Answer Posted / b. b. shukla

In a synchronous motor, a rotating magnetic field is produced in stator by giving 3 phase AC supply.Now as per working principle of syn. motor,rotor must have a magnetic locking with stator field. For this purpose we need permanent magnetic poles in rotor so that rotor pole will form magnetic locking with nearest opposite pole of rotating stator field and rotates with stator field at synchronous speed.
Now to form permanent magnetic poles in rotor, DC excitation is fed to rotor winding of synchronous motor.

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