Which type of autotransformer starter utilizes autotransformers in only two legs of the three-phase supply?

The correct answer is that a closed transition autotransformer starter utilizes autotransformers in only two legs of the three-phase supply. This type of starter is designed to reduce the voltage applied to the motor at startup, minimizing inrush current and allowing for smoother acceleration.

In a closed transition autotransformer starter, the motor is initially connected to the reduced voltage from the autotransformer. Once the motor reaches a certain speed, the transition occurs to the line power supply while maintaining the electrical connection. This process helps to prevent any interruptions in the power supply to the motor, which can happen during the transition phase, thereby ensuring a seamless connection while still benefiting from the lower starting voltage.

This method helps to provide the benefits of reduced starting current and torque without the sudden changes in power that are a risk with other types of starters. Understanding the role of the autotransformer in this context aids in recognizing why this option is specifically used for applications where a three-phase motor requires controlled starting conditions.