Magnetic Effects of DC Signal Injection on Induction Motors for Thermal Evaluation of Stator Windings

Active thermal protection techniques have been proposed for the thermal protection of induction motors. By injecting dc signals, the dc model of the 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 unbalanced magnetic saturation, which may degrade a motor's performance and may 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 an induction motor during 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. Such analysis is illustrated from both the finite-element simulation 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. In addition, it is shown that the injected dc signals have a little impact on the thermal performance of induction motors, which proves that the dc signal injection can be widely used for thermal protection without causing much thermal stress in induction motors.