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The winding insulation of low-voltage induction motors in an adjustable-speed drive system with IGBT pulse width modulated (PWM) inverters is substantially stressed due to the uneven voltage distribution and excessive voltage stress (dv/dt), which result in the premature insulation breakdown. In this paper, the detailed insulation test results of 48 low-voltage induction motors are presented. Different types of insulation techniques are applied to 48 motors. The insulation characteristics are analyzed with partial discharge, discharge inception voltage, and dissipation factor tests. Also, breakdown tests by high voltage pulses are performed.