Author:

Abstract:

The market of small or micro electric energy generation units, often combined with local heat production, is becoming increasingly important. This growth is driven by several factors such as the increasing fossil fuel prices and several European legislative initiatives, such as certificates and feed-in tariffs, stimulating consumers to look for alternative heat and power production and to use the available fossil fuels in a more efficient way. This explains the increased interest in small-scale alternative and renewable energy generation technologies, such as CHP (combined heat and power) production, small wind or hydro turbines, etc. 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. It does not need synchronizing equipment and when short-circuited, the machine delivers little or no sustained power. No power electronic interface between the generator and the grid is required when speed can be kept almost fixed: rotor speed changes are absorbed by a shift in the slip. Drawbacks are the limited allowed speed range of the prime mover and the amount of reactive power they draw from the grid in order to be magnetized. Since the induction machine mostly is used as a motor, the manufacturers' catalogues only mention the motor efficiency values. Whereas for the designer of small-scale energy production units it is important to have correct information in order to be able to compare the different solutions properly. In order to determine the efficiency, different standards are used. An induction motor testing program conducted over the past years in the research group, to which the authors are associated, revealed that depending on the standard used, the same machine's measured efficiency can differ by more than one 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. The paper uncovers that nevertheless induction machines can be used either in motor or generator mode, the efficiency curves for these modes are not necessarily the same. A high- and a low-efficiency induction machine of similar power rating were compared for motor and generator mode. We concluded that the machines with lower stator and rotor resistances 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. This is caused by the stator voltage drop that decreases the airgap voltage in motor mode whereas it increases the airgap voltage in generator mode. This causes higher magnetizing current and increased core losses. When using catalogues for induction motors, efficiency values differ according to the standards the manufacturers use. One should also consider that nevertheless machines can be used either in motor or generator mode, the efficiency curves for these modes are not necessarily the same and also depend on the operating conditions.