Induction machine efficiency in motor and generator mode

Since induction machines are mostly used as a motors, the manufacturers’ catalogues only mention motor efficiency values, but no values for generator mode. The paper uncovers that if induction machines are used in motor or generator mode, efficiency curves for these modes are not necessarily the same. Several high- and low-efficiency induction machines with different power ratings are compared for motor and generator mode. These experiments show that depending on the machine’s design, and thus also efficiency class, the efficiency for motor and generator mode can differ significantly. Special attention is paid to differences between copper and aluminium rotors. In the paper, underlying phenomena causing this effect are elucidated. It can be concluded that machines with lower stator and a non-saturated 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. Whether for an induction motor or a generator, an efficiency increase of just one percent can represent a considerable economic benefit over the whole life-time.