Motor repair shop testing-how we stress insulation

Insulated motor windings arrive at motor shops in various states of cleanliness, age, and damage. The authors describe the typical tests and procedures applied in EASA (Electrical Apparatus Service Association) shops to check the old insulation system, and the new winding insulation if required. The results of a survey taken in the Southeastern Chapter of the EASA are used to illustrate the range of capabilities and procedures. Repair tests on a typical three-phase, AC induction motor are described. Many comments also apply to wound rotor and DC motors. Of the shops surveyed, 63% had the capability of fully testing 4-kV insulation. All (98%) could test 600-V insulation. It is noted that AC ground testing is potentially hazardous to windings if it is applied by touching the leads and drawing an arc. Similarly, faulty contacts in motor starting can cause high voltage transients. The magnitude of the winding-to-ground capacitance is an important factor in insulation stress during motor starting