SOLUTION

• Synchronous motor is made self-starting by providing a squirrel cage winding (like that of an induction motor) along with the dc field winding on the rotor.
• In such a case when a three-phase supply is applied across the stator windings the rotor starts rotating as an induction motor and when it reaches near its final speed (near synchronous speed), dc field winding is energized and the rotor thus pulls into synchronism with the revolving field and continues to run at synchronous speed.
• At synchronous speed, there will be no current in the squirrel cage winding since at synchronous speed slip is zero.
• The squirrel cage winding, therefore, is designed only for short-duty services.
• During the starting period, the dc field winding has to be Kept shorted through a discharge resistance. This is done so as to avoid building up an extremely high voltage in the winding.
• If the field is left open-circuited a high voltage will develop in the open field winding as it has a large number of turns and the relative speed of stator flux to the windings of the poles is high during starting.
• But this induced high voltage will gradually decrease as the motor will be picking up speed.
• The induced emf in the field winding is kept to a safe value by shorting the winding.
• This would limit the demagnetizing effect on the main flux otherwise caused due to the current flowing in the dc field winding as a result of induced emf in it. This demagnetizing effect, if allowed to happen will reduce the starting torque of the motor.
• If in some special applications a higher starting torque is required the field winding can be left open-circuited, but should be sectionalized, to have reduced voltage induced across the separated portions.