A Novel Leakage-Current Monitoring Method for Inverter-Fed AC Motors

The insulation capacitance is an evaluation criterion for insulation aging, which is calculated using the common mode voltage and common mode leakage current. Compared to the phase current, the common mode leakage current is negligible, resulting in inaccurate measurements. Moreover, the nonuniqueness of the flow path makes it impossible to measure leakage current directly. In this article, we present a novel method, based on Kirchhoff's current law, to indirectly measure the common mode leakage current. Furthermore, this method utilizes the spatial symmetry of the coaxial conductor to eliminate the influence of the phase current on the leakage current measurement. The relation among the dc bus voltage, modulation ratio, and common mode voltage is theoretically derived to reduce the voltage probe. The result verifies that the proposed method can monitor the variation in the common mode leakage current, and the analytical common mode voltage parallels the measured value. Consequently, the equivalent capacitance, which represents the condition of the motor insulation can be accurately estimated. The maximum estimated error of the capacitance is less than 5%.