Efforts to improve capacity and efficiency present new design challenges. As older generators and motors are upgraded, the required higher electrical stresses increase the possibility of slot discharges. Evidence of these slot discharges typically shows up as deterioration in the semiconducting coatings, both in the slot itself and in the voltage gradient area. Also, increases in capacity require better heat transfer between the coil surfaces and the laminated core.
In response to these design challenges, NATIONAL ELECTRIC COIL has developed NECCOPAK-Z, a flexible coil side packing system that reduces the effects of slot discharge and enhances heat transfer to the core. Integral with the coil, NECCOPAK-Z eliminates the need for additional side packing materials. The impetus for design of the NECCOPAK-Z came through the insight, which NEC gained through numerous upgrades, repairs and rewinds for motors and generators of various types and brands.
Over the last 15 years, generator and motor ratings for units in their same frame sizes have increased tremendously. Electrical stresses in insulation have increased to 60 to 75 volts per mil with localized temperatures exceeding 155° C. Larger wire sizes or higher rated insulation, or both, must be accommodated in the original slots. Optimal rewind design, therefore, may demand a decrease of 10 to 30 percent in the thickness of ground wall insulation. The challenge for NATIONAL ELECTRIC COIL's engineering department: Improve unit performance and longevity — get more current through the coils and achieve better heat transfer between the coils and core lamination — without sacrificing the life of the associated corona suppression system.
NECCOPAK-Z, NEC's new zero-clearance side packing system is the answer. Made of flexible, conductive silicon rubber with an integral, semiconducting, protective exterior surface, NECCOPAK-Z closes air gaps at contact surfaces, and it withstands the higher temperatures at contact points. NECCOPAK-Z's elastic nature insures the maintenance of tight slots, despite the expansions and contractions of coils during operation. Tests have shown that coils installed with NECCOPAK-Z have an improved conductive contact to ground, greater heat dissipation and reduced corona.
National Electric Coil, 800 King Avenue P.O. Box 370, Columbus, OH 43216. Tel: 614-488-1151; Fax: 614-488-8892.