Stator Voltage Control in Induction Motors

Stator Voltage Control in Induction Motors

Induction motors are a common choice for industrial, appliance, and heavy industry applications due to their low cost, reliability, and efficiency. However, these motors are typically used for constant speed applications, making them unsuitable for variable speed applications without the use of AC power controllers. One method of speed control in three-phase induction motors is stator voltage control, which allows for the adjustment of motor speed by varying the voltage applied to the stator windings.

Theoretical Background

Stator voltage control is based on the fact that the torque in an induction motor is directly proportional to the square of the applied voltage. By reducing the applied voltage, the torque and thus the rotational speed of the motor decreases. Conversely, increasing the applied voltage increases the motor's speed. This relationship allows for the control of motor speed by adjusting the stator voltage.

The torque equation for a three-phase induction motor is given by:

T = (Vs / π) * √(3) * (1 - sin²(πft))

where T is the torque, Vs is the stator voltage, f is the frequency, and t is the time. From this equation, it can be seen that the torque is proportional to the square of the stator voltage.

Practical Implementation

To control the stator voltage in a three-phase induction motor, a 3-phase AC voltage controller is typically employed. This controller regulates the firing angle of the thyristors connected in anti-parallel in each phase, allowing for the adjustment of the rms value of the stator voltage. By controlling the firing angle, the motor torque and speed can be regulated accordingly.

Fig. 1 shows a 3-phase voltage controller feeding a 3-phase induction motor. The firing angle of the thyristors is controlled to regulate the rms value of the stator voltage.

Advantages and Limitations

Stator voltage control is suitable for speed control in a reasonably wide range, especially for applications where the intermittent operation of the drive is required. It is also suitable for fan and pump drives, as the load torque varies as the square of the speed. However, this method only provides speed control below the normal rated speed, and operation at voltages higher than the rated voltage is not admissible.

Conclusion

Stator voltage control is an effective method for achieving speed control in three-phase induction motors. By adjusting the stator voltage, the torque and thus the speed of the motor can be regulated, allowing for the control of motor speed in a wide range of applications. This method is particularly useful for fan and pump drives, where the load torque varies as the square of the speed.