Efficiency is the most important technical and economic index of motor Low voltage asynchronous motor load rate is generally between 50%~80%, mostly in the range of 60%~70%. The efficiency of asynchronous motor variable changes in different load rate and the loss of motor (stator and rotor winding copper loss) and constant loss (iron loss and mechanical loss) of the proportion, in total loss, variable loss increases, constant loss is reduced and the highest efficiency is equal to the low load rate. Therefore, in the design of the motor, increasing electromagnetic load, variable loss will be increased, and the constant loss should be controlled, which can make the motor in a wide range of load rate efficiency characteristics of relatively flat, is conducive to the efficiency of the motor is higher at a relatively low rate under load.