Influence of Delays on Digitally Controlled IM Drive with Inverter Output LC Filter

This paper presents the influence of delays on closed-loop control of digitally-controlled induction motor (IM) systems with an inverter output LC filter. The delays are inevitable in a digitally controlled system; it influences the closed-loop performance, frequency-response, and, consequently, the closed-loop system stability. Therefore, the delay (sampling and processing delays) is modeled and incorporated into the open-loop IM system while designing the closed-loop controllers to address the forenamed issues. Furthermore, when an LC filter is connected at inverter output terminals to convert the pulse width modulated voltages into sinusoidal voltages, it further increases the delay in the closed loop; and eventually turns the closed-loop system more prone to attain instability. These delays and impending stability issues are thoroughly analyzed here through the frequency-domain indices (Gain-Margin, Phase-Margin) for the IM system with an LC filter. The presented analysis is validated through simulations and experiments on a 5 Hp IM system.