Impulse Strength of High Voltage Motor Coil Turn Insulation

Breakdown of the turn insulation in large ac motors eventually leads to the failure of the stator winding. The turn insulation can fail if the motor coils are subjected to steep-fronted surges. The impulse strength of the turn insulation was determined for fifty-three typical coils procured from different manufacturers and tested singly in air. The coils had six different insulation systems and their specifications (voltage, power, size, turns, strands etc) cover a very wide range. Most coils were found to have an impulse strength exceeding 5 times peak line-to-ground voltage (5 pu).