Iron loss in rotor-flux-oriented induction machines: identification, assessment of detuning, and compensation

Iron loss, traditionally ignored in vector control schemes, has recently attracted more attention as a cause of detuned operation of rotor-flux-oriented induction machines. Appropriate mathematical tools, that enable evaluation of detuning due to iron loss, have become available, and these have been used so far only in assessment of detuning for rated speed operation in the constant flux region. The available studies are based on the measurement of iron losses with voltage supply of rated frequency. This paper attempts to provide a more detailed treatment of iron loss induced detuning in rotor-flux-oriented induction machines by presenting at first an experimental method of iron loss identification over the entire frequency (speed) range of interest. The experimental results enable calculation of the equivalent iron loss resistance that is subsequently used in evaluation of detuning. The regimes dealt with encompass motoring and braking operation in the base speed range and motoring in the field-weakening region up to the five times rated speed. It is shown that detuning in the base speed range will be the highest at rated speed operation and will exhibit opposite trends in motoring and braking regions. Detuning in the field-weakening region is found to be significantly in excess of the one at rated speed, provided that the machine operates at high speeds with relatively light loads. As compensation of iron loss seems to be necessary in this case, the concluding part of the paper presents a novel rotor flux estimator that utilizes experimentally identified equivalent iron loss resistance values and enables elimination of detuning that is otherwise present. The estimator is a modified version of the well-known scheme that operates on the basis of measurement of stator currents and rotor speed (position). Its ability to compensate for iron loss is verified by simulation.