All the advantages of synchronous motors are equally balanced by the drawbacks of being non-self starting. As the synchronous motors are non self-starting in nature, they have to be started by some external means and has to brought to synchronous speed. Both a.c and d.c supplies are used to start the synchronous motor. Synchronous motors can have only two possible speeds – synchronous speed or zero speed.
In synchronous motor, stator winding is provided with the a.c supply and rotor winding is provided with d.c supply.The torque acting on the rotor of the synchronous motor is not uni directional but a pulsating one. Due to inertia of the rotor, the rotor of the synchronous rotor will not rotate in any direction and will be stationary. Therefore in order to start synchronous motor and bring it to synchronous speed some external means are employed.
Starting by DC Source:
Synchronous motor can be stated with the help of dc supply and dc compound motor, by coupling the synchronous motor to the dc motor. DC motor is started and the speed of the dc motor is adjusted by the speed regulator. The synchronous motor is then excited and synchronized with the ac supply mains. At the moment of synchronizing, the synchronous motor is switched on with the ac mains and dc motor is disconnected from the dc supply mains. Now the synchronous machine operate as synchronous motor and dc motor which helped to start the synchronous motor acts as load to the synchronous motor.
The synchronous motor can also be stated by the exciter mounted on an synchronous motor shaft extension. Here also, an available dc supply source operate the exciter as a motor during the starting period of the synchronous motor until it attains synchronous speed. Then once again exciter (dc generator connected on the shaft of the synchronous motor for providing dc field current) can carry on its normal function.
Starting by AC Motor:
A small directly coupled induction motor (pony motor) may be used for the starting of synchronous motor. Induction motor should have two poles less than the synchronous motor so that induction motor can able to run at synchronous speed of the synchronous motor (If both induction motor and synchronous motors have same number of poles, induction motor cannot be used as it cannot attain synchronous speed because induction motor runs below synchronous speeds). Before switching the ac supply to synchronous motor, it must be synchronized with the busbar. After normal operation is established, pony motor can be de-coupled from the synchronous motor shaft. This method is not satisfactory and not suited for industrial applications.
Starting by Damper Windings:
Modern synchronous machines are usually are of self starting type and are arranged to stat as induction motors. The synchronous motor is made self starting winding by providing damper windings or squirrel cage winding in the rotor poles. In the faces of the field poles damper windings are provided. Damper windings are the short circuited copper bars inserted in pole faces of field poles. 3-phase AC supply provided to stator windings of the synchronous motor generates a rotating magnetic field which rotates at synchronous speeds. This rotating magnetic field cuts the damper windings in the rotor. An emf is induced in the damper windings and current flows through the damper windings. This damper winding current produces a magnetic field which on interaction with the rotating magnetic field generates a torque which helps to rotate the synchronous motor as induction motor during stating. When the motor attains a speed of about 95% of the synchronous speed, the rotor windings are connected to the exciter terminals which locks the rotating field of the stator and rotating rotor of synchronous motor.