An improved observer-based current controller for inverter fed AC machines with single DC-link current measurement

In modern industrial applications even low cost drives have excellent dynamic behaviour which is achieved by field oriented control combined with a high dynamic current control. Usually at least two phase current sensors are necessary for such an operation. In this paper, a new current control scheme is presented, which enables 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 realisations 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. Measurement results performed on an induction machine drive at different points of operation show the applicability of the current controller and furthermore enables a grading of different structures.