Many motors driving various equipment operate a constant speed. To control the amount of work the motor is providing at any given time, devices are used to reduce the machine's output through artificial loading. Examples include oulet valves or dampers, which restrict output to decrease the flow rate to a desired level for the system, or a bypass loop which simply diverts the fluid. In some cases, the machine runs at a higher speed than is necessary to do the job. All these attempts esentially waste energy.
Variable Frequency drives or VFDs can reduce output by controlling the motor rather than having the motor work at a constant, almost full load and adjusting the system to reach the desired result.
Variable speed drives are especially effective at reducing power and energy consumption to centifugal equipment such as pumps and fans. This is because a reduction in flow is directly proportional to a reduction in speed, while the reduction in power is proportional to the cube of the change in speed.
For centrifugal loads, small decreases in equipment rotating speed or fluid flow yield significant reductions in energy use. For example, reducing the speed or flow by 20% can reduce power requirements by approximately 50%.