When the load of the asynchronous motor is not heavy, we can achieve energy saving by reducing the outside power. If the asynchronous motor runs normally and do not take no-load torque into thinking, the electromagnetic torque must be equal to the load torque. After the power is reduced, the main flux will decrease. At this moment the motor’s torque will decrease and the motor’s rotate speed will be reduced. Although this can increase the rotor current, however, when the load of the motor is light, the increasing of the rotor current will be reduced. By the electromagnetic theory, we can see that, the stator current should equal to the magnetization current and the rotor current of the motor. There into, the motor’s magnetization current will reduce as the main magnetic reduces. Under a certain load, only if the reducing of the motor’s magnetization current can play a leading role, then the reducing of the stator current will equal to the rotor current vector and the motor’s magnetization current vector. As a result, the stator copper loss will decrease and the motor’s efficiency will rise.

In practice, when the load is lower than 50%, the connection of the asynchronous motors of triangular connection can be replaced by the star shaped connection. At this time, the phase voltage of the motor isof the original voltage, so as to achieve the energy saving effect.