The reliability of medium voltage (MV) motors used for driving pumps, fans, and other loads in the power generation industry is critical for continued power production. Periodic testing of stator winding insulation, which is the most vulnerable component in MV motors, is an essential part of predictive maintenance for preventing unplanned reduction in the power generation capability. Korea Electric Power Corporation (KEPCO) Research Institute's 17+ years of experience on predictive maintenance of stator insulation for MV motors is presented in this paper. KEPCO's stator insulation test program and statistics of test records are summarized, and cases of recent turn and phase insulation failures that resulted in motor forced outages are investigated. Analysis of the turn or phase insulation failures and insulation test records performed on the failed motors show that it is difficult to predict turn or phase insulation failures with the insulation tests currently performed. Turn or phase insulation test methods are summarized and evaluated based on the findings of the investigation to help target future research efforts towards industrial needs.