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Reliability of inverter-fed motors depends upon partial discharge (PD) behavior of stator windings. A new measuring system of PD inception voltage (PDIV) was developed with repetitive impulses which simulate turn-to-turn voltage of the motors. Using the system, PDIV was measured repeatedly on twisted-pairs of magnet wires. With bipolar repetitive impulses PDIV fluctuated with large scatterings in early stage and converged to stable values, while such tendency was not observed with unipolar repetitive impulses. Pre-stress condition for stable PDIV measurement was discussed from a practical point of view.