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A Method for Testing Enameled Copper Wire for Surface Quality and Electrical Integrity Author: Brian J Alexander |
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Enameled copper wire has been produced for the past one hundred years. It has been known as winding wire in Europe and magnet wire in the U.S.A, descriptive terms since the purpose of the wound wire is to produce a magnetic field. Earlier winding wires were wound with silk or cotton, which progressed to similar constructions impregnated with varnishes based on natural resins and oils. As the varnishes improved, the need for silk and cotton was reduced and further developments have led to the synthetic coatings we have today which combine toughness, flexibility, and electrical insulation at elevated temperatures up to 200 C and even higher. The process for producing the enameled wires is essentially the same as it was one hundred hears ago. The drawn copper wire is pulled through a trough of enamel and the surplus wiped off before passing through an oven that firstly evaporates the solvents and then cures the resinous film. This wire passes again through the same enamel and a second coat is applied. This continues until the required build is obtained which often necessitates up to ten coats. The enameled wire is then coated with a small quantity of oil or wax lubricant needed to aid windability and then wound on to a spool via a capstan that is the pulling force drawing the wire through the enameling machine. Although the process has changed little there have been many refinements with the chemical composition of the enamel coatings and with the quality of the copper. There have been similar advances with the method of application of the enamel to the wire. Up to diameters around 0.15mm wire, the ovens are horizontal and the enamel is applied with felt wipes to which controlled amounts of enamel are fed from metering pumps. Between 0.15 and 0.30mm the application is still horizontal but dies are used to control the enamel build. Above that size, usually to diameters of about 5.0mm, the ovens are vertical and the enamel build is controlled using similar accurate dies that have a few microns clearance. The earlier ovens were simply chimneys with the organic solvents and decomposition products being vented to the atmosphere. Modern vertical ovens have complex catalytic burning units, which convert the organic waste products to heat that in turn heats the oven. The exhaust gases are now odorless and environmentally acceptable. The ovens are longer, up to ten meters with a further eight meters cooling zone and run at higher temperatures, say 800 C. Since the enamel curing reaction is a function of time and temperature, the running speed of modern ovens is much greater that the old type convection ovens. An increase of speed by a factor of ten is not uncommon. Just as line speeds and quality have improved, so have the demands for larger and larger packages. It is now common for magnet wire producers to supply 1.00mm enameled wire on spools that are almost one meter in height and hold up to 850 kilos weight of wire or 122 kilometers length with a filling time of fifteen hours or so…(Fig 1)
The increases in enameling speeds have been accompanied by ever increasing demands from the users who have themselves developed faster and more demanding coil winding equipment. Irregular surface faults in the form of beads or blisters on the wire may have been accommodated on older winders but this is no longer the case. In the early 1980's Lear Engineering developed a hand held diameter measuring device at the request of a magnet wire company in the United States (the Lear Gun). Through this association Lear was asked in the early 1990's if they could develop a procedure for monitoring the surface of magnet wire during manufacture, which would detect surface irregularities.
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