Design of fractional-order PI controllers and comparative analysis of these controllers with linearized, nonlinear integer-order and nonlinear fractional-order representations of PMSM

Abstract

Mathematical model of permanent magnet synchronous motor (PMSM), which is also popularly known as sinusoidal BLDC motor, is highly nonlinear. Speed control of this motor is a cascade control structure which has two distinct loops: an inner current control loop and outer speed control loop. In this paper, we present the design of fractional proportional-integral (PI) controller using small-signal model of PMSM. The basic scheme used for speed control is field-oriented control wherein constant torque angle control is obtained by zero direct-axis current. This requires three PI controllers: one for direct-axis current to keep at zero; another for quadrature axis current to maintain at maximum; and the third for the speed to keep at the reference level. In this paper, we have replaced all three PI controllers with fractional-order (FO) PI controllers, where order of integrator is fractional. These three FOPI controllers are tested on integer-order nonlinear model as well as fractional-order nonlinear model of PMSM using MATLAB simulink and FOMCON toolbox. The comparative analysis shows superior performance of the FOPI controllers for FO PMSM.