Synchronous Motors:

Synchronous motors are such as induction motors within they both have stator windings which generates a rotating magnetic field.  Dissimilar an induction motor, the synchronous motor is excited through an external DC source and, thus, needed slip rings and brushes to give current to the rotor. Inside the synchronous motor, the rotor locks into step along with the rotating magnetic field and rotates at synchronous speed.  No torque is developed if the synchronous motor is loaded to the point where the rotor is pulled out of step along with the rotating magnetic field, and the motor will stop.  A synchronous motor is not a self-starting motor since torque is only established when running at synchronous speed; thus, the motor requires a few type of device to bring the rotor to synchronous speed.

Synchronous motors use a wound rotor.  That category of rotor holds coils of wire placed in the rotor slots.  Brushes and Slip rings are used to supply current to the rotor.