Synchronous motors are often started as induction motors. To implement induction motor starting, the rotor has windings or bars called damper windings that are separate from and additional to the field excitation windings. The damper windings are shorted just as the rotor bars in an induction motor are shorted. With this starting method, the rotor is accelerated by induction motor operation to nearly synchronous speed. At that point the rotor excitation is applied and the motor pulls into synchronism. Once the motor is operating at synchronous speed, induction motor operation ceases naturally because no current is induced in the damper windings once there is no slip.
While the motor is starting, the field winding is shorted by connecting a small resistance across the field terminals. That protects the field windings and slip-ring assembly from excessive voltage that would be induced during starting. The induced current also adds to the induction motor accelerating torque.