The steep fronted pulses at high switching frequencies occurring in adjustable speed drives (ASD) of new generation may produce premature failures of the winding insulation of induction motors, as those adopted for traction applications. Life curves of such systems are typically not available for waveforms generated by the modern inverters characterised by steep fronts and high harmonic content. Indeed, these data are of great importance for an accurate and reliable design of the insulation in new generation ASD. For this reason an experimental investigation has been carried out on several insulation systems. The effects of the applied voltage wave-shapes parameters, such as the peak value, the slew rate (dV/dt) and the duty cycle, are analysed for different insulation thickness and impregnation resins adopted