In this method, an auto-transformer is placed in series with the motor during staring. The transformer action reduces the voltage applied to the motor terminals. Because of the transformer action, the line current is less than the motor current. For a given starting current on the line side, the motor terminal voltage can be higher than for other reduced voltage start methods. Thus, this method gives the highest motor torque per line ampere.
START: R OPEN, S & Y CLOSED
RUN: R CLOSED, S & Y OPEN
Auto-Transformer Reduced Voltage Starting