IEC 60370: Insulating Varnish Thermal Endurance — Temperature Index Determination for Motor Impregnating Varnish

How Motor Impregnating Varnish Thermal Class Is Actually Determined — Using Real Coil Specimens

IEC 60370:2017 specifies insulating varnish thermal endurance test methods. Unlike generic material thermal ageing tests, varnish endurance must be evaluated under conditions simulating real application — enamelled wire twisted pairs, impregnated and cured, then aged at three temperatures while measuring breakdown voltage over time.

The varnish works in a composite insulation system with the wire enamel and insulating paper/film. Its thermal endurance is influenced by substrate catalytic effects, degree of cure, and mechanical stress — none of which are captured by testing the varnish film alone. IEC 60370 uses twisted-pair coils — impregnated, cured, then aged — exactly matching motor winding manufacturing processes.

TNLab — Impregnating varnish thermal endurance must be evaluated in a “real motor winding composite insulation system,” not on the varnish film alone.