Frequency control method of three-phase asynchronous motor: frequency control is to change the frequency of motor stator power supply, so as to change its synchronous speed. The main equipment of the frequency conversion and speed regulation system is the frequency converter which provides the frequency conversion power. The frequency converter can be divided into AC-DC-AC frequency converter and AC-AC frequency converter. At present, the AC-DC-AC frequency converter is widely used in China. This method is suitable for the occasion of high precision and good speed regulation performance. Features: 1. The slip loss in the process of speed regulation can be fed back to the power grid or production machinery with high efficiency. 2. The capacity of the device is proportional to the speed range, and the investment is saved. It is suitable for the production machinery with the speed range of 70% - 90% of the rated speed. 3. When the speed regulating device fails, it can be switched to full speed operation to avoid shutdown. 4. The power factor of thyristor cascade speed regulation is low, and the influence of harmonics is large. Three phase asynchronous motor cascade speed regulation method: cascade speed regulation refers to the adjustable additional electromotive force in series in the rotor circuit of wound motor to change the slip of motor and achieve the purpose of speed regulation. Most of the slip power is absorbed by the additional electromotive force in series, and then the additional device is generated by reuse. The absorbed slip power is returned to the grid or converted energy for use. According to the absorption and utilization mode of slip power, cascade speed regulation can be divided into motor cascade speed regulation, mechanical cascade speed regulation and thyristor cascade speed regulation, and most of them adopt thyristor cascade speed regulation. The method is suitable for use on fans, water pumps, rolling mills, mine hoists and extruders. Features: 1. High efficiency, no additional loss in the process of speed regulation. 2. Large speed range, hard characteristic and high precision. 3. It can be used in cage asynchronous motor. 4. The technology is complex, the cost is high and the maintenance is difficult.