Shaft voltages caused by alternating flux encircling the shaft

If a voltmeter is connected across the two ends of the shaft of a rotating electrical machine, a small voltage can normally be detected. In many cases this voltage is too small to cause problems, but if it reaches a level at which significant current starts to flow through bearings or other components attached to the shaft, then precautions such as insulating the bearings will be required. A brief review of the various mechanisms for the generation of shaft voltage is given, and one of these, involving a nonuniform air gap and alternating flux linkages with the shaft, is considered in detail for a particular machine. Simplified theory is presented together with a calculation procedure that is tested against measured results.