What is the primary purpose of a capacitor in a single-phase motor?

The primary purpose of a capacitor in a single-phase motor is to create a phase shift for starting torque. In single-phase motors, there is typically only one winding, which does not produce a rotating magnetic field necessary to start the motor. By introducing a capacitor in a particular manner, a second, auxiliary magnetic field is generated. This phase shift between the main winding and the auxiliary winding creates a torque that starts the motor.

When the capacitor is connected in series with the auxiliary winding, it causes the current to lead the voltage, thereby shifting the phase. This phase difference allows the motor to produce the necessary starting torque to overcome inertia and start rotating. Once the motor reaches a certain speed, a switch (like a centrifugal switch) often disconnects the auxiliary winding and the capacitor from the circuit.

While the other options may seem plausible within the context of motor function, they do not accurately represent the primary role of a capacitor in this application. Increasing voltage, preventing overheating, and controlling speed are functions typically associated with different components or mechanisms in a motor system.