Analyzing and Compensating for Rotor Time Constant Variation in Induction Machines at Flux-Weakening Operations

Vector control methods have made accurate and fast control of induction machines possible. The success of indirect rotor-flux-oriented-control (RFOC) depends greatly on the accuracy of the machine parameters, especially the rotor time constant. In this paper, rotor resistance data is extracted from standard current-slip tests and analyzed. The data show that the rotor resistance appears to change with slip frequencies. The variation can be attributed to skin effects in the rotor bars and cross-coupling effect between d-q inductances. The torque error when entering flux-weakening caused by neglecting the variance of rotor resistance is analyzed. An induction machine model with compensated rotor resistance is proposed and proved to improve the torque linearity, particularly during flux-weakening operations.