The auto- transformer is used to reduce the phase voltage to the fraction x of normal value. The advantage of the method lies in the fact that the voltage is reduced by transformation, not by dropping the excess in impedance, so that the line current and power input are reduced in comparison with stator resistance starting.
Neglecting magnetizing current and losses in the auto-transformer, the current on the output side is Is in the fraction of the turns, and on the input side will consequently be xIs = x2Ix in all the turns of each phase. Thus for a starting torque of x2 of that obtainable by direct switching, only x2 of the short-circuit current is taken from the line. Alternatively, for the same line current, the starting torque with the auto-transformer is 1/x times as great as that obtainable with the stator impedance startor.
The system of connections shown provides voltage steps during starting, the auto-transformer being cut completely out of service after the start has been completed. Since it is only in use for short periods, the current density in the windings may be increased considerably above that normal for power transformers. At the same time, since the starting torque depends on the square of the current, the impedance of the transformer should be kept small. The auto-connection is here of particular advantage.
