Studies on Surface Defects of Enamel Coated Magnet Wires

Abstract

Enamel coated magnet wire is commonly used for electric motors of automotive engine, e.g. for fuel injector and pump. Polyesterimide (PEI) type of enamel coating can be damaged and brought about the motor's performance drop, in case of using volatile organic solvent (methanol, ethanol) in biofuel applied engine. In this work, The process condition of manufacturing for fuel injector and pump, e.g. temperature and duration, is investigated to know the effect on the coated surfaces. Furthermore diverse volatile organic solvents, e.g. ethanol, isopropyl alcohol (IPA), acetone, are tested about those affecting on enameled surfaces. In addition, various factors are varified in terms of electric properties of magnet wires. Typical PEI [tris (2-hydroxyethyl) isocyanurate (THEIC)-modified PEI, according to IEC (International Electrotechnical Commission) 60317–13] has the temperature index of 180-200°C (IEC 60172). To find out the temperature and cooling time effect of PEI during injection molding process, the temperature (280/320/350°C) and cooling time (4/240 s) were varied. At 320°C with cooling time of 240 s, the defects, e.g. voids and pin holes, of PEI coating were accelerated, compared with current condition (280°C, 4 s cooling). As further investigation step, the wound wires were immersed into organic solvents (ethanol, IPA and acetone). Subsequently those surface damages were observed by three-dimensional (3D) confocal microscopy and optical microscopy, and measured those electric resistances as well. Multiple crack lines, in form of surface crazing, were occurred with reaction of IPA, acetone; especially the most severe was acetone. Moreover pre-extended wires were damaged at ethanol, as well. To enhance the chemical resistance of PEI coating, polyamideimide (PAI) is applied and then shows promised results, e.g. thermal and chemical properties and durability. This PAI can be expected to apply in biofuel used engine components and next generation fuel injector and pump.