An analysis of magnetic saturation in induction motors during dc signal injection

Active thermal protection techniques have been proposed for the thermal protection of induction motors. By injecting dc signals, the dc model of induction motors can be used to estimate the stator winding resistance, and thus the average stator winding temperature. The injected dc signals may lead to magnetic saturation, which may degrade a motor´s performance and increase heat dissipation in the motor. This paper presents a detailed analysis of the magnetic saturation caused by dc signal injection with its effects discussed. It is shown that the injected dc signal induces a varying magnetic saturation in each cycle. The effects of the magnetic saturation on the motor currents, the motor´s thermal behavior and the estimation of the stator winding resistance and temperature are analyzed. A mathematical model of induction motor during dc signal injection is proposed with considerations of magnetic saturation. Such analysis is illustrated from finite-element simulation results and experimental results. It is shown that the magnetic saturation does not affect the performance of the stator resistance and temperature estimation, which proves the feasibility of these active thermal protection techniques under different load conditions.