The Real World of Magnet Wire Tests

A look into the history of developing devices to detect bead and HVC faults in magnet wire.

Detect Insulation Faults

An often heard complaint was that some HVC test equipment was damaging the wire by exploding a pinhole into a crater. If true, this would certainly relate to the magnitude of current the test equipment was dumping into a fault. The NEMA standard does specifically define a current limit requirement which we found to be mostly ignored by some suppliers of HVC test equipment.

Current limit will make a world of difference in the number of faults found depending on the "rapid recovery" interval and in the likelihood of damaging the wire. Should strict compliance with a current limit be an important design parameter? It does add a lot of complication to the design if done sharply and without one last shot of stored energy into the fault resulting in an exploded pinhole, now a crater.

This was especially important to our design approach which was to emphasize a very short "test footprint" along the length of the wire and provide extremely high linear resolution. A side benefit is that this approach should limit damage to the pinhole by shortening the exposure time of the test current (and therefore, the energy) delivered to the pinhole fault.

Looking for more information that might guide the way, we called Norman Swain, Director of Technoloy of the Magnet Wire Group of Alcatel. He quickly informed us that he was Chairman of a NEMA committee working on these very problems with old standards and help was on the way. He also welcomed any input we had to offer that might help resolve some of these issues for the committee and said that they still had lots to do before going to print.

At this point, we decided that further research into present practices was not leading us anywhere. The real world of equipment design was again pulling us off base from our original plan. We decided to wing it and go for broke.

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