Wideband impedance measurements of DC motors under dynamic load conditions

One of the principal conducted EMI (electromagnetic interferences) sources of low voltage DC (direct current) motors is the commutation occurring during rotation. In this paper the small-signal impedance of low voltage DC motors under different functioning modes, including the dynamic one is studied over a large frequency range [0 Hz-200 MHz]. Different test benches inspired from military and European standards (MIL-STD) have been set up. The measurements were performed on the motor's armature, on the DC motor under static mode and during idle rotation. The setups provided repeatable and precise impedance results. They also highlight the impact of the stator and suppression filters in the low frequency range. The role of the parasitic inductances and capacitances in the high frequency range is also pointed out. The results were compared with those of other measurement techniques.