Rotor Winding Distortion and Turn Displacement
Problem: Severe rotor winding distortion and turn displacement can cause shorted turns or electrical grounds. Distortion can result from poor design of the end-turn blocking supports or by lack of an adequate slip plane between the top copper turns and the retaining ring, causing the top turns to elongate as shown in photo at top left.
Prevention: Proper design of the rotor coils and the blocking supporting the coils under axial loads is essential. In some instances, if only the top turn is distorted, it can be placed back into its proper position. Rotors should be tested for turn-to-turn shorts with a flux probe at operating speed. Retaining-ring ground insulation should include a Teflon® surface, so that the coils can expand and contract with temperature.