How to Control Speed of Induction Motor

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Speed control methods of Induction Motor

The basic operating principle of the induction motor is: By supplying the three-phase balanced voltage source to the three-phase stator windings of the induction motor, there is the flow of three-phase current across the stator windings. As a result, the produced three-phase current magnetizes the stator core. There are various techniques by which we can control the speed of Induction motor.
Generally, the speed of the induction motor is controlled by changing or alternating the frequency of applied voltage. The previous frequency applied to the induction motor should be changed with the new frequency in order to change the speed of the induction motor.

To alter or change the frequency of the applied voltage, we should use variable frequency inverter. The variable frequency inverter is also called a variable frequency drive which controls the speed of the motor and adjusts numerous frequencies applied to the induction motor.

Ways to control the speed of the induction motor

1. One of the ways to control the speed of the induction motor is by changing the applied voltage to the motor. The recent voltage of the motor should be replaced by the new one in order to control the speed of the induction motor. We should apply the desired and balanced voltage to the motor in order to achieve the desired speed of the induction motor.

2. Another way or technique to control the speed of the induction motor is of course by changing the applied frequency. As I already mentioned that the variable frequency inverter or variable frequency drive is the best solution for this purpose. It controls speed as well as torque and direction of the induction motor.

3. One of the important techniques to control the speed of the induction motor is by changing the number of stator poles. In the induction motor, the main function of stator poles is to induce a desired magnetic field in the motor. By inducing a magnetic field in the induction motor, the torque is produced. Therefore, the available number of stator poles should be changed to control the speed of the induction motor.

4. The basic methods used to control the speed of induction motor are rotor rheostat method and cascade connection method. Both the methods are equally important in controlling the speed of the induction motor.

Speed control of DC shunt motor

To control the speed of DC shunt motor, generally, there are two important techniques in use. They are termed as flux control method and armature control method. In the flux control method, the controller resistance is reduced so that the full rated field current flows in the motor. As a result, the DC shunt motor rotates with rated normal speed. In this way, we can control the speed of the motor by the flux control method.

The armature control method is quite different than the flux control method. In this method, the control resistance is not reduced but is increased keeping the torque of load constant. As a result, the current flowing through the armature remains constant. Also, the potential difference across the armature of the DC shunt motor decreases and we can control or decrease the speed of DC shunt motor by decreasing the speed of the armature.

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