The figure on the right shows the diagram of a start-up by means of an auto-transformer with a single intermediate tap. Note that u, v and w are the start terminals of the motor phases. The protection devices are not indicated.
The sequence of operations has to be carried out to start the motor is as follows:
First of all switch T1 is closed so that the auto-transformer is set up for star-type operation. T2 is then closed (keeping T1 closed) and the motor is powered via the auto-transformer at a voltage of 0.7Vn. Then, T1 is opened so that the motor is fed through the auto-transformer reactors which are in series with the line. Finally, the T3 is closed and the motor is powered at full voltage as the reactors are short-circuited.
In the case we have considered of an auto-transformer with a single intermediate tap, the starting torque is reduced to 1/2 (as the voltage is 70% of the nominal value) and current consumption is also reduced to 1/2 of the nominal level (provided the motor is taken as a constant impedance load; in fact, 70% of its nominal voltage is applied to the motor and it will absorb 70% of the normal starting current, and this current is then reduced by a further 70% as the auto-transformer voltage ratio is 1 : 0.7).
As they are intended for a time-limited service (10-20 sec. approx.), the auto-transformers for this type of use operate with high values of magnetic induction and with high current density values. For this reason they are much more compact and less expensive than auto-transformers for continuous service.
The table below indicates the dimensions (mm) of some auto-transformers that we have standardized with a line voltage of 400V and an output of 280V at 50-60 Hz.