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

Another bane to magnet wire producers, high voltage continuity (HVC) faults are a weakness or failure of the insulating ability of the enamel coating and includes small "pinholes" to the bare copper underneath as well as continuous bare wire.

We were aware of several reputable manufactures of test equipment who manufactured off-line equipment to do this as well as others who manufactured online equipment. We felt it would be interesting to use a very different approach and design an online test system that had a very short "footprint" along the tested length of wire-allowing very high linear resolution of faults. This means that two faults are not counted as one and the fault currents are not summed.

As this field of test seemed to be rather mature and with well thought out testing standards accepted worldwide (we thought), we set out to make a "science" of high resolution online HVC fault detection as opposed to the bead project where there were no accepted parameters.

We started by researching the existing test standards so that the design would, at least, perform and correlate with these requirements and hopefully go a lot further. We definitely had no intentions of creating new standards and looking for universal acceptance.

Surprise-existing standards were ambiguous to say the least. For example, physical dimensions of the fault area are vaguely implied for bench testers, but not very useful for online testers. When should one big fault be considered as two or more? What was the definition of bare wire? Likewise, the electrical test itself was not well defined and could allow variations in design, although not in violation, to produce very different results on the same tested wire.

In contrast, the mechanical details of a bench tester were spelled out in great detail, however, this is the area where we planned to deviate the most from previous designs. We intended to design a sensor emphasizing much higher linear resolution and to test the wire as it was being made.

Let's talk to the industry. After all these years, they will have filled in the blank space and lit up all the dark corners. Surprise-in interviews with engineers at many magnet wire producers, we found virtually no agreement as to how the test parameters, as found in NEMA and IEC, were to be interpreted, off-line or in-line. None had much of an idea of the electrical performance details of their in-house test equipment. Most wanted to be in compliance with whatever standards existed while complaining that these standards were not really adequate for their needs as evidenced by product returns for HVC problems.

In some plants, we knew the existing test equipment in use did not even come close to performing the tests in compliance with the standards. There seemed to be no real concern by the plant personnel in how the test was conducted so long as returns were at or below acceptable levels. No surprise here.

The only point of universal agreement was that the test equipment results should match the performance of the magnet wire user's equipment as used for incoming inspections. No surprise here either, but what was that performance in design terms?

More and more, the bead project was being repeated since we could not nail down performance requirements that the world would agree to and that was our intended approach.

OK, since there are no qualifying standards for the product designs, lets assume there are no disqualifying standards either and fire our best shot.

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