Autotransformer starting of Induction motor
This method also aims at connecting the induction motor to a reduced supply at starting and then connecting it to the full voltage as the motor picks up sufficient speed. Fig. (1) 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.
Relation between starting And F.L. torques:
Consider a star-connected squirrel-cage induction motor. If V is the line voltage, then voltage across motor phase on direct switching is V/√ 3 and starting current is Ist = Isc. In case of autotransformer, if a tapping of transformation ratio K (a fraction) is used, then phase voltage across motor is KV/√ 3 and Ist = K Isc,
The current taken from the supply or by autotransformer is I1 = KI2 = K2Isc. Note that motor current is K times, the supply line current is K2 times and the starting torque is K2 times the value it would have been on direct-on-line starting.