The application of inverter-fed drives provides additional electrical stresses in induction motors' insulation system. This is due to steep-fronted voltage pulses responsible for high overvoltages at the motor's input terminals and to an uneven voltage distribution along the windings. This article presents a model for the analysis and characterization of the internal voltage distribution and complex impedance in random wound coils. Experimental and theoretical results are presented using frequency response analysis (FRA) and model effectiveness using different approximations is studied. Finally, the frequency domain characterization will allow a time domain analysis, by means of inverse fast Fourier transform (iFFT).