The torque of asynchronous motor is generated by interaction between magnetic flux of motor and current in rotor. In rated frequency of motors, using constant voltage with reduced frequency, it will result in over-magnetic flux, magnetic saturation, and even cause motor damage. Therefore, frequency and voltage have to be changed in proportion. In other words, the output voltage should be changed with frequency. To fix the magnetic flux of motor, it can avoid weaken flux and magnetic saturation. This control method is usually used in energy-saving drives for fan and pump.