The power factor of the asynchronous motor is both lagging behind when it starts and starts. Therefore, when the pump station is driven by the asynchronous motor, it will cause large pressure drop when starting the second and later motors. The starting of synchronous motor is generally divided into 2 stages. In the first stage, the stator windings are connected to the AC power grid to make the synchronous motor start (asynchronous starting) as an asynchronous motor. In the second stage, when the motor speed reaches 95% of the synchronous speed, the excitation coil of the rotor is connected to the DC power supply, so that the motor pulls into the synchronization and moves to stable operation.
In asynchronous start stage, besides the torque MD generated by the starting winding, the induction current in the equivalent phase of a single phase winding will interact with the air gap magnetic field, which will cause the single axis torque. The characteristic of the uniaxial torque is that a negative torque is generated at the transfer rate. Therefore, if the starting load is heavier enough to exceed the minimum torque of the motor, the motor will be jammed near the half synchronous speed of NS, and it can not continue to speed up. In order to reduce the single axis torque, a current limiting resistor can be strung in the excitation winding. After the speed exceeds 0.5ns, the resistance is gradually removed.
When the speed is close to 95% synchronous speed (i.e. slip ratio s=0.05, the corresponding electromagnetic torque is called traction torque, if M2>M can be input excitation, pull the torque into the rotor to synchronize. When the synchronous motor starts asynchronously, the excitation winding can not be opened. Otherwise, when the starting voltage is aroused, the dangerous high voltage will be broken through the winding, causing the accident. Therefore, it is necessary to pay attention to the excitation link of the excitation device.
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