The main starting methods that are applied to synchronous motors are direct starting, starting with reduced line voltage (star-delta key, autotransformer, ballast), starting with auxiliary motor coupled to the shaft (or ponymotor), starting with soft-starter , starting with frequency converter. https://www.mrosupply.com/v-belts/v-belts/b-section/178341_b123_gates-rubber/
The direct starting and low voltage methods of the network are the traditional ones already widely used in induction motors, except when there is need of excitation for the synchronous motor, both for the moment of synchronization and in regime, for maintenance or control of its factor of power.
The auxiliary motor starting coupled to the shaft is a smooth, generally slow starting, where the assembly is having the rotational speed gradually increased by the auxiliary motor, coupled to the second shaft end of the synchronous motor. This starting method is used especially to reduce the impact of starting the synchronous motor on the mains and also in some cases to reduce high transient torques to the motor-load mechanical assembly. This starting method can be achieved by using as an auxiliary motor a winding rotor induction motor with liquid rheostat, or a cage rotor induction motor driven by a frequency converter or even a DC motor.