Limitations of the PI Control with Respect to Parameter Variation in PMSM Motor Drive Systems

Most of the industries today employ the PI based control algorithms for their drive systems. These methods although successful in majority cases tend to suffer with limitations under certain operational conditions of the machine. The parameter uncertainties of the machine due to part to part variation can also limit the performance of the PI controller. The PI control is capable of correcting various uncertainties to maintain the reference current until its output is saturated. After saturation, the PI is not able to maintain current and the effect of this on the torque production needs to be clearly understood for the optimal control of the drive system. The effect of integral windup limit in such cases also needs to be understood. This paper discusses the effect of each parameter variation with respect to the PI output and the torque production. The discussion includes the PI behavior for the entire operating conditions that can expose the limitation of the PI control. The effective PI controllable region of the torque speed curve of the machine can be much lower than the nominal torque speed curve for which the machine was designed.