Operating restrictions for third harmonic control of flux in induction machines

Flux control in AC cage induction motor drives using third harmonic voltages and currents in star and delta connected machines, respectively, has been proposed and developed by a number of authors. To date, measured third harmonic components have enabled an accurate estimation of the airgap flux magnitude and a good estimate for its phase angle. This paper investigates the basis and the operating range of control techniques utilising these components, and describes steady state and dynamic characteristics for constant flux controllers based on observations of the triplen currents circulating in the delta winding of a machine. It is shown that the principal assumption inherent in the method, namely that there is a direct relationship between the airgap flux and the measured third harmonic component is not always valid so that the general applicability of the technique is restricted. Experimental results are presented for both steady state and transient conditions to determine the useful operating range that may be expected of drives under this strategy.