The torque of an asynchronous motor is generated by the interaction between the magnetic flux of the motor and the current flowing through the rotor. At the rated frequency, if the voltage is fixed but only the frequency is reduced, then the magnetic flux will be too large, the magnetic circuit will be saturated, and the motor will be damaged.Therefore, the frequency and voltage should be changed proportionally. The frequency should be changed at the same time to control the output voltage of the frequency inverter. It keep the magnetic flux of the motor constant and avoid the phenomenon of weak magnetism and magnetic saturation.This kind of control mode is mainly used in energy-saving inverter of fan and pump.