A Variable Speed Drive (drive) is an electronic device that controls the characteristics of a motor's electrical supply allowing the speed and torque of the motor to be matched with the requirements of the machine it is driving.
Varying the speed of a motor with a drive can reduce energy use substantially. Of all the applications for variable speed control, centrifugal fans and pumps offer the greatest potential for energy savings. In addition to saving money, drives can also improve process control, whilst reducing waste, maintenance costs and carbon emissions.
Source: Control Techniques News