Why three phase induction motor is self starting?

The Self-Starting Capability of Three-Phase Induction Motors

Three-phase induction motors are widely recognized for their reliability, durability, and simplicity in various industrial applications. One of the most important features that sets them apart is their self-starting capability, which enables the motor to begin rotating automatically upon being powered. This feature eliminates the need for external starting devices or mechanisms. Below is a structured explanation of how this self-starting capability works.

1. Generation of Rotating Magnetic Field

   When a three-phase AC supply is connected to the stator windings of the motor, it creates a rotating magnetic field. This magnetic field rotates around the stator at a constant speed known as the synchronous speed, which depends on the frequency of the supply and the number of poles in the stator.

2. Induction of Electromotive Force (EMF) in the Rotor

   As the rotating magnetic field sweeps across the rotor (which is typically a squirrel-cage rotor or a wound rotor), it induces an electromotive force (EMF) in the rotor conductors according to Faraday’s Law of Electromagnetic Induction. This induced EMF causes current to flow in the rotor conductors.

3. Creation of Rotor Magnetic Field

   The current flowing in the rotor conductors generates a magnetic field around the rotor. This rotor magnetic field interacts with the rotating magnetic field produced by the stator.

4. Production of Torque

   The interaction between the rotor's magnetic field and the stator's rotating magnetic field produces torque, which causes the rotor to start turning. The rotor accelerates in the direction of the stator’s rotating magnetic field until it reaches its operating speed, which is just below the synchronous speed.

5. Self-Starting Mechanism

   This entire process happens automatically when the motor is connected to the power supply, giving the motor its self-starting capability. No external components such as starting capacitors or relays are required to initiate rotation, making three-phase induction motors straightforward and efficient in design.

The Importance of a Three-Phase Configuration

The three-phase configuration of the stator's windings is essential for the self-starting capability of a three-phase induction motor. In a single-phase induction motor, the stator's windings are arranged in a single-phase configuration, and this single-phase configuration does not produce a rotating magnetic field. As a result, a single-phase induction motor requires an external device, such as a capacitor, to produce a rotating magnetic field and to start the rotor of the motor.

In contrast, a three-phase induction motor does not require an external device to produce a rotating magnetic field and to start the rotor of the motor. The three-phase configuration of the stator's windings produces a rotating magnetic field strong enough to start the rotor of the motor, allowing a three-phase induction motor to start without the need for an external starting device.

Summary

In summary, a three-phase induction motor is self-starting because the stator's windings are arranged in a three-phase configuration, and this three-phase configuration produces a rotating magnetic field. The rotating magnetic field induces a current in the rotor of the motor, and the current flowing through the rotor produces its own magnetic field. The interaction between the stator's magnetic field and the rotor's magnetic field causes the rotor to rotate, which in turn drives the shaft of the motor. The three-phase configuration of the stator's windings is essential for the self-starting capability of a three-phase induction motor, allowing the motor to start without an external starting device.