Abstract The replacement of environmentally unfriendly refrigerants and the energy saving demand have recently caused changes of the components and operation of the vapour compression plants; in particular, the compressors have been experiencing upgrades and modifications. The compression systems are usually designed for working under maximum load conditions, but most of the time these plants work under partial load conditions with compressor on–off cycles regulated by a thermostat. As for the variable speed compressor, the speed is continuously controlled to match the compressor capacity to the load required; this allows to save energy when compared to the thermostatic control. The aim of this paper is to identify the compressor current frequency that optimizes the energy, exergy and economy aspects. The determination of the optimum frequency for each working condition is key to build a control algorithm that allows the compressor speed to be continuously regulated by an inverter. This analysis has been applied to the reciprocating and scroll compressors and high energy savings have been achieved.