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Iron loss identification and detuning evaluation in rotor flux oriented induction machines

Iron loss, as a source of detuned operation of rotor flux oriented induction machines, has attracted much attention. Appropriate mathematical expressions, that enable evaluation of detuning, have become available and performance deterioration due to iron loss has been evaluated for rated speed operation in the constant flux region. The available studies are based on the measurement of iron loss with sinusoidal supply of rated frequency. This paper at first describes an experimental procedure for determination of iron loss and hence an equivalent iron loss resistance over the frequency range of interest. The procedure is based on no-load tests with PWM inverter supply and it can be executed during commissioning of the drive. The second part of the paper evaluates detuning introduced by iron loss in a rotor flux oriented induction machine, utilising the experimentally determined values of equivalent iron loss resistance. The emphasis is placed on operation in the braking region and on operation in the field weakening region. It is shown that detuning characteristics in the braking region differ from those valid in the motoring region. Detuning in the field-weakening region is found to significantly exceed the one at rated flux, rated speed operation, especially if the machine runs at high speeds with light loads. Finally, a modified rotor flux estimator, that enables compensation of the iron loss, is introduced and verified by simulation.