Effects of flux level on a CSI-fed field-oriented induction motor

A definite relation exists between the flux level, torque and slip speed of a vector controlled induction motor. An untuned vector controller generates an inappropriate slip frequency that changes the operating flux of the machine. This affects the electromagnetic torque linearity and dynamic performance of the drive system. For an induction motor with magnetic saturation, the ratio of flux and torque-producing components of stator current differs from unity for the maximum electromagnetic torque. With a linear model this ratio is unity. The dependence of this ratio for maximum torque output is explained for linear and nonlinear models of the machine. The torque characteristic is analysed with three aspects of magnetic state: true saturation curve, hard-limit saturation curve, and constant inductance model. Degradation in dynamic performance due to saturation for a CSI-fed system is presented.