Autotransformer starting

 Autotransformer starting

This method also aims at connecting the induction motor to a reduced supply at starting and then connecting it to the full voltage us the motor picks up sufficient speed. 

Figure shows the circuit arrangement for autotransformer starting. 

The tapping on the autotransformer is so set that when it is in the circuit, 65% to 80% of line voltage is applied to the motor

At the instant of starting, the change-over switch is thrown to "start" position. 

This puts the autotransformer in the circuit and thus reduced voltage is applied to the circuit.

Consequently, starting current is limited to safe value. 

When the motor attains about 80% of normal speed. the changeover switch is thrown to "run" position. 

This takes out the autotransformer from the circuit and puts the motor to full line voltage. 

Autotransformer starting has several advantages viz low power loss, low starting current and less radiated heat. 

For large machines (over 25 H.P.), this method of starting is often used. 

This method can be used for both star and delta connected motors.