Incorporating variable frequency drives into applications such as fans, pumps, and cooling towers can reduce energy use up to 50 percent at partial loads by matching motor speed to the changing load and system requirements.Electric motors' driving equipment, such as pumps and fans, normally operate at a constant speed. Some form of mechanical throttling - a valve on the outlet, in the case of a pump, or the slats in a louver, in the case of a fan - controlled the water or airflow speed and volume.Using these flow-control methods, the motor continues to operate at full speed and uses electric energy at the full-load rate, even though it was performing less useful work. In the process, it wasted a great deal of energy.Variable frequency drives have introduced a more efficient way to provide load control when the load varies, which is most of the time when moving fluids.
The two most commonly used methods to vary speed electrically, rather than mechanically, depend on whether the VFD is AC or DC. If it is AC, which encompasses most motors, varying the frequency of the electrical energy supplied to the motor controls the speed. Examples include AC motors' driving fans, pumps, and compressors.
If the motor is DC - still a large portion of equipment - VFD motors - a typical solution is to apply an AC power source to a silicon-controlled rectifier (SCR) motor control, converting the AC to rectified DC. DC power, speed, torque and horsepower then vary to meet the demand.
Technicians can convert AC induction motors to variable-speed applications by inserting a VFD controller - basically, a power supply and a computer - into the circuit, either as the original equipment specification or as an upgrade, matching the controller and motor with demand variations required by the application.