AC MOTOR TYPES
Figure 7 Wound Rotor
Synchronous motors are like induction motors in that they both have stator windings that produce
a rotating magnetic field. Unlike an induction motor, the synchronous motor is excited by an
external DC source and, therefore, requires slip rings and brushes to provide current to the rotor.
In the synchronous motor, the rotor locks into step with the rotating magnetic field and rotates
at synchronous speed. If the synchronous motor is loaded to the point where the rotor is pulled
out of step with the rotating magnetic field, no torque is developed, and the motor will stop. A
synchronous motor is not a self-starting motor because torque is only developed when running
at synchronous speed; therefore, the motor needs some type of device to bring the rotor to
Synchronous motors use a wound rotor. This type of rotor contains coils of wire placed in the
rotor slots. Slip rings and brushes are used to supply current to the rotor. (Figure 7).