Three-phase asynchronous motor's very logarithmic p and s slip can be too much to change speed. From the perspective of the nature of speed regulation, different ways of governing is nothing but change synchronous ac motor synchronous speed or not turn two. Widely used in the production of machinery is not the change in speed control method of synchronous speed of wound rotor motor rotor series resistance speed governing, chopping speed regulation, cascade speed regulation and application of the electromagnetic slip clutch, hydraulic coupling, such as oil film clutch speed regulation. Changes of the synchronous speed logarithmic multi-speed motor, stator can change of the stator voltage, frequency, frequency control of motor speed has no commutator motor speed regulation and so on. From energy point of view, when there is high speed control method and low speed control method of two kinds: an induction refers to the effective speed control is constant, so there is no slip loss, such as multi-speed motor, frequency control of motor speed and slip loss can be recovered speed control methods (such as cascade speed regulation, etc.). Have slip loss speed regulation method belongs to the low speed, such as rotor series resistance speed control method, the energy loss in the rotor circuit; Electromagnetic clutch speed regulation method of energy loss in the clutch coil; Hydraulic coupler speed regulation and energy loss in the oil of hydraulic coupling. In general slip loss increases with expanding range of speed regulation, if the speed range is not big, energy loss is small. This control method is used to change the stator winding of red way to change the cage ac motor stator polar logarithmic to achieve speed, features below: With a hard mechanical properties, good stability; No slip loss, high efficiency; Wiring is simple, convenient control, low price; Have used differential is bigger, cannot obtain smooth speed regulation; Can be used with regulating speed control, electromagnetic slip clutch, smooth speed characteristics of high efficiency. This method is suitable for don't need a stepless speed regulation small electric motors of production machinery, such as metal cutting machine tools, elevators, lifting equipment, fan, water pump, etc. Cascade speed regulation refers to the motor rotor winding circuit in series with adjustable additional potential to change the slip of the motor, to Motor vehicle achieve the purpose of speed control. Most of the slip power is absorbed by series of additional potential to recycle produce additional device, the absorption of returning the slip power grid or conversion of energy use. According to the slip power utilization way, cascade speed regulation can be divided into Y series three-phase asynchronous motor cascade speed regulation, mechanical cascade speed regulation and thyristor cascade speed regulation form, adopts thyristor cascade speed regulation, its characteristic is: Slip loss in the process of speed regulation can be back to the grid or production machinery, high efficiency; Installed capacity is proportional to the speed range, the investment province, apply to speed range in production machinery from 70% to 90% of the rated speed; Speed regulating device failure can switch to run at full speed, avoid production; Thyristor cascade speed regulating power factor is low, the influence on harmonic. This method is suitable for the fan, water pump and the rolling mill, mine hoist, the extrusion machine use. Frequency control method Frequency control of motor speed is to change the frequency of the motor stator power, to change its synchronous speed control method. Variable frequency speed regulation system is the main equipment to provide frequency conversion power of inverter, the inverter can be divided into ac - dc - ac frequency converter and ac - ac frequency converter two kinds big, cross - straight - to pay inverter are mostly used in China at present. Its features: There is no additional loss of high efficiency and speed the process; Application range is wide, can be used in the cage type asynchronous motor; Large range of speed regulation, and the characteristics of hard and high precision; Technology is complex, high cost, maintenance difficulties. This method is suitable for the requirement of high precision, better speed control performance.