Rotor-flux-oriented control of induction machines considering the stray load losses

Detuned operation of rotor-flux-oriented (11170) induction machines results due to motor parameter variation effects and due to the phenomena that are not modelled and are therefore unaccounted for in the controller. The first category encompasses variations of rotor and stator resistance, mutual inductance and leakage inductances, while the second category includes stator iron (core) losses. All of these sources of detuned operation have been studied in a considerable depth in the past and the mechanisms by which they affect accuracy of RFO control are nowadays well understood. It appears that the only potential source of detuned operation, which has never been studied before, is the stray load loss, which belongs to the category of unmodelled phenomena. This paper therefore develops an analytical model that characterises detuning due to stray load losses in indirect RFO induction motor drives, by means of the orientation angle error, actual to reference rotor flux ratio, and actual to reference torque ratio. In order to evaluate the stray load loss induced detuning, stray load losses are at first experimentally determined using the procedure outlined in IEEE 112-B standard. The parameters of a motor equivalent circuit that accounts for the stray load losses are further fitted to the measurement data. Motor parameters obtained in this way are used to assess quantitatively the impact of stray load losses on accuracy of field orientation. It is shown that, although stray load losses are comparable to the iron losses in the studied machine (i.e., of approximately the Same value in the rated operating point), their impact on RFO control is much smaller when compared to the iron loss induced detuning.