Transfer of Motor Loads Between Out-of-Phase Sources

An inrush current normally occurs when a motor is momentarily disconnected from a source of power and is immediately reconnected to an alternate source of power. The magnitude of the in-rush current may substantially exceed the normal starting current of the motor. The mechanical shock to the system may create cumulative damage to the motor shaft and windings resulting in reduced life if not immediate failure. The thermal and/or magnetic shock to the system can result in nuisance opening of overcurrent protective devices. Test data are presented showing how in-rush currents drawn by a motor vary for different transfer conditions, and one method for minimizing such currents is explained.