In a single-phase induction motor, the function of a capacitor is to provide the necessary phase shift in the starting winding, which creates a rotating magnetic field that initiates the motor's rotation. Single-phase induction motors typically require a starting mechanism to overcome the challenges of starting without a true rotating magnetic field.
The single-phase power supply provides an alternating current (AC) voltage, which means the magnetic field it generates oscillates back and forth in one direction. To produce a rotating magnetic field in the motor's stator, a phase shift is needed.
The capacitor is connected in series with the starting winding of the motor. When the motor is initially powered on, the capacitor charges, causing a phase shift between the current in the main winding and the starting winding. This phase shift creates a rotating magnetic field, which allows the motor to start and gain momentum.
Once the motor reaches a certain speed, a centrifugal switch or a starting relay disconnects the capacitor from the circuit. The motor then operates with just the main winding, which is not ideal for high efficiency or power factor, but it is sufficient to maintain the rotation of the motor.
Without the capacitor, the single-phase induction motor would have limited starting torque and might not be able to start on its own. The capacitor provides the necessary phase shift during startup to overcome this limitation and get the motor running smoothly.