Which of the following is used to alter the speed of a 3-phase squirrel cage induction motor?

The use of frequency to alter the speed of a 3-phase squirrel cage induction motor is vital because the speed of such motors is directly related to the frequency of the power supply. The synchronous speed of the motor can be calculated using the formula:

Synchronous Speed (N) = (120 × Frequency) / Number of Poles

This relationship indicates that by changing the frequency of the supply voltage, one can effectively change the synchronous speed at which the motor operates. For instance, increasing the frequency will increase the motor speed, while decreasing it will lower the speed. This principle is commonly utilized in variable frequency drives (VFDs), which allow for precise control over the motor speed by manipulating the frequency supplied to the motor.

In contrast, altering the voltage will not significantly change the speed of a squirrel cage induction motor, as these motors primarily rely on frequency for speed regulation. While changing current can affect torque and performance, it does not provide a reliable method for adjusting speed. Lastly, adding resistance would typically be associated with starting or improving dynamic performance, but again, it does not directly affect the rated speed of the motor. Hence, frequency is the most appropriate choice for altering the speed of a 3-phase squirrel cage induction motor.