Flux-Decay Test: A Viable Solution to Evaluate the Induction Motor Rotor Time-Constant

This article deals with the determination of the rotor time-constant of an induction machine by performing a flux-decay test in generic load conditions. The flux-decay transient occurs when the supply stator voltage is turned off suddenly. In this way, from the decay envelope of stator back-electromotive forces and, in the case of the wound rotor machine, from that of the rotor currents, the rotor time-constant can be extrapolated with a high level of accuracy. In this article, an in-depth analysis of this methodology is reported, whose validation has been carried out on both the squirrel cage and wound rotor motors. The estimated value of rotor time-constant can be used for the machine torque regulation, using the well-known control schemes presented in the literature and whose performance in the low-speed operation is affected by the detuning of this parameter. Since the inverter-fed motors nowadays characterize most of the share in the market of adjustable speed drives, experimental results concerning the flux decay test performed under the pulsewidth modulation supply are also presented.