The universal field oriented (UFO) controller applied to wide speed range induction motor drives

The principle of the universal field-oriented (UFO) controller operating in an arbitrary reference frame is applied to a PWM inverter feeding a standard induction motor. An alternative synchronous current regulator is proposed that provides quick current response during transient torque conditions, that has the capability to transition to six-step operation, and that is characterized by a low harmonic current content during steady-state operation. The fundamentals of this current regulator are based on the space vector theory. In the linear pulsewidth modulation (PWM) region, the voltage reference is obtained through current feedback regulation in the synchronous reference frame. To attain full DC bus utilization at high motor speed (flux weakening), it is necessary to switch the regulator from linear PWM regulation to square-wave (six-step) operation. The transition between PWM and square-wave operation has to be controllable and should not cause undesirable currents and torque transients. To avoid these transients a feedforward voltage strategy is applied during this transition. Complete digital simulation results are carried out