Minimizing shaft currents in dynamoelectric machines

Details Minimizing shaft currents in dynamoelectric machines Minimizing shaft currents in dynamoelectric machines

1998-11-30

2001-04-17

LaBalle, Clayton (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S06700R, C310S071000, C310S07500D, C310S06000A, C310S061000, C310S079000

Reexamination Certificate

active

06218757

ABSTRACT:

BACKGROUND OF THE INVENTION
The presence of shaft currents in dynamoelectric machines (DEM's) has long been known, but is difficult to measure. Designers have postulated on methods of eliminating the troublesome circulating shaft currents which in some instances can rise to such levels that damage to the bearings, which journal the rotating shaft, often occurs.
Because of the difficulty in assessing the cause of bearing failure in operating DEM's, many bearings have been replaced in DEM's, the demise of which has often been attributed to bearing misalignment or lubrication failure when in reality, the cause of the bearing failure was caused by current circulation through the bearings of the machine and into the DEM pedestals where it could find a convenient return path.
In some troublesome machines, the useful life of a bearing set has been reduced to the order of hours or days by the presence of large circulating currents through the machine. The exact cause of the generation of shaft EMF's is not well understood, but is believed that the production of unbalanced magnetic fields in the DEM's result in generation of sizable zero sequence shaft voltages being induced in the DEM rotor shafts. The magnitude of the resulting circulatory currents in the shaft of the DEM is largely determined by the path resistance through the bearings and the ground return systems and not by the resistance of the shaft itself.
Many solutions have been attempted to ameliorate the circulating current problem and usually the most direct solution lay in increasing the resistance of the ground return paths.
The simplest solution utilized in the past, involved mounting part of the bearing structure in such a manner that the bearings themselves were insulated from their pedestals, or the pedestals were insulated from ground.
SUMMARY OF THE INVENTION
The machine of the invention is modified in its construction to minimize the circulating currents flowing in the shaft of the DEM. The modification consists of providing a hollow shaft for the DEM and passing an insulated conductor through the passageway in the hollow shaft which is rigidly connected to the ends of the rotor shaft. The insulated conductor inside the DEM rotor shaft rotates with the shaft and inhibits any circulating currents from entering the bearings of the machine by providing a low resistance return path. In the end, the insulating conductor functions as a transformer secondary in which any induced current in the insulated conductor opposes the magnetic flux which would induce a circulating current.
RELEVANT PRIOR ART
PUBLICATIONS
WALKER, P. “Preventing Motor Shaft-Current Bearing Failures” Plant Engineering, Oct. 4, 1990.
The Walker reference above provides a lucid explanation of the generation of shaft EMF's and provides various solutions to prevent large circulating shaft currents from being developed. Amongst the solutions are: insulating the bearing pedestals of a DEM; insulating the components of a bearing to insert large resistance in the flow path through the bearings of the DEM; providing an alternative path for current around the machine. The last solution requires the use of brushes mounted on the DEM at each location where the shaft leaves the DEM housing, so that a conductor may be connected to the brushes to “short-circuit” the brushes together and prevent any significant current through the bearings of the DEM. COSTELLO, M. J. “Shaft Voltages & Rotating Machinery” IEEE Paper No. PCIC -91-13, 1991.
This paper lists a number of causes of circulating shaft currents in both electrical and non-electrical machinery. Besides describing the physical damage phenomena resulting from the troublesome circulating shaft currents, the reference teaches the various stimuli which result in circulating shaft currents.
The two types of stimuli that are discussed which are of interest to this application are “sources of residual magnetism” and “magnetic dissymmetry” of the ac machine. Both stimuli result in circulating currents in the rotor shaft and various techniques of inserting insulation into the current circulation loop between the shaft and ground are described.


REFERENCES:
patent: 4609251 (1986-09-01), Nakaoka et al.
patent: 5136195 (1992-08-01), Allen, III et al.
patent: 5625244 (1997-04-01), Bradfield
patent: 5661352 (1997-08-01), Oguchi et al.
patent: 5705868 (1998-01-01), Cox et al.

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