An option for the activation of three-phase induction motors with load is the starting with auto-trafo compensator, commonly called compensator. The start is in two stages, and in the first, the power supply to the motor happens with reduced voltage through the autotransformer. At the start of the starting, the current peak and the conjugate are decreased proportionally to the square of the transformation ratio and according to the tap of the transformer, this transformation ratio can be 65 or 80%. The motor torque bearings still reaches the first stage, which is faster than that achieved in the star-delta system. In the second stage, after the initial start time, the neutral point of the autotransformer is opened, the motor is turned on under full voltage, returning to its nominal characteristics.
The motor voltage is reduced proportionally with the transformer taps, and when using the 65% position, the line current is almost equal to that of the star-delta starting system. However, in the transition from the reduced voltage to the full voltage, the motor will not be switched off, so the second peak current is greatly reduced, because the auto-trap for a short period of time becomes a reactance connected in series with the motor.