A normal synchronous motor rotor having a damper winding arranged such like slip ring IM. On applying 3-phase supply to the armature winging of Synchronous motor, its start as an induction motor(IM).
On achieving about 95% of synchronous speed, the rotor winging is connecter to exciter terminal (by changeover) and rotor is magnetically locked with stator r.m.f. and motor runs as a synchronous motor.
If starting and excitation is done automatically the motor is known as Auto-Synchronous Motor.
Fig. shows the circuit diagram of Auto Synchronous motor rotor.
In this method synchronous motor start as a Slip Ring IM.
This method of starting used when synchronous motor started ON Load.
Modified Rotor winding (Three phase winding) just like slip ring induction rotor are placed in rotor face/slot and brought out through slip ring & connect with variable resistant (Fig.)
Hence, rotor start rotating just like induction motor and reached very near to synchronous speed. As rotor reached near to synchronous speed, change over switch will operate from start position to run potion & DC supply apply to the field winding.
Due to this stator pole attract the rotor pole and magnetic locking (synchronism) take place automatically. As changeover take place, slip ring winding get DC excitation. Hence, starting winding has now became field winding after starting period.
By variable resistance, High starting torque and Less Starting Current can obtain. Motor can start on load.