What is the ideal phase difference between the starting winding and the running winding?

The correct answer is based on the functionality and design of single-phase induction motors. In these motors, the starting winding and the running winding must create a phase shift that generates a rotating magnetic field, which is crucial for proper motor operation.

When the starting winding is energized, it needs to operate out of phase with the running winding to produce this rotating magnetic field effectively. A phase difference of 90 degrees is ideal because it ensures the maximum torque is developed when the motor starts. This arrangement allows the motor to turn; when the rotor begins to spin, the starting winding can be disconnected (often using a centrifugal switch or an electronic control) because the running winding alone can sustain the motor's operation effectively.

A phase difference of 90 degrees optimizes the motor's performance by ensuring a smooth transition from start-up to running conditions, enabling efficient operation. Options like 0 degrees, 45 degrees, and 180 degrees do not facilitate the necessary torque or can lead to inefficient motor operation, as they do not create the required rotating magnetic field effectively. Thus, a 90-degree phase difference is the ideal scenario for achieving the desired performance in single-phase induction motors.