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AC Current Controller with Error-Free Feedback Acquisition System

In this paper, we introduce an improved ac current controller with robustness against the noise in the feedback path. This three-phase current controller is suited for inverter supplied ac machines and grid-connected power converters. Conventional solutions make use of symmetric pulse-width modulation (PWM) techniques with feedback sampling in the middle of the voltage pulses. Single-sample-based feedback acquisition gets affected by the noise and parasitic phenomena. We perform a thorough analytical and experimental study on feedback errors with conventional sampling, and we consider the impact of the lockout time, motor cables, winding capacitance, and anti-aliasing filters. In order to suppress a significant spectral content in the area of low-order harmonics, we apply an improved acquisition technique, which uses a period-average and removes any PWM noise from the feedback signals. Both anti-aliasing filter and proposed period-average filter introduce delays, which impair the control-loop performance. The conventional current controller is then extended to suppress the effects of the delays. Parameter setting procedure is devised to achieve both the bandwidth and the robustness against the parameter changes. Analytical and experimental studies prove that the proposed feedback acquisition technique improves the robustness in the presence of variable delays and switching transients. Experimental tests show that the extended current controller with period-average feedback acquisition reaches the same bandwidth and robustness as the state-of-the-art controller.