Universal field oriented controller based on air gap flux sensing via third harmonic stator voltage

A universal field oriented (UFO) controller operating in the air gap flux reference frame is applied to a current regulated pulse width modulated (PWM) induction motor drive. The air gap flux is measured by the stator third harmonic (TH) voltage component. It is shown that, due to motor saturation, a TH flux component is generated in the air gap. Since the TH flux component is a function of the fundamental air gap flux and is always in phase with it for any load condition, it constitutes an accurate way of measuring the fundamental air gap flux. This harmonic flux component induces a TH in each one of the motor phase voltages, and it is shown how this voltage is processed for the measurement of the air gap flux and consequent implementation of the UFO controller. Due to the robustness of the TH sensing method and to the structure of the UFO controller, the resulting drive is not affected by detuning errors in steady-state operation. In addition, the UFO drive has excellent dynamic performance, even under strongly detuned conditions