Variable Frequency Drives (VFDs) as the name suggest vary the frequency of the waveform supplied to the motor to control the amount of power that is used by the motor for its requirement.  The electronic modulation of the frequency allows efficient usage of power consumed by reducing initial inrush current and electronic speed control of the motor.

You can save a significant amount in energy costs when replacing gear boxes or air dampers with a Variable Frequency Drive to control the product flow.