An important consideration in specification of an automatic transfer switch (ATS) is the method used to safely transfer motor loads between two energized sources, such as when returning from the generator to the utility after an outage or a test.
If a motor is transferred between two sources that are not in phase and the second source is applied before the residual voltage has decayed, transient current and torque can be as high as 20 times rated values. These transients can damage the motor, coupling, or driven equipment. When the sources to an ATS are a utility and a standby generator, there is no controlled phase relationship between them and it must be assumed that they will not be in phase. Consequently, some means of preventing out-of-phase transfers between energized sources must be provided whenever an ATS is to be used to transfer motor loads.