Autotransformer start – Features

This starting is done without interruption of the current in the electric motor, thus avoiding the transient phenomena resulting from the interruption. However, transient phenomena of the same nature may be produced at the time of full voltage connection if certain precautions are not taken. In fact, the inductance value connected in series with the motor after the opening of the star connection is high compared to that of the motor. Hence the consequence of the high voltage drop, which causes a peak of transient current at the moment of connection to full voltage. To avoid this inconvenience, there is an air gap in the magnetic circuit of the autotransformer whose presence results in a reduction of the inductance value.

This value is calculated in such a way that, at the time of opening the star connection, in the second time, there is no voltage change at the motor terminals. The presence of the air gap results in an increase in the magnetizing current of the autotranformer, which raises the required current in the network during the first start time. This starting mode is normally applied to motors with a power greater than 10 kW. It implies, however, in the use of relatively expensive equipment due to the high price of the autotranformer.