Asynchronous motor is a kind of ac motor. Its load speed and the frequency of the network is not a constant relationship. It also varies with the size of the load. The higher the load torque, the lower the rotor speed. Induction motor includes induction motor, double - fed induction motor and ac commutator motor. Induction motor is the most widely used in the absence of misunderstanding or confusion, which generally can be called induction motor.
The stator winding of common induction motor is connected with ac power grid. Therefore, it has the advantages of simple structure, convenient manufacture, use and maintenance, reliable operation, low quality and low cost. Asynchronous motor has higher operating efficiency and better working characteristics. It runs at almost constant speed from no load to full load, which can meet the transmission requirements of most industrial and agricultural production machines. Induction motors are also easy to be derived into various types of protection to adapt to different environmental conditions. When the induction motor is running, the reactive excitation power must be drawn from the grid to make the power factor of the grid go bad. Therefore, the synchronous motor is often used to drive the high-power and low-speed mechanical equipment such as ball mill and compressor. As the speed of the induction motor and its rotating magnetic field speed have a certain rotation difference, its speed control performance is poor (except the alternating current commutator motor). It is more economical and convenient to use dc motor for transportation machinery, rolling mill, large machine, printing and dyeing machine and paper making machine. However, with the development of high-power electronic devices and ac speed control system, the speed control performance and economy of the induction motor for wide speed control can be compared with that of the dc motor.