In single-phase induction motors, a capacitor is used to create a phase shift between the main winding and the auxiliary winding, which is essential for generating a rotating magnetic field and starting the motor. Here's how it works:
Single-phase power supply: The single-phase induction motor is designed to operate from a single-phase AC power supply. In a single-phase supply, the voltage alternates between positive and negative cycles, resulting in a back-and-forth motion of the current.
Main winding: The motor's main winding is responsible for producing the magnetic field that drives the rotor to rotate. It is connected directly to the power supply and carries the current that produces the magnetic field.
Auxiliary winding: The auxiliary winding, also known as the starting winding or the starting capacitor winding, is a second winding in the motor. This winding is positioned at an angle relative to the main winding and creates a phase shift between the currents in the two windings.
Capacitor: The capacitor is connected in series with the auxiliary winding. When the motor is powered, the capacitor causes the current in the auxiliary winding to lead the voltage across it. This leads to a phase shift between the currents in the main winding and the auxiliary winding.
Rotating magnetic field: The phase shift generated by the capacitor causes the magnetic fields produced by the main and auxiliary windings to be out of phase with each other. The combination of these two magnetic fields results in a rotating magnetic field that sweeps around the inside of the motor.
Rotor interaction: The rotating magnetic field induces currents in the rotor bars through electromagnetic induction. These currents interact with the magnetic field, creating a torque that drives the rotor to rotate.
Starting the motor: Initially, the motor might be stationary or rotating slowly. As the rotor starts to rotate, it gains momentum and synchronizes with the rotating magnetic field. Once the rotor speed matches the speed of the rotating magnetic field, the motor reaches its full operating speed.
The capacitor helps create the necessary phase shift between the main and auxiliary windings, which is critical for inducing the starting torque and allowing the motor to accelerate to its running speed. After reaching the running speed, the auxiliary winding and capacitor are often disconnected using a centrifugal switch, as they are no longer required for the motor's operation.