3. Auto Transformer Motor Starting

1. Operated by a two position switch i.e. manually / automatically using a timer to change over from start to run position.
2. In starting position supply is connected to stator windings through an auto-transformer which reduces applied voltage to 50, 60, and 70% of normal value depending on tapping used.
3. Reduced voltage reduces current in motor windings with 50% tapping used motor current is halved and supply current will be half of the motor current. Thus starting current taken from supply will only be 25% of the taken by DOL starter.
4. For an induction motor, torque T is developed by V2, thus on 50% tapping, torque at starting is only (0.5V)2 of the obtained by DOL starting. Hence 25% torque is produced.
5. Starters used in lager industries, it is larger in size and expensive.
6. Switching from start to run positions causing transient current, which can be greater in value than those obtained by DOL starting.