To change the speed of an induction motor, you must either change the frequency of the power supply or change the ____________.

Changing the number of poles in an induction motor is a fundamental method for altering its speed. The synchronous speed of an induction motor is determined by the formula: Speed (RPM) = (120 × Frequency) / Number of Poles. By adjusting the number of poles, you can effectively change the motor’s synchronous speed for a given frequency.

For instance, if the frequency of the power supply remains constant and the number of poles is increased, the motor will operate at a lower speed. Conversely, reducing the number of poles allows the motor to achieve higher speeds. This principle is central to motor design and performance in relation to desired operational speeds.

While changing the voltage, current, or resistance can impact motor performance in other ways—such as efficiency and torque—these adjustments do not inherently change the motor's synchronous speed like altering the number of poles does. Therefore, to specifically address changes in speed based on operational design and requirements, modifying the number of poles is the correct approach.