Limitations in Inner-loop PI Controller for Induction Motor Drive with an LC Filter & Its Remedies

An LC filter placed in-between a voltage source inverter (VSI) and an induction motor (IM) introduces resonance frequency oscillations in filtered voltage and, at times, leads to close-loop instability. To mitigate this drawback, the PI controller for d- q axes current in vector-control (VC) need to be redesigned by considering LC filter dynamics along with the IM dynamics. However, the PI controller being a first-order controller, does not guarantee sufficient stability margins and performance criteria for an IM + LC filter system. This paper illustrates this limitation by deriving the complete range of stabilizing controller sets (K
_p
, K
_i
) for the IM + LC filter system and then determines the stabilizing region satisfying the desired performance criteria (Phase Margin, Gain Margin). Therefore, to achieve the controller with better stability margins and satisfying desired performance criteria a second-order controller may be required. A method to design a second-order controller is proposed in this paper, and the closed-loop stability has been verified. The system performance for the newly designed controller has been discussed, and results are presented to verify the same.