Thermal Analysis of a Directly Grid-Fed Induction Machine with Floating Neutral Point, Operating under Unbalanced Voltage Conditions

Significant changes may occur in the thermal behaviour of a directly grid-fed induction machine when subjected to unbalances in the voltage supply. This paper studies and analyses the thermal behaviour of a low power, three-phase, squirrel-cage induction motor, connected in star configuration with floating neutral point, when subjected to different levels of unbalanced voltage. The dependence of the thermal motor behaviour on the severity level of the unbalance is studied and analysed. In addition to amplitude unbalances, this paper focuses on the effects of phase unbalances, as well, which were not addressed in detail in previous published studies. Moreover, situations of mixed unbalance, where amplitude and phase unbalances occur simultaneously, are also studied. The Finite Element Method (FEM) was used to simulate the thermal behaviour of the machine. The experimental setup consists of a three-phase programmable AC power supply, suitable to precisely emulate unbalance conditions that may occur in real-scale power systems, supplying a 2.2 kW induction motor. Experimental data were acquired resorting to Resistance Temperature Detectors (RTDs) PT100, placed in the machine phase whose supply current value changed the most. Finally, the simulation results are verified and critically discussed through experimentally obtained results.