Comparison of Carbonised Foam to Other Testing Media for Insulation Verification of Hairpin Windings

Insulation testing is a vital part of electric motor manufacturing, because insulation failure can cause short circuits, overheating and premature motor failure. Traditional motors use thin round enamelled wire with multiple strands to form the coils, however a newer type of motor, the hairpin motor uses rectangular cross section conductors coated in enamel. This has the advantage of greater conductor area within a slot, partly due to the rectangular packaging offering greater packing density, and partly due to lower enamel area within the slot. However, the disadvantage of the hairpin motor design is that the wire must be formed to fit into the slot. Insulation can be deformed or broken as a result of this forming. Testing for insulation failures is usually performed by immersing the hairpin in conductive metal shot which can erode enamel, is hard to automate and has a limited contact area. This paper presents a comparison of alternative grounding media for hairpin insulation testing, including graphite powder, copper tape, carbonised foam and mild steel. Carbonised foam as a grounding medium is shown to facilitate a low cost, non abrasive, residue free, high speed method of testing insulation parameters.