Improved constant phase fractional order approximation method for induction motor FOPI speed controller

This paper presents the digital realization of the fractional-order PI (FOPI) controller fractional integrator element using the improved constant phase approximation method. A new pole-zero rationalization approach is proposed within the bandwidth of the Bode phase plot. The voltage source inverter fed induction motor fractional-order model has been identified. A novel mathematical FOPI controller design procedure for the identified FO plant model has been proposed. The designed FOPI controller fractional integrator element is implemented using the constant phase approximation method. A detailed comparative performance study of the proposed approximation method with the existing Charef, Oustaloup, and refined Oustaloup methods has been carried out using the Bode, Nyquist, and Nichols plots. The proposed FOPI controller is placed in the speed feedback loop of the rotor field-oriented control algorithm of IM. To confirm the proposed FOPI controller approximation method's robustness, a hardware study is carried out on the laboratory hardware-in-loop (HIL) platform. In addition, variation in the number of pole-zero pair's effect on the speed tracking performance, robustness against parameter and load variations are also analyzed.