Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2000-04-24
2001-05-29
Mullins, Burton (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S261100, C310S156030, C310S06000A, C310S052000
Reexamination Certificate
active
06239520
ABSTRACT:
TECHNICAL FIELD
This invention relates to the general field of permanent magnet generator/motors and more particularly to an improved system and method for cooling the permanent magnet rotor of a permanent magnet generator/motor.
BACKGROUND OF THE INVENTION
A permanent magnet generator/motor generally includes a rotor assembly having a plurality of equally spaced magnet poles of alternating polarity around the outer periphery of the rotor or, in more recent times, a solid structure of samarium cobalt or neodymium-iron-boron. The rotor is rotatable within a stator which generally includes a plurality of windings and magnetic poles of alternating polarity. In a generator mode, rotation of the rotor causes the permanent magnets to pass by the stator poles and coils and thereby induces an electric current to flow in each of the coils. Alternately, if an electric current is passed through the stator coils, the energized coils will cause the rotor to rotate and thus the generator will perform as a motor.
The permanent magnet materials utilized in the permanent magnet rotor do not hold their magnetic properties above a given temperature, that is, they will lose their magnetic properties if they are subjected to a temperature above a certain limit. Neodymium-iron-cobalt permanent magnets, for example, may be permanently demagnetized if subjected to a temperature above 350 degrees Fahrenheit, while samarium-cobalt permanent magnets may be permanently demagnetized if subjected to temperatures above 650 degrees Fahrenheit. Even operating at temperatures close to the above limits can degrade performance of and/or damage the permanent magnet material.
In use, the permanent magnet material is usually enclosed within a sleeve of non-magnetic material. Intimate contact between the non-magnetic material sleeve and the permanent magnet material is achieved by inserting the permanent magnet into the permanent magnet sleeve with a radial interference fit by any number of conventional techniques.
During operation of the permanent magnet generator/motor, the non-magnetic sleeve is subjected to eddy currents and aerodynamic heating. Because of the intimate contact between the non-metallic sleeve and the permanent magnet, the heat from the non-magnetic sleeve is transferred to the permanent magnet. Such heating must be taken into consideration when selecting permanent magnet materials for a permanent magnet generator/motor.
Since the radial air gap between the outer diameter of the permanent magnet rotor sleeve and the inner diameter of the stator is deliberately kept small to enhance magnetic performance, the resulting annular clearance through which cooling air can travel axially severely restricts the flow of cooling air. When this air gap is on the order of 0.050 inches, there simply is not enough annular clearance for sufficient cooling air to flow.
One of the applications of a permanent magnet generator/motor is referred to as a turbogenerator/motor which includes a power head mounted on the same shaft as the permanent magnet generator/motor, and also includes a combustor and recuperator. The turbogenerator/motor power head would normally include a compressor, a turbine and a bearing rotor through which the permanent magnet generator/motor tie rod passes. The compressor is driven by the turbine which receives heated exhaust gases from the combustor supplied with preheated air from the recuperator.
SUMMARY OF THE INVENTION
The open end of the permanent magnet rotor includes an end cap having an axially extending bore with a plurality of radially extending holes aligned with holes in the sleeve around the permanent magnet. Air flowing through the holes provides cooling in the air gap between the permanent magnet sleeve and the stator. A second smaller diameter bore, having a second plurality of radially extending holes, offset from the first plurality of holes, may also be provided. The housing may also include an annular extension over the plurality of holes in the non-magnetic sleeve to direct the flow of cooling air into the air gap between said rotor and said stator.
REFERENCES:
patent: 4546279 (1985-10-01), Hammer et al.
patent: 4649303 (1987-03-01), Hirao
patent: 4902922 (1990-02-01), Annovazzi
patent: 4933583 (1990-06-01), Ripplinger
patent: 5801470 (1998-09-01), Johnson et al.
patent: 5925960 (1999-07-01), Hayes
patent: 5994804 (1999-11-01), Grennan et al.
Stahl David A.
Vessa Phillip B.
Brunell Norman E.
Capstone Turbine Corporation
Irell & Manella LLP
Mullins Burton
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