Autotransformer Starting

Autotransformer starting automatically switching between taps of an autotransformer reduces the voltage, current and torque to the motor according to the tap setting used on the autotransformer. There are two very distinctive characteristics of an autotransformer starter.

1. The motor terminal voltage is not a function of load current and remains constant during the acceleration time.

2. Due to the turn ratio advantages, the primary line current is less than the secondary motor currents. A three-coil autotransformer is connected in a wye configuration and connected to the motor in such a way as to supply reduced voltage to the motor when the line voltage is applied to the Autotransformer. Several sets of taps are usually available to the user to provide different values of reduced voltage.

(NEMA standards are 80%, 65% and 50% of the full line voltage).

But, auto transformer starter suffers form various drawbacks as mentioned below.

a] It has mechanical components for changing of taps and hence it requires frequent maintenance and inspection.

b] Voltage applied to the motor terminals is in step manner and not continuous. Hence, voltage flickers and mechanical jerks are a part of auto transformer starting.

c] A very dangerous spike, as shown in figure is experienced by the system during starting.