The autotransformer steps down the voltage and the rotor is added to the excitation starting circuit according to the frequency change.

The circuit is shown in Figure 3-187. When starting, connect the autotransformer ZQB to reduce the voltage and start. After a period of delay,
ZQB exits operation and the motor speeds up under full pressure. When the speed rises to the quasi-synchronous speed and above, cut off (short-circuit) the discharge resistor R:, add DC excitation
to the rotor winding , the synchronous motor is led to run synchronously, and the starting process ends.