Thermal and slip effects on rotor time constant in vector controlled induction motor drives

In this paper, the fast variation of rotor resistance due to winding temperature is shown. Thus, the rotor time constant in the vector controlled induction motor drives, in contrary to common belief, changes fast via temperature. Moreover, it depends on the motor slip. The slip dependency of rotor time constant is due to motor loss that is usually ignored. The iron and stray loss is introduced in the induction machine dynamic and static models and a new expression of the rotor time constant is derived, that contains the motor slip. Thus, the rotor time constant rapidly varies at load torque changes. A novel slip frequency calculation procedure is proposed that ensures the accurate and fast estimation of the valid machine rotor time constant. The above aspects have been verified by extensive simulation and experimental tests in a wide speed-torque range.