Why is an autotransformer used in a motor starting circuit?

An autotransformer is utilized in a motor starting circuit primarily to limit inrush current. When an electric motor starts, it can draw significantly more current than its rated full-load current, which is referred to as inrush current. This surge can create challenges, such as damaging the motor windings, tripping circuit breakers, or causing voltage drops in the power supply.

By using an autotransformer, the voltage applied to the motor can be reduced during the initial starting phase. This reduction in voltage translates to lower current drawn by the motor at startup, effectively limiting the inrush current to safer levels. Once the motor reaches a certain speed and the current stabilizes, the autotransformer can be switched out of the circuit, allowing the motor to operate at full voltage.

Overall, limiting inrush current is crucial for protecting both the motor and the electrical infrastructure, making this the primary reason for using an autotransformer in these applications.