A variable frequency drive (VFD) is a device that controls the speed of an electric motor, so that the motor uses only the power the job needs. It is one of the largest energy savers in a factory or a building.
An ordinary motor connected straight to the mains runs at one speed, whatever the job needs. If a pump needs only half the flow, the usual solution is to run the motor at full speed and close a valve. That wastes power. A VFD changes the frequency of the electricity it gives the motor, and the motor speed follows. So the motor turns slower when less is needed.
The saving is large because of a rule of physics for pumps and fans. The power needed falls with the cube of the speed. At 80% of the speed, a pump needs only about 51% of the power, since 0.8 × 0.8 × 0.8 is 0.512.
The rule has a limit. It holds well for fans and for pumps that only push the liquid along a pipe. It holds less well for a pump that must lift water to a great height, because that lifting needs a fixed amount of power, however slowly the pump runs.
For instance, a 15 kW pump motor supplies water to a large building in Kochi. The building needs full flow only in the morning. For most of the day, the pump runs at 80% speed. It then draws about 7.7 kW, not 15 kW, and saves about 7.3 kW. Over 16 hours, that is around 117 units of electricity a day.
A VFD also protects the motor and the machine. A motor connected straight to the mains draws several times its normal current for a moment when it starts, and it gives the machine a hard jerk. A VFD raises the speed gradually from zero, which is called a soft start. This reduces the stress on belts, gearboxes and the electricity supply, and it is the second reason that plants fit a VFD, after the saving on electricity bills.
The PLC sends the VFD the speed it should hold. The VFD sets the speed of motors that turn pumps, compressors and the fans of HVAC systems. It is a lasting product, as factory equipment is rarely replaced.