Induction Motor Rotor Time-Constant Determination using Flux-Decay Test

This paper deals with the determination of the rotor time-constant of an induction machine by performing a flux-decay test in no-load conditions. The flux-decay transient happens when the supply stator voltage is turned off suddenly, allowing the computation of the rotor time-constant at the rated supply conditions. From the envelope of stator back-electromotive force decay and, in the case of the wound rotor machine, from the envelope of the rotor current decay, the rotor time-constant can be obtained accurately. In this work, the proposed method is deeply analyzed, and experimental measurements are reported and discussed. Besides, the phenomena that can influence the values of the rotor time-constant are described. The results of the proposed methodology can be used in the design of the machine control scheme, where the accuracy into knowing the rotor time-constant value strongly influences the performance of the torque control in the low-speed range of the drive.