Demonstrative Video
Methods of Starting of Synchronous Motors
Since SM are inherently not self-starting, the following methods are generally adopted to start :
By means of Auxiliary Motor
By providing damper winding
By means of Auxiliary Motor :
A small IM called the pony motor (auxiliary motor) is coupled to SM on the same shaft.
The auxiliary motor should have the same number of poles as that of SM or preferably one pole pair less so that it can rotate the motor nearly at \(N_s\).
First supply is given to the pony motor and it rotates the rotor of the SM near to the \(N_s\) then the main switch and DC switch of the main SM are closed.
The rotor poles are pulled into synchronism with the rotating field (poles) of the armature (stator) of the main motor. Then supply to the auxiliary motor is disconnected and it acts as a load on the main motor.
By providing damper winding:
Most common method of starting a SM.
Motor is first started as a SCIM by providing a special winding on the rotor poles, known as damper winding.
DW consists of number of copper bars embedded into the slots provided on the outer periphery of the pole shoes, where salient poles are employed, and then short circuiting these bars by brazing them to end rings
In a non-salient machine, the DW conductors are placed in the rotor slots above the main field winding and short circuited by the end rings.
When the SM is connected to \(3\phi\) supply, a revolving field is set up which causes the rotor to rotate as a SCIM.
As soon as motor attains about 65% \(N_s\), the rotor winding is connected to DC mains and the rotor field is magnetically locked with the stator rotating field and the motor starts running runs as a SM.