The Universal Field Oriented (UFO) controller operating in the air gap flux reference frame is applied to a current regulated PWM induction motor drive. The air gap flux is measured by the stator third harmonic voltage component. It is shown in the paper that due to motor saturation, a third harmonic flux component is generated in the air gap. Since the third harmonic 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 to measure the fundamental air gap flux. This harmonic flux component induces a third harmonic in each one of the motor phase voltages, and the paper shows 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 third harmonic 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. To avoid any transition between the Indirect Universal Field Orientation (IUFO) (suitable controller structure for low speed) and the Direct Universal Field Orientation (DUFO) (suitable controller structure for high speed), a new type of UFO, i.e. a combination of the two methods is presented.
