On Control of Back-to-Back Converters and Sensorless Induction Machine Drives

Doktorsavhandling, 2003

This thesis focuses on design and analysis of the control system structure for back-to-back converter squirrel-cage induction machine drives. Particularly, sensorless control of induction machines, meaning vector control without a mechanical shaft sensor, and vector control of pulsewidth-modulated (PWM) rectifiers are considered. The back electromotive force is used as the basis for sensorless control in this thesis. A variant of the classical "voltage model" is adopted for sensorless flux estimation. It is shown that the estimator must be redesigned for the purpose of arbitrarily placement of the closed-loop poles. A thorough stability analysis of the redesigned estimator shows that asymptotical stability can be guaranteed at nominal speeds. The stability at very low frequencies is, however, largely affected by the knowledge of the stator resistance. The presence of a singularity for zero stator frequency is found, which makes it impossible to guarantee stable operation at very low frequencies, except for the case of zero external load torque. The underlying mechanisms behind the two widely acknowledged instability phenomena for sensorless control at low frequencies are revealed. The most critical form of instability is the infamous flux collapse: the flux collapses to approximately zero, giving nearly total loss of torque, and uncontrolled rotation in the direction of the external load torque. The less well-known instability phenomenon frequency lockup is not as critical: the field orientation deteriorates, such that the torque reduces but not vanish, and the stator frequency and rotor speed lock on to constant values close to zero. A control system structure is developed for the PWM rectifier. The previously proposed concept of virtual flux is adopted for grid-voltage synchronization, and three different synchronization algorithms are analyzed. The PWM rectifier is also considered for an active filtering application, for which a vector current control system designed for the deadbeat response is designed. An analysis shows that the resulting deadbeat control system is equivalent to previously proposed Smith predictor structures.