Current-controller with single DC link current measurement for inverter-fed AC machines based on an improved observer-structure

In modern industrial applications even low-cost drives have excellent dynamic behavior, which is achieved by field-oriented control combined with high-dynamic current regulation. Usually at least two phase-current sensors are necessary to realize such an operation. In this paper, a new current-control scheme is presented, which enables high-dynamic control of ac machine-line currents with improved accuracy by measuring only the current of the dc-link. The controller is based on three individual and adaptive phase-current observers, which guarantee very accurate phase-current estimates even at low-modulation indices. Thus, operation is possible even at low-speed without modifying the switching pattern of the controller and the necessary modification at zero-speed can be clearly reduced. Different realizations of the proposed scheme are given and a comparison is made to previously published schemes which are also based on a single dc-link current sensor. Measurements performed on an induction-machine drive at different points of operation show the applicability of the proposed structure. It is shown that the long-term accuracy is clearly increased and the maximum deviation of the observer estimate from the actual phase-current is reduced when using the proposed scheme.