Why Three Phase Induction Motor Self-Starting but Synchronous Motor not?

Starting of Induction Motor and Synchronous Motor

The Phase Induction Motor does not need any starter for starting, or in other words, the phase induction motor is self-starting. A three-phase synchronous motor has almost the same in construction as a three-phase induction motor. However, the synchronous motor is not self-starting. So a question arises that Why Three Phase Induction Motor Self-Starting but the Synchronous Motor is not self-starting?

Why Three-Phase Induction Motor Self-Starting?

In the three-phase induction motor, the stator winding needs a three-phase supply. There are three windings in the stator each are placed in 120 degrees apart.

So when we give a three-phase power supply to the stator winding, a rotating magnetic field is created. At the starting time, the rotor is constant, so an emf is induced in the rotor winding for the transformer action. As the rotor winding is short-circuited so a short circuit current will flow through the rotor winding which is caused by the induced emf.

The short circuit current creates a magnetic flux in the rotor winding which is opposite to the stator flux according to Lenz's Law. Now the rotor flux tries to oppose the stator flux and to reduce the relative speed between the rotating magnetic field and the rotor speed, the rotor tends to run in the direction of the rotating magnetic field.

So, in simple words, the three-phase induction motor is self-starting because,

1. The stator creates a rotating magnetic field

2. The rotor flux is produced due to the stator flux and according to Lenz's law, the rotor flux tries to oppose the stator flux and creates a torque.

Why Three-Phase Synchronous Motor is not Self-Starting?

Like a phase induction motor, in a three-phase synchronous motor also rotating magnetic field is produced but the synchronous motor is not self-starting.

Induction motor works in the principle of electromagnetic induction but synchronous motor works in the principle of attraction or locking.

 As the rotor in the synchronous motor rotates at the synchronous speed so we need to create a locking between the rotor and the rotating magnetic field.

We know that the synchronous motor needs two power supplies for its operation one is the main three-phase AC supply in its stator winding and another is the DC supply to its rotor winding.

When we give the DC supply to the rotor winding, it creates a magnetic field, and its poles are always constant. But as the AC supply is given to the stator winding, the stator poles do not get constant, they continuously rotate as the magnetic field rotates.

Suppose when the north pole of the stator comes near to the north pole of the rotor they repulse to each other. When the south pole of the stator comes near to the north pole of the rotor they attract to each other. So there is a continuous reversible torque will be produced on the rotor and there no locking will be created between the stator and rotor.

But if we initially rotate the rotor by means of the external prime mover at synchronous speed, the locking will be created and it will continuously rotate even after removing the prime mover.

So this was the reason Why the phase Induction Motor was Self-Starting but the Synchronous Motor was not Self-Starting.

 Read Also:

• Why DC is used not AC for excitation of Synchronous Alternator and Motor?
• [Actual Difference] Between Synchronous Motor and Induction Motor
• Why Damper Winding is used in Synchronous Alternator and Motor?
• Why Alternator Armature Winding always connected in STAR?
• Why Capacitor is used in Single phase Induction Motor?Full Explanation.

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