawing FIG. 1 is an exploded perspective view of one embodiment of the rotor according to the present invention.

FIG. 2 is a front plan view of the rotor shown in FIG. 1.

Detailed Description of the Drawing FIGS. 1 and 2 illustrate the elements of the rotor 10 of an embodiment of the present invention, which include the rotor hub 12 and the magnet arcuate segments 14. The rotor hub maybe any shape compatible with a motor stator. In the depicted embodiment, the rotor hub 12 is generally cylindrical and includes two end faces 16 and a cylindrical surface or rim 18. The meeting edges 20 of the rotor faces 16 and the rim 18 are preferably, but do not have to be, chamfered. Moreover, axial extending keyways 22 are located in the rim 18.

The rotor hub 12 may be solid or hollow and formed from a variety of materials possessing suitable physical properties including structural integrity and durability, such as steel or other metal. In the present embodiment, the rotor hub 12 is formed from a powdered metal whereby the powder is pressed and fired to form the rotor hub 12.

While pre-formed magnet segments could be attached to the rotor hub 12 using adhesive or mechanical locking means, the magnet segments 14 are preferably injection-moulded about the rotor hub 12. Injection-moulding provides a stronger bond between the magnets 14 and the rotor hub 12. Magnet powder and plastic powder are mixed and heated until the plastic becomes molten. The molten mixture is then injection moulded about the hub rim 18. The mixture fills the keyways 22 and covers the rotor rim 18 and chamfered hub edges 20. Hardening of the molten mixture occurs almost instantaneously to form the magnet segments 14.

Expansion joints 24 are located between the magnet segments 14. The expansion joints 24 may be devoid of any magnet material so that the rotor rim IS is exposed within the expansion joints 24 (as depicted in FIGS. 1 and 2), or, alternatively, the expansion joints may be formed, by reducing the thickness of the magnet so that the thickness of the magnet at the expansion joints 24 is less than the thickness of the magnet segments 14. While FIGS. 1 and 2 illustrate four magnet segments 14, any number of magnet segments may be formed on the rotor hub 12. Moreover, only four keyways 22, one for each magnet arc 14, are disclosed. However, more than one keyway 22 may be provided for each magnet arc 14.

Preferably, keyways 22 are present in rim 18, as they strengthen the connection between the rotor hub 12 and the magnet segments 14. The keyways 22 may assume any shape. Upon rotation of the rotor 10, the keyways 22 help hold the magnet segments 14 in place and prevent the segments 14 from flying off, or otherwise separating from, the rotor hub 12. Each keyway 22 is an axial slot 26 having axial edges 28 in the hub rim 28. As clearly illustrated in FIG. 2, keyway 22 has a radially outwardly tapering cross-section with the slot width 26 (i.e. the distance between the axial edges 28) less than the width of the keyway at its widest portion. This narrowed slot 26 further helps prevent the magnet material from escaping from the keyway 22. Moreover, the chamfered hub edges 20 help ensure that the magnet segments 14 remain affixed to the rotor hub 12 should the rotor hub 12 shift in a direction normal to its rotation during operation. The foregoing is provided for the purpose of illustrating, explaining and describing embodiments of the present invention.

Claims

CLAIMS:

1. A rotor comprising: a. a rotor hub (12) having a keyway (22); and, b. at magnet (14) within the keyway; characterised in that c. the magnet (14) is located at least partially about the rotor hub (12) and within the keyway (22).

2. A rotor as claimed in claim 1 and further characterised in that the magnet (12) is injection-moulded about at least part of the rotor hub (12) and at least partially fills the keyway (22) .

3. A rotor as claimed in claim 1 or claim 2 and further characterised in that the keyway (12) has a radially outwardly tapering cross-section.

4. A rotor as claimed in any of claims 1 to 3 and further characterised in thatthe rotor hub (12) is cylindrical with two end faces (16) and a rim (18) and the keyway (22) is formed in the rim.

5. A rotor as claimed in claim 4 and further characterised in that the meeting edge (20) between at least one of the two end faces (16) and the rim (18) is chamfered.

6. A rotor as claimed in any of claims 2, 4 and 5 and further characterised in that the magnet (12) is injection-moulded about at least one of the meeting edges (20).

7. A rotor as claimed in any of claims 1 to 6 and further characterised in thatat least two magnets (22) are located on the rotor hub (16) and are at least partially separated by an expansion joint (24).

S A rotor as claimed 7 and further characterised in that at least two keyways (22) are formed in the rim (18) and at least two magnets (14) are each injection- moulded about at least part of the rotor hub (12) and each to at least partially fill one of the keyways.

9. A method of manufacturing a rotor comprising affixing a magnet (14) to a rotor hub (12) having a keyway (22) with the magnet at least partially within the keyway characterised by the step of locating the magnet (14) at least partially about the rotor hub (12) and within the keyway (22).

10. A method as claimed in claim 10 and further characterised in thatthe rotor hub (12) is cylindrical with two end faces (16) and a rim (18) and the keyway (22) is formed in the rim.

11. A method as claimed in claim 9 or claim 10 and further characterised by the step of heating a mixture comprising plastic powder and magnetic powder until the mixture is molten and injection-moulding the mixture onto the rotor hub (16) and at least partially into the keyway (22) to form the magnet (14).

12. A method as claimed in claim 11 and further characterised by the step of injection-moulding the mixture onto about a meeting edge (20) between at least one of the two end faces (16) and the rim (18).

13. A method as claimed in claim 12 and further characterised by the step of injection-moulding at least two magnets (14) onto the rotor hub (16).

14. A method as claimed in claim 13 and further characterised in that the two magnets (14) are at least partially separated by an expansion joint (24).