Raleigh, NC--A milestone in the high-voltage performance of thermoplastic-encapsulated coil devices. That's how ABB Electric Metering and Control describes the new coils used in its single-phase and polyphase electromechanical watt-hour meters for residential and commercial applications. The new version replaces an older design overmolded with thermoset epoxy.
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ABB puts the new coils through 8 kV tests twice, using a specific wave form that simulates a powerful lightning surge. Not only do the coils pass, but they achieve this high-voltage performance at a cost savings. ABB engineers attribute this to the use of Zytel(reg) nylon resin supplied by DuPont Engineering Polymers (Wilmington, DE), astute component design, and a shift to an automated manufacturing and assembly operation.
"We took advantage of the toughness, strength, and high-productivity molding characteristics of Zytel nylon in designing the coil bobbin with automated winding, termination, and encapsulation in mind," explains ABB Project Engineer Russell Broome. The grade used: Zytel 70G33HS1l, a heat-stabilized nylon 66 resin with 33% glass-fiber reinforcement.
When the coils are wound, the winding machine ties off lead wires to temporary posts molded as part of the bobbin's terminal area. The posts shear off during automated insertion of insulation displacement (ID) terminals into pockets provided in the bobbin for just that purpose.
The grade of nylon used for encapsulation is the same as that of the bobbin. On the recommendation of DuPont engineers, the ABB team designed the bobbin with a groove on each flange in the area where the encapsulation material must seal to it. That area has a groove with a reverse taper. During encapsulation, material flows into the reverse-tapered groove, causing the thinnest tapered areas of the bobbin to soften and bond with the encapsulation material. This arrangement provides more output with less labor than the thermoset molding process used in the past, according to Broome. In addition, the change from thermoset molding powder to pelletized nylon eliminates airborne dust, while the hot-runner mold does away with runner scrap.
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