When the stator voltage of the motor is changed, a set of different mechanical characteristic curves can be obtained, and different rotating speed can be obtained. Because the torque of the motor is proportional to the square of the voltage, the maximum torque decreases a lot, and its speed range is small, which makes it difficult to apply the general cage motor.

In order to expand the range of speed regulation, cage motor with large rotor resistance should be used, such as torque motor specially used for voltage regulation and speed regulation, or series frequency sensitive resistance on wound motor. In order to expand the stable operation range, feedback control should be adopted when the speed regulation is above 2:1 to achieve the purpose of automatic speed regulation. The main device of voltage regulation and speed regulation is a power supply which can provide voltage change. At present, there are several commonly used voltage regulation methods, such as series saturation reactor, autotransformer and thyristor voltage regulation. The thyristor voltage regulation mode is the best. Voltage regulation and speed regulation is generally suitable for production machinery below 100KW.

The characteristics of voltage regulation and speed regulation are as follows. 

1, The voltage regulation and speed regulation circuit is simple and easy to realize automatic control. 

2, In the process of voltage regulation, the slip power is consumed in the form of heating in the rotor resistance, and the efficiency is low.