Speed Control of Three Phase Induction Motor (1)
Variable slip speed control
1. Changing the stator voltage
The torque of an induction motor is proportional to the square of the stator voltage. That is, the change of the stator voltage can change the mechanical characteristic and torque of the motor.
This method is not suitable for the ordinary squirrel-cage motor, because its rotor resistance is very small and the current will rise rapidly at the low speed.
But it can be used for the wound-type induction motor by series resistance or frequent varistor in the rotor circuit to reduce the heat of the motor.
2. Changing the rotor resistance
This method of speed regulation is only applicable to the winding motor. In the rotor circuit of induction motor in series with resistance, when the load is fixed, the greater the resistance, the lower the speed of the motor. The smaller the resistance, the higher the speed.
This method is simple, easy to control, and has low initial investment. But the slip power is consumed on the resistance by heating. It also has soft mechanical characteristics.
3. Cascade speed control
At present, the cascade speed control takes use of SCR inverter cascade control circuit, and has the following advantages: get stronger mechanical characteristic, low rectifier voltage drop, small space, no rotating part, low noise, simple maintenance. It is one of the speed control methods of a wound-type motor.
It also has its own disadvantage. That is, the rotor circuit is equipped with a reactor for the filter, so the power factor is low.