Three-phase asynchronous motor speed formula: n = 60f / p (1-s) 

    From the formula, we can see that change in supply frequency f, the number of motor pole pairs p and slip s can be changed to achieve the purpose of speed. Judging from the nature of speed, the difference about speed control is nothing more than two ways.To change the way the exchange of synchronous motor synchronous speed or not to change the speed. 

    First，the number of pole-changing speed of the method 

    This speed control method is used to change the connection mode of the stator windings in order to reached on the purpose which is change the cage motor stator pole speed. The features is following: 

    With hard mechanical properties, good stability; 

    No wear and tear and deterioration of quality, high efficiency; 

    Wiring is simple, convenient control and low prices     

There are class speed, a larger differential, speed can not be smooth; can be used with speed regulator, electromagnetic clutch deteriorates, the smooth efficiency of access to higher speed characteristics. 

    This method is suitable for stepless speed regulation which does not require the production of machinery, such as metal-cutting machine tools, lifts, lifting equipment, fans, pumps and so on.

   Second，Frequency Control Method 

   Frequency Control is to change the frequency of power of the motor stator, thereby to change its method of synchronous speed of the governor. Frequency Control System is to provide the main equipment of the inverter power inverter, inverter can be divided into AC - DC - AC converter and exchange - the exchange of two major categories of frequency converter, using the most current pay - Direct - cycloconverter. Its characteristics: 

    High efficiency, not speed the process of additional wear and tear; 

    Wide range of applications, can be used for cage induction motor; 

    Speed range, features a hard, high precision; 

    Technical complexity, high cost, difficult maintenance overhaul. 

    This method is suitable for applications requiring high accuracy and good speed performance occasions.

    Third, Cascade Control Method 

    Cascade Control is the rotor winding loop of wound motor of string attached to the adjustable potential to change the motor to the poor, to achieve the purpose of speed. And deterioration of quality of power by the majority of string attached to the absorption potential, and then the absorbing power of the poor to return to power grids or conversion to the use of energy by the additional device . According to poor power absorption in accordance with the use of power, speed cascade cascade can be divided into motor speed control, mechanical and thyristor cascade speed regulation governing the form of the use of thyristor cascade speed regulation, which is characterized as follows:

    Speed can be in the process of wear and tear and deterioration of quality of feedback to the grid or production machinery, more efficient; 

    The scope of device capacity and speed in direct proportion ,save investment, applied to the production machinery which rated speed range of 70% -90% speed; 

    Speed control device failure can be switched to run at full speed to avoid the cut-off; 

    Cascade Speed Control Thyristor low power factor, harmonic influence. 

    This method is suitable for fans, pumps and steel rolling machines, mine hoist, the use of extruder. 

    Fourth, Rotor string resistor speed control method for wound motor speed resistor string method  

    Wound rotor induction motor attached to the resistor string so that the motor slip up, at a lower speed motor running. The greater the resistance to the string, the lower the motor speed. This method is simple, convenient control, but to poor power consumption in the form of heat in the resistor. Belong to step speed regulation, mechanical properties of soft.

Seven speed control means for three-phase asynchronous motor (The Second) 

The fifth, Method of stator speed regulator 

    When changing the voltage voltage of motor's stator can get a different set of mechanical characteristic curve to obtain different speed. As the motor torque and voltage proportional to the square, so the biggest drop in a lot of torque, the smaller the scope of its governor, It is difficult to apply on the cage motor. In order to expand the scope of regulation, Voltage control and speed contron should be use the cage motors which rotor resistance value is large, such as the regulator for speed control of the torque motor used, or the winding-type motor resistor in series-min frequency. In order to expand the scope of stable operation, when the governor of the occasion in more than 2:1 feedback control should be adopted in order to achieve the purpose of automatic adjustment of speed.

    The main speed regulator is a device to provide power supply voltage changes, At present, the way common regulator series saturable reactor, autotransformer, etc., as well as several thyristor regulator. Thyristor voltage regulator is the best. Speed regulator features: 

    Speed regulator circuit is simple and easy to achieve automatic control; 

    The process of power transfer regulator poor to heat the rotor resistance in the form of consumption, and less efficient.

    Speed regulator 100KW below are generally applicable to the production machinery.

    6, electromagnetic motor speed control method 

    By the electromagnetic motor cage motor, electromagnetic clutch switch and DC excitation power (controller) is composed of three parts. DC Power Excitation smaller, usually from single-phase half-wave or full-wave thyristor rectifier component to change the thyristor conduction angle, you can change the size of excitation current.

    Electromagnetic clutch switch from the armature, pole and field winding is composed of three parts. Armature and there is no mechanical link which can rotate free. Coaxial rotor and motor armature connected to the initiative as part driven by the motor; magnetic coupling with dock and load axis as the driven part. When the armature and magnetic pole are static, such as a DC excitation winding pass, then along the circumferential surface of air-gap will be formed by a number of N, S alternating polarity of the magnetic pole, the magnetic flux through the armature. When the motor armature rotates with the drag, as the armature and the relative motion between the magnetic pole, thus resulting armature vortex sensor, the eddy current interaction with the magnetic torque, the rotor poles are driven by rotating the same direction, but its Constant speed is lower than the speed of the armature N1, which is a speed difference to the way of change and deterioration of quality of the DC excitation current clutch, the clutch will be able to change the output torque and speed. Speed of electromagnetic motor characteristics:

     Device structure and the control circuit is simple, reliable operation, easy maintenance; 

     Smooth speed control, stepless speed regulation; 

     Harmonic impact on power system; 

     Rate of loss of large, low efficiency. 

     This method is suitable for medium and small power requirements of a smooth move, short-term low-speed operation of production machinery.

     7, hydraulic speed control method of coupling  

     Hydraulic coupler is a hydraulic transmission device, generally composed of pump and turbine wheel, they work collectively, round, shell sealed. Shell in charge a certain amount of work into the liquid, when the pump wheel, driven by prime mover in the spins, in which the liquid driven by the rotating blades, under the centrifugal force along the outer ring into the turbine wheel pump when turned on in the same to thrust to the turbine blades to drive the functioning of production machinery. Coupler hydraulic power transfer capability and a relatively fluid-filled shell the size of the volume is consistent. In the course of their work, the rate of change in fluid-filled coupler can be changed on the turbine speed, stepless speed regulation to do, which is characterized as follows:  

     Power to adapt to a wide range to meet from a few dozen to several thousand kW kW different power needs; 

     Simple structure, reliable operation, use and maintenance of convenience and low cost; 

     Small size, can allow large; 

     Control regulation of the easy realization of automatic control. 

     This method is suitable for fans, pumps Speed Control