A Method for Testing Enameled Copper Wire for Surface Quality and Electrical Integrity

Author: Brian J Alexander
Lear Technical Consultant, Europe

This proved to be an intriguing problem since there is no definition of what constitutes a bead or blister although the latter is more obvious. The way in which the industry determined the surface quality was simply by touch followed by a visual examination on the spool. Everyone familiar with magnet wire plants will be used to seeing plant personnel holding the wire between forefinger and thumb usually just before it enters the spool. Such decisions are judgmental and a small bead on a fine wire running at say 250 meters per minute will have a totally different feel from the same size bead on a larger diameter wire running slower at 60 meters per minute. There is no quantitative definition of a bead. Lear had to find some way of producing data that would be meaningful to magnet wire producers and users that would correspond with the information they deduced from their own thumb and forefinger test. As line speeds have increased, the wire itself is now very hot and it is often physically impossible to apply the touch test.

A typical magnet wire plant will have at least 150 lines running and Lear quickly realized that since all lines have to be continuously monitored, the process, on a line-by-line basis, must not be too expensive. Magnet wire plants are not the friendliest environments and Lear also realized that the equipment must be robust and virtually maintenance free. This rules out optical non-contact systems when initial capital and maintenance costs are considered.

Before discussing the solution, perhaps we should consider the causes of surface defects that usually occur alone but can coincide with electrical breakdown failure. The most obvious cause is the surface quality and shape of the input copper. If this is rough or not perfectly circular in cross section, then the enamel will not be concentric and where the enamel is thickest, the solvents have difficulty escaping during the baking process and produce blisters in the enamel film. At the same time the corresponding thinner enamel film may be deficient in electrical properties.

The majority of enameled wire is now produced by drawing the basic 8mm rod on a conventional rod breakdown machine and annealing it in line on a contact sheave annealer. Close attention has to be paid to the process since the final output wire can be damaged by physical abuse from worn drawing dies and capstans and electrical erosion in the annealer. Usual drawing speeds are 25 meters per second. This output wire, typically 2.0mm, is then used as the feed wire to the enameling machines where it is drawn to the required size by a small in-line drawing machine of say 8 to 10 dies running at the much slower enameling speeds. Any faults on the surface of this wire are not removed by this secondary drawing but are simply spread out along the surface. Any copper spikes that occur will protrude through all or most of the enamel film and manifest themselves as electrical faults. They may or may not cause a bead or blister.

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