Bus transfer of AC induction motors, a perspective

The author attempts 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. Some results of a study of bus transfer on a 250 hp motor for various reclosure times are shown. The complex electromagnetic transient and the resulting shaft torque transient for various durations of power interruption are illustrated. It is noted that there is no simple method of analysis available for determining the effects on the rotating components of the motor, the couplings, and the driven equipment. There are several general guidelines 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 per unit 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% to 25% of its initial value may not be practical for critical equipment. It is concluded that when one does not have the time to delay reclosure, the NEMA MG1 recommendation that a detailed analysis be performed is appropriate