Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1999-08-19
2001-11-06
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S178000, C310S254100, C310S181000
Reexamination Certificate
active
06313555
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to magnetic bearings and more particularly to a low-loss pole configuration thereof.
BACKGROUND OF THE INVENTION
A magnetic bearing, which includes a rotor and a stator concentrically located with respect to each other, typically controls the radial or axial distance between the rotating rotor and the stationary stator. More specifically, adjustable electro-magnetic forces generated by current flowing through coils wrapped around the stator poles, as controlled by a control circuit adjusts distances between the stator and rotor. U.S. Pat. No. 4,387,935 and 4,082,376 describe details of the magnetic bearing.
Although superior to mechanical bearings in terms of rotational losses, magnetic bearings exhibit rotational losses caused mainly by eddy-current losses generated when non-uniform flux distributions exist along the rotor surface. A heteropolar magnetic bearing requires reversal of the magnetic bias-flux direction as seen by the rotor at each stator pole location resulting in a greatly varying flux distribution around the rotor. Laminated soft-magnetic material is commonly used for the rotor construction to reduce eddy-current losses.
Homopolar magnetic bearings are known to minimize these losses by utilizing two stators: one feeds magnetic bias flux into its rotor and the other feeds magnetic bias flux out of its rotor making the flux distributions around the rotors much more uniform than for the heteropolar case. As the rotors rotate, flux reversal, which is inherent in heteropolar magnetic bearings, will then not occur. However, flux levels as seen by the rotors will modulate with rotation because fluxes will drop off at rotor locations between the stator poles. The drop off of flux levels as seen by the rotors will maintain high eddy-current and rotational losses, but these losses will be less than those of heteropolar magnetic bearings.
The pole-to-pole gap of a magnetic bearing is generally orders of magnitude larger than its pole-to-rotor gap. There are two reasons for having a large pole-to-pole spacing. First, in order to develop force, it is necessary for flux to flow from pole to rotor (pole-to-rotor) and from rotor to pole (rotor-to-pole) to an opposing pole. A parallel flux path exists pole-to-pole which will not generate force and waste generated pole flux. Maintaining a relatively large pole-to-pole gap will cause this parallel flux path to have a high reluctance and be insignificant. Second, it is more convenient for assembly to install coil windings into a relatively large pole-to-pole gap.
If the two reasons for having a large pole-to-pole gap can be tolerated or dealt with in some manners, it can be possible to reduce the pole-to-pole gap to create a more uniform rotor flux distribution. This can result in a reduction of eddy-current losses and rotational losses.
Thus, it is an object of this invention to provide a multi-pole homopolar magnetic bearing with uniform rotor flux distribution.
It is a further object of this invention to provide a multi-pole homopolar magnetic bearing that reduces eddy current and rotational losses therein.
It is yet a further object of this invention to provide a multi-pole homopolar magnetic bearing with sectored-pole-pieces.
It is still another object of this invention to provide a multi-pole homopolar magnetic bearing with reduced pole-to-pole gaps in accordance with flux allocation ratio.
SUMMARY
The present invention provides a multi-pole homopolar magnetic bearing pole configuration that reduces rotational losses caused by eddy-current generated when non-uniform flux distributions exist along the rotor surface. As proposed by this invention, a homopolar magnetic bearing with a pole-to-pole gap reduced to the same order as its pole-to-rotor gap exhibits a much more uniform rotor flux than with relatively large pole-to-pole gaps.
The present invention proposes a magnetic bearing with a reduced pole-to-pole gap. The poles are shaped where they meet pole-to-pole so as to form a defined gap with a defined area resulting in a defined reluctance. By shaping the poles so as to form controlled reluctances for non-force producing parallel fluxes which flow from pole-to-pole, it is possible to create a more uniform bias flux distribution while allowing for enough pole-to-pole reluctance to minimize non-force producing parallel fluxes. The pole feature necessary to perform this task is called the pole-link. As the pole-to-pole gap is reduced, the bias flux distribution becomes more uniform, but the parallel reluctance path reduces as well causing more control flux to be wasted in a parallel path resulting from the parallel fluxes which flow from pole to pole. The Actuation Constant (Newtons/{square root over (watt)}) of the magnetic bearing will fall to inefficient levels as the parallel reluctance path approaches the reluctance path of the primary force-producing control flux. An optimal design will have pole-links dimensioned such that, for example, 85% to 95% of the control flux will generate force and 5% to 15% of the control flux will be in the parallel path. In this manner, the majority of the flux is used for force, and the pole-to-pole gaps are minimized to ensure a uniform flux distribution on the rotor surface.
As a result of development of a low-loss pole configuration for magnetic bearings which results in a relatively small pole-to-pole gap, it became necessary to develop a new stator configuration to facilitate installation of coil windings. Typical stator designs are single piece units fabricated from a solid or a stack of laminations of soft-magnetic material. U.S. Pat. No. 5,570,503 describes how to make a stator by winding coils on a stack of magnetic materials. If a stator is a single multi-pole unit, each turn of a winding must pass through its pole-to-pole gaps. This process is very tedious to do manually, but special automatic winding equipment developed for motor winding can be utilized. However, it is not practical to wind a precision layer wound coil either manually or with automatic equipment. Precision layer wound coils achieve the highest possible fill factor, thus highest efficiency, and exhibit the highest reliability, especially under severe thermal conditions.
In one preferred embodiment, the stator is divided into sectors, into as many pieces as there are poles, allowing for each coil to be separately wound. In this way, it is possible to mount each sectored-pole-piece onto a standard coil winding machine, and it is practical to wind a precision layer wound coil. For maximum coil efficiency, it is desired to use all the available space for the coil. The coils then can be wound into a tapered shape. After winding, the sectored-pole-pieces are installed into and fastened by bonding or other means, to a ring of material which encloses the sectored-pole-pieces, forming a complete stator.
Many advantages are inherent in this sectored-pole-piece configuration. First, assembly is simplified when small pole-to-pole spacing would inhibit winding of the stator. Second, precision layer winding techniques can be applied, resulting in higher actuation efficiency due to a reduction of resistive losses, and resulting in higher reliability windings necessary for aerospace, cryogenic, and other demanding applications. Third, tapered windings are practical to wind, utilizing the natural geometric space available. This further results in higher actuation efficiency due to a reduction of resistive losses. Fourth, the tapered shape of the sectored-pole-piece restrains the location of each sectored-pole-piece so as to provide proper alignment before and after fastening.
REFERENCES:
patent: 4082376 (1978-04-01), Wehde et al.
patent: 4387935 (1983-06-01), Studer
patent: 5111102 (1992-05-01), Meeks
patent: 5319273 (1994-06-01), Hockney et al.
patent: 5942829 (1999-08-01), Huynh
patent: 5962940 (1999-10-01), Imlach
Blumenstock Kenneth A.
Hakun Claef F.
Mullins Burton S.
The United States of America as represented by the Administrator
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