autotransformerstart

Autotransformer starting, or autotransformer start, is a method of starting induction motors that uses an autotransformer to apply a reduced voltage to the motor during the start-up phase. The reduced voltage lowers not only the starting current drawn from the supply but also the mechanical stress during acceleration. After the motor reaches a predetermined speed, the transformer is bypassed and full line voltage is applied.

Operating principle: The motor is connected to a tapping on an autotransformer that provides a fraction a of the line voltage, where 0 < a < 1. At start, the motor current is roughly proportional to the applied voltage, so the motor draws I_start ≈ a times the direct-start current, and the supply current from the grid is reduced further by the transformer action, roughly I_line ≈ a^2 times the direct-start current. The starting torque is proportional to the square of the applied voltage, so T_start ≈ a^2 T_DOL. When the motor reaches about 80–90% of rated speed, the circuit switches to full voltage.

Advantages: The method reduces inrush current and voltage drop on the supply, yielding smoother acceleration and reduced electrical stress on cables and switchgear. It can be used with larger motors without needing full-voltage control gear. Limitations: It requires an autotransformer and associated switching equipment, which adds cost, space, and transformer losses. It is less flexible than a variable-frequency drive for speed control and not ideal when high starting torque is required.

Applications: Autotransformer starting has been used for induction motors in fans, pumps, conveyors, compressors, and other heavy-duty applications where the electrical system cannot tolerate direct-on-line starting and where simple speed control is not needed. It was more common before inexpensive variable-frequency drives became available.