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

Author: Brian J Alexander
Lear Technical Consultant, Europe

Larger sizes of wire are not drawn in-line on the enameling machines. In these cases the wire comes directly from the rod breakdown machine and is annealed immediately before the enameling machine. The softened wire is quenched in water to prevent oxidation and this annealer water must be kept clean to prevent deposits forming on the wire which will prevent smooth application of enamel causing beads on the wire and premature ageing of the enamel covering.

Once these basic faults with copper have been overcome, problems arise in the enameling process. The application of enamel is a multicoating process in which as many as ten coats of enamel can be applied which can often be seven coats of one enamel with three coats of a different topcoat. For self bond wires, even a third enamel can be necessary. Each of these coats is applied to a specific thickness by means of a solid die that floats on the enamel column and only allows the correct amount of enamel on to the wire at each pass. Should just one of these dies become displaced, then an uneven enamel build will result and more often than not a string of beads or blisters will appear. Remember there can be up to 1500 dies in use at any one time in the plant and it is difficult to ensure that all are operating successfully.

The different types of wet enamel are not compatible and small spillages into the applicators will result in poor coating and beads and blisters. Sometimes wires break in the enameling oven because the tensile strength of the wire at these elevated temperatures is approached especially if there is some obstruction with the feed wire. A broken wire will cause the burnt residues of enamel to enter the oven air circulating system together with copper oxide produced on uncoated wire in the oven. Such debris can again affect the application dies of adjacent wires with disastrous consequences. In some countries, flying insects are a problem and occasionally these have been known to be the cause of surface defects.

Those who have seen enameling machines will remember just how many pulleys or sheaves are involved - twenty or more for each wire line. The bearings on these pulleys fail from time to time causing vibration in the wire line that again leads to an irregular surface.

It is clear that there are many causes of beads, blister and electrical failures. If known by the operator, some of these should be well within his control but some can be transient in nature and difficult to control. It is these latter ones with which we are mainly concerned. A string of beads in the middle of a large spool at 4.00 AM is likely to go undetected until some unsuspecting customer finds them in his winding shop and temporarily closes down his production. If repeated, future orders for wire from that supplier aren't going to happen.. Equally, a long string of small beads will have a deleterious effect on the space factor on critically wound coils.

Lear set out to replicate the “touch” test upon which the industry relies, using proprietary techniques invented by Lear.

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