Final answer:
Variable frequency drives reduce the speed of AC motors by varying the frequency of the supplied power, allowing for precise speed control and energy savings.
Step-by-step explanation:
Variable frequency drives (VFD) reduce the speed of AC motors by varying the frequency of the supplied power. By adjusting the frequency, VFDs can control the speed of the motor without changing the motor's voltage or altering its physical characteristics, such as the number of poles. It does not involve any mechanical changes like lubricating the bearings. The frequency supplied to an AC motor determines the rotation speed, according to the relationship between the number of poles the motor has and the frequency of the electrical current. A VFD modifies the frequency based on the desired speed, providing precise speed control and potential energy savings.