The speed of an asynchronous motor depends on the mains frequency; at 50 Hz it turns at a fixed speed. The inverter first rectifies the mains voltage and then regenerates it at the desired frequency. When the frequency falls the motor slows down, when it rises it speeds up. In refrigeration this means matching the compressor’s capacity to the load: instead of a compressor that cycles on and off, a compressor that turns as much as is needed and keeps the suction pressure constant. The room temperature stays within a narrower band, the starting current falls and the contactor and motor suffer less wear.
The typical operating range is 25–70 Hz in reciprocating compressors and, depending on the model, 20–75 Hz in screw and scroll compressors; capacity varies roughly in direct proportion to frequency. In central systems an inverter is usually fitted to only one compressor; the other compressors come in as steps and the one with the inverter fills the gap in between. Because the suction pressure rises and the condensing pressure falls at part load, annual consumption falls by 15–30% in most plants. With fans and pumps the gain is even greater, because power varies with the cube of the speed.
Not every compressor can be run with an inverter. The frequency range approved by the manufacturer is observed: at very low speed the oil pump does not deliver enough and the gas velocity in the pipes cannot carry the oil back; at very high speed the motor current and valve load increase. It is common for the pipework and frame to go into resonance and vibrate at certain frequencies; these frequencies are defined in the drive as skip bands. In systems that run at low speed for long periods the drive raises the speed at set intervals to gather the oil back to the compressor.
On the electrical side a screened cable is run between the inverter and the motor, the screen is earthed at both ends and the cable is kept as short as possible; an interference filter is fitted on the mains side. The inverter produces heat (about 3% of its power); the panel is ventilated and protected from dust. A contactor fitted at the inverter output is not opened under load. The residual current device must be of a type suitable for a circuit with an inverter. The motor nameplate values are entered in full in the drive parameters; the acceleration and deceleration times are set according to the limits given by the compressor manufacturer.