Bus transfer of AC induction motors: a perspective

An attempt is made to put the problem of bus transfer of induction motors into perspective by presenting, in general terms, the overall concerns and the methods of analysis recently developed. It is pointed out that a decision to transfer one or more induction motors from one power source to another is a difficult one to make. It is complicated by the conditions under which it must be made, whether in a planned or emergency situation, and by the variation in the length of time between separating the motor from the first power source and connecting it to the second. The decision is influenced by the operation of the breakers, either simultaneous pole opening or staggered openings. No simple method of analysis is available for determining the effects on the rotating components of the motor, the couplings, and the driven equipment. It is noted that several general guidelines are in use, but care must be applied when determining if they are applicable to the conditions under consideration. The ANSI C50.41 recommendation of using a 1.33 pu limit for the resultant vectorial volts per hertz as a determining factor appears to reduce the complexities of the decision too much. The NEMA MG1 recommendation to wait until the motor residual voltage decays to 30-25% of its initial value may not be practical for critical equipment. When one does not have the time to delay reclosure, the NEMA MG1 recommendation that a detailed analysis be performed is appropriate