Auto Transformer

An Auto Transformer Starter is a widely used starting device for three-phase induction motors, designed to reduce the high inrush current that occurs during motor startup. It operates using a three-phase star-connected auto transformer along with a changeover switching mechanism to provide a controlled and reduced voltage to the motor during starting.

At the time of starting, the stator winding of the induction motor is supplied with a reduced voltage through the auto transformer, selected through suitable tapping points. By lowering the applied voltage, the starting current drawn from the supply is significantly reduced, protecting both the motor and the electrical distribution system from excessive current and voltage dips.

Typically, the auto transformer is provided with multiple tapping points per phase to allow different starting voltage levels. Standard tapping combinations generally include 50%, 65%, and 80% or 60%, 70%, and 80% of the line voltage. These tapping options enable the motor to start smoothly under varying load conditions. Customized tapping configurations can also be provided as per customer requirements.

Once the motor accelerates and reaches approximately 70–80% of its rated speed, the starting circuit automatically disconnects the auto transformer from the system. At this stage, the motor is directly connected to the full supply voltage, allowing it to operate under normal running conditions.

Auto Transformer Starters are particularly suitable for large capacity motors where direct-on-line (DOL) starting is not feasible due to high starting current. They ensure smooth motor acceleration, reduced electrical stress, and improved system stability. Our Auto Transformer Starters are designed for reliable operation, robust construction, and efficient performance, making them suitable for a wide range of industrial applications such as pumps, compressors, conveyors, and heavy machinery.