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The winding insulation of low-voltage induction motors in adjustable-speed drive systems with voltage-fed inverters are substantially stressed by the uneven voltage distribution and excessive voltage stress (dv/dt), which result in the premature insulation breakdown. This paper describes test results of the insulation characteristics of a 5 HP, 380 V induction motors for inverter applications. Eight different types of insulation techniques are applied to 40 induction motors respectively. Insulation characteristics are compared with respect to partial discharge, capacitance, current, and dissipation factor tests. Also, the insulation breakdown tests by high voltage pulses are performed and the corresponding breakdown voltages are obtained.