The first results of investigations having the purpose to evaluate how Sweep Frequency Response Analysis (SFRA) and Frequency Dielectric Spectroscopy (FDS) can reflect the changes of insulation system of MV induction motors, are discussed in this paper. These methods, together with dissipation factor and partial discharge (PD) measurements, were applied to evaluate complete machines having moisture contamination, internal delaminations, end-winding displacements and other defect typologies. The results are compared with those obtained testing twin machines in good conditions, used as a reference. It is shown that a more complete diagnostic information can be obtained integrating the test results of PD, SFRA and FDS test methods
