Estimation of the rotor time constant of an induction machine at constant speed

Indirect field oriented control of an induction machine requires knowledge of the rotor time constant to estimate the rotor flux linkages. Here an online method is presented for estimating the rotor time constant and the stator resistance both of which vary during operation of the machine due to ohmic heating. With the additional assumption of collecting the data while the motor runs at constant speed under load, two different methods are presented to estimate the rotor time constant and stator resistance. The first method formulates the problem using a nonlinear least-squares criterion and shows that the parameters that minimize the least-squares error can be found in a finite number of steps. The resulting algorithm requires significantly less computation than a previously reported algorithm by the authors developed under varying-speed conditions. The second method shows the parameters can also be found using a linear least-squares criterion provided that the rotor flux magnitude is varied with the speed held constant.