In single-phase induction motors, a capacitor is often used to create a phase shift between the main winding (running winding) and an auxiliary winding (start winding) in order to generate a rotating magnetic field. This rotating magnetic field is crucial for the motor to start and operate properly.
Here's how a capacitor assists in creating a phase shift and generating a rotating magnetic field:
Single-Phase Power Supply: Single-phase induction motors are typically used in applications where only a single-phase power supply is available. However, single-phase power supplies cannot directly create a rotating magnetic field like three-phase power supplies can. In a three-phase system, the phases are naturally offset by 120 degrees, creating a rotating magnetic field. But in a single-phase system, the magnetic field generated by a single winding does not rotate; it just oscillates back and forth.
Auxiliary Winding and Capacitor: To create a rotating magnetic field in a single-phase motor, an auxiliary winding is used in addition to the main winding. The auxiliary winding is connected in series with a capacitor. The capacitor introduces a phase shift between the voltage across the main winding and the auxiliary winding.
Phase Shift: The phase shift introduced by the capacitor is around 90 degrees. This means that the current flowing through the auxiliary winding lags behind the voltage across it by about 90 degrees in terms of phase angle. The main winding carries the primary current and provides the main torque for the motor, but due to the phase difference created by the capacitor and auxiliary winding, the auxiliary winding current leads to the development of a second magnetic field.
Rotating Magnetic Field: The combination of the two magnetic fields (from the main winding and the auxiliary winding) creates a resultant magnetic field that is rotating. This rotating magnetic field interacts with the squirrel cage rotor (a short-circuited winding) within the motor, inducing currents in the rotor bars. These induced currents create a second magnetic field in the rotor, which interacts with the rotating magnetic field, causing the rotor to start moving and follow the rotating field.
Starting and Running: Initially, the motor may require some external force or starting mechanism to overcome the inertia and get the rotor moving. However, once the rotor starts moving, it will continue to follow the rotating magnetic field created by the phase-shifted windings and capacitor.
In summary, a capacitor assists in creating a phase shift between the windings of a single-phase induction motor. This phase shift, in combination with the main winding, generates a rotating magnetic field that enables the motor to start and operate as a self-starting induction motor. The capacitor's value and the winding design are important factors that determine the motor's performance characteristics.