Energy Efficiency of small Induction Machines: Comparison between Motor and Generator Mode

The market of small or micro electric energy generation units, often combined with local heat production, is becoming increasingly important. For these applications the classical asynchronous machine with cage rotor is an interesting generator. The grid-connected induction generator has some advantages over the synchronous generators: e.g. it is cheap, robust and maintenance-free. Since the induction machine is mostly used as a motor, the manufacturers’ catalogues only mention the motor efficiency values. The paper uncovers why nevertheless induction machines can be used either in motor or generator mode, the efficiency curves, based on IEEE 112-B, for these modes are not necessarily the same. We conclude that the machines with lower stator and rotor resistances and a nonsaturated core, typical characteristics for high efficiency or machines with a high power rating, will have comparable efficiencies in both generator and motor mode. This is not the case for low power rated, low efficiency machines, where the efficiency for generator mode can drop several percent.