A three-phase synchronous motor will have

• A. No slip-rings
• B. One slip-ring
• C. Two slip-rings
• D. Three slip-rings

Answer: (C)

A three-phase synchronous motor basically consists of a stator core with a three-phase winding (similar to an induction motor), a revolving DC field with an auxiliary or amortisseur winding and slip rings, brushes and brush holders, and two end shields housing the bearings that support the rotor shaft. An amortisseur winding consists of copper bars embedded in the cores of the poles. The copper bars of this special type of “squirrel-cage winding” is welded to end rings on each side of the rotor. The function of a slip ring is to transfer electrical signals from rotary to stationary  components or systems
 
Both the stator winding and the core of a synchronous motor are similar to those of the three-phase, squirrel-cage induction motor, and the wound-rotor induction motor.
The rotor of the synchronous motor has salient field poles. The field coils are connected in series for alternate polarity. The number of rotor field poles must equal the number of stator field poles. The field circuit leads are brought out to two slip rings mounted on the rotor shaft for brush-type motors. Carbon brushes mounted in brush holders make contact with the two slip rings. The terminals of the field circuit are brought out from the brush holders to a second terminal box mounted on the frame of the motor. A squirrel-cage, or amortisseur, winding is provided for starting because the synchronous motor is not self-starting without this feature.