Synchronous motor asynchronous starting circuit

When a synchronous motor is running normally, the rotor rotates at a synchronous speed. At this time, the rotor magnetic field and the stator magnetic field are in a relatively static state.
Their interaction can produce electromagnetic torque of constant direction and constant magnitude. But when starting, the situation is completely different. In fact,
if no other measures are taken, the synchronous motor itself has starting torque. As shown in Figure 4-43, first connect an additional resistor R about 10 times the excitation winding
resistance     in series in the excitation circuit of the synchronous motor, and then set the switch os2 to position 1 on the stage to form a closed loop I. Close the switch Qsl to give When the stator winding is supplied with three-phase alternating current, the synchronous motor will start asynchronously under the action of the starting winding , and the speed will gradually increase. When the speed rises to close to the synchronous speed (about 0.95n), quickly turn switch QS2 from position 1 to position 2 to energize the rotor with DC current, relying on the attraction between the stator rotating magnetic field and the rotor magnetic poles. , pull the synchronous motor to run at synchronous speed. After the rotor reaches synchronous speed, the relationship between the rotor conductor and the rotating magnetic field is in a relatively static state. No current flows through the squirrel cage starting winding, so the starting winding loses its function and the starting process ends.