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Why single phase induction motor is not self starting ?

The single-phase induction motor is not self-starting due to its inherent design limitations, particularly with respect to the production of a rotating magnetic field.

In a three-phase induction motor, a rotating magnetic field is easily generated by the three-phase AC power supply. The magnetic fields produced by each phase are spaced evenly in terms of time, creating a rotating effect that propels the motor into motion. This self-starting capability is a key advantage of three-phase induction motors.

However, in a single-phase induction motor, generating a rotating magnetic field is more challenging because there is only one alternating current (AC) supply. The magnetic fields produced by a single-phase supply are not evenly spaced in time, resulting in an oscillating or pulsating magnetic field rather than a continuous rotation. This pulsating magnetic field fails to produce sufficient torque to initiate motor rotation.

To overcome this limitation and enable self-starting in single-phase induction motors, auxiliary mechanisms are often employed. One common method is the inclusion of an additional winding, known as a starting winding, which is positioned at an angle to the main winding. This creates a phase difference and helps produce a rotating magnetic field during startup. Once the motor reaches a certain speed, a centrifugal switch disconnects the starting winding, and the motor continues to run on the main winding.

Another approach involves the use of a capacitor in conjunction with the starting winding to create a phase shift and establish a rotating magnetic field during startup. Capacitor-start single-phase induction motors are commonly used in various applications where self-starting capability is essential.

In conclusion, the absence of a naturally rotating magnetic field in a single-phase AC supply makes single-phase induction motors not self-starting. Additional components, such as starting windings or capacitors, are incorporated to create the necessary phase shifts and initiate rotation during motor startup.

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