LOSSES IN INDUCTION MACHINES
Losses in induction machines occur in windings, magnetic cores, besides mechanical friction and windage losses. They determine the efficiency of energy conversion in the machine and the cooling system that is required to keep the temperatures under control. In the design stages, it is natural to try to calculate the various types of losses as precisely as possible. After the machine is manufactured, the losses have to be determined by tests. Loss segregation has become a standard method to determine the various components of losses, because such an approach does not require shaft-loading the machine. Consequently, the labour and energy costs for testings are low. On the other hand, when prototyping or for more demanding applications, it is required to validate the design calculations and the loss segregation method. The input-output method has become standard for the scope. It is argued that, for high efficiency machines, measuring of the input and output Pin, Pout to determine losses Σp on-load outin PPp −=Σ (11.1)
requires high precision measurements. This is true as for a 90% efficiency machine a 1% error in Pin and Pout leads to a 10% error in the losses. However, by now, less than (0.1 to 0.2)% error in power measurements is available so this objection has been reduced considerably. On the other hand, shaft-loading the IM requires a dynamometer, takes time, and energy. Still, as soon as 1912  it was noticed that there was notable difference between the total losses determined from the loss segregation method (no-load + short – circuit tests) and from direct load tests. This difference is called “stray load losses”. The dispute on the origin of “stray load losses” and how to measure them correctly is still on today after so numerous attempts made so far [2 − 8]. To reconcile such differences, standards have been proposed. Even today, only in USA (IEEE Standard 112B) the combined loss segregation and input-output tests are used to calculate a posteriori for each motor type the “stray load losses” and thus guarantee the efficiency. In many other standards the “stray load losses” are assigned 0.5% or 1% of rated power despite the fact that all measurements done suggest much higher values. The use of static power converters to feed IMs for variable speed drives complicates the situation even more, as the voltage time harmonics are producing additional winding and core losses in the IM. Faced with such a situation we decided to retain only the components of losses which proved notable (greater than (3 to 5%) of total losses) and explore their computation one by one by analytical methods. Further on numerical, finite element, loss calculation results are given.