Synchronous motor

     An alternator can  may convert to a motor by connecting its armature winding to a 3-phase supply .Then this is called a synchronous motor.As the name indicate this motor will runs at synchronous speed (Ns =(120f/P)).That is there is a synchronism with the speed of motor and  field produced by the 3-phase supply .The speed of rotation is therefore depends on the frequency of the supply .Since the frequency of the 3- phase supply is constant the motor speed also constant irrespective of the load or supply voltage .