Inverter (variable frequency drive)
Also known as: frequency inverter, VFD, variable speed drive
Short definitionAn electronic drive that changes the frequency of the current fed to a motor and so matches the speed of a compressor, fan or pump to the load. Capacity is adjusted steplessly and the electricity bill falls.
An induction motor connected straight to the mains turns at a single speed on 50 Hz. The drive rectifies the voltage and then rebuilds it at the required frequency; as frequency falls, so does speed. For a compressor this means a machine that turns according to the load instead of cycling on and off: suction pressure stays steady, room temperature holds a narrow band and starting current is lower. The usual range for reciprocating compressors is 25–70 Hz.
In quotes for multi-compressor central systems, a drive is usually fitted to one compressor and the others cut in as stages. The frequency range allowed by the compressor manufacturer is not exceeded; at very low speed the oil does not return. Frequencies that cause vibration are skipped in the drive. The motor cable is screened and short. The drive rejects about 3% of its power as heat, so the panel is ventilated. A contactor on the output is not opened under load.