The choice of motor types and forms depends on AC or DC, mechanical characteristics, speed and starting performance, maintenance and price. The three-phase asynchronous motor with squirrel cage rotors and two types of selection may refer to the following principles: (1) should first consider the selection of three-phase squirrel cage asynchronous motors, because it has the advantages of simple structure, durable, reliable, low price and convenient maintenance etc.. But its main disadvantage is that the adjustment is difficult, the power factor is low, the starting current is large and the starting torque is small. Therefore, it is mainly suitable for dragging general production machines with mechanical characteristics rather hard and without special adjustment requirements, such as general machine tools, pumps and fans with power less than 100kW. For some production machinery with large starting torque, such as carding machine, loom, compressor and belt conveyor, the high starting torque cage motor can be selected. The cage type multi speed motor can be used in the situation where only the adjustment is required. It is suitable for machine tools such as machine tools and elevators. (2) the price of wound rotor motor is higher than that of cage motor, but its mechanical characteristics can be regulated by external resistance of rotor, which can limit starting current and improve starting torque. So it can be applied to the situation where the power supply is small and the power of the motor is large or the adjustment is required. For example, some cranes, hoisting equipment, forging machines and moving beams of heavy machine tools. (3) when the adjustment range is lower than 1:10, and also requires a smooth adjustment of the production situation, the first selection of slip motor.