Whenever the synchronous motor is started using any of the starting methods, the field winding is excited as soon as it comes nearly-to the synchronous speed. Now it is not essential that both the poles of stator
and rotor are enlined. The rotor pole might be trailing or leading the stator opposite pole. Thus an angular displacement is observed. In that
case the force of attraction between the opposite poles comes into existence and the rotor pole is pulled into enline as shown in fig. 20.5.
Fig. 20.5.
So the amount of the torque that a motor exerts when pulling into synchronism is called the pull-in-torque. Its magnitude differs motor to motor. As the driving source is taken off the rotor is retarded or pulled back by a small angle α, called torque angle and with this angle the rotor runs to the synchronous speed.