An improved AC drive using a third harmonic flux estimator

A constant flux controller, which uses zero sequence third harmonic currents as a flux estimator to control a conventional cage induction motor (CIM) using a pulse width modulated (PWM) AC inverter, is described. The third harmonic current is extracted in real time using an online discrete Fourier analysis method. A simple PI control strategy is then implemented within the microprocessor based controller which, under the most favourable conditions, enables complete recovery, to within 1-2% of the rated flux, for any load change within the rating of the machine, in less than 80 ms. At lower speeds, however, longer response times are required to maintain stability, and to improve further the performance of the controller at these low frequencies, two simple decoupling techniques are employed. When both techniques are introduced the measured recovery time of a machine rated at 50 Nm and 50 Hz, but operating under worst case conditions at 15 Hz, to various sudden 20 Nm load changes, is found to decrease from typically 200 ms to less than 130 ms while maintaining flux accuracy to better than 2%