Insulation of enameled copper wire

It's hard to answer this question without knowing more about your oven. There was a time when a V/D
for 1 mm wire was as low as 10-15. It looks like right now for 0.75 you have a V/d of about 52. What did the manufacturer tell you it should be for that size wire.

Dear tech1,
Thank you very much for your information. Our oven lenght is 9 m and its temperature is 550°c. We produce 0.75 mm with speed 70m/min. What should be the speed for producing 1.50 mm using the formula V/D.
Best regards,
Wire

Wire,
What you are experiencing with the polyester-imide sounds like a cure issue. 200c polyester-imide has a high glass transistion temperature along with a high curing temperature to achieve cross linking of the polymer. There are two tests that you can perform to test for cure. Firstly, you can check the tangent delta ( DF vs temperature ). This test should show two transitions. One at 200c and the other greater than 220c. The second test is old school. Place the coated wire in alcohol for approximately 2 hrs and then bend the wire. If it cracks ( the coating ) when you bend it, it is not cured properly. Increase temperature and reduce coating speeds. One last thing, you mentioned that one size does it and the other does not. For a starting point on production speed try this formula ( V/D ). Velocity times Diameter and then proprtion the other size.

The type of copper used should not make a difference. I have never heard of this happening with this type enamel.
One thought that occurred is that this type enamel especially the overcoat is hydroscopic meaning that it picks up moisture from air. If you have a high ambient air humidity and a large open supply tank with slow turn over of the enamel, then if too much moisture is adsorbed, this may be the cause of the problem. Might be a stretch thought but if you can put a lid on your tank or reduce the volume, perhaps it would help.

rb

Dear Richard,
Thank you very much for your information and help. We would like to inform you that we use a copper ETP grade A and polyester-imide enamels thermal class 200. We have noticed that the breakdown voltage remains unchanged for 0.75 mm and falls down after weeks for higher diameters (such 1.50 mm).

Best regards

The answer to your question is neither. The annealer or oven only provide heat to ore-anneal the copper or to drive the solvents out of the liquid enamel or to dry it. The effect is short term or momentary at best unless you are over or under curing a product. If the product is good when it left the oven there is nothing that the oven can do to effect the wire once it is no longer in the oven.

I think your answer may be in your material. Is it a thermoplastic? The difference between thermoplastics and thermosetting plastics is that thermoplastics become soft, remoldable and weldable when heat is added. Thermosetting plastics however, when heated, will chemically decompose, so they can not be welded or remolded. On the other hand, once a thermoset is cured it tends to be stronger than a thermoplastic.

It is probably continuing to cure/downgrade even when sitting on the shelf.

This is not necessarily a problem if you are encapsulating it or putting it in an oil filled transformer. It is a big problem if you are trying to use it for motor windings and coil windings that are not totally encapsulated.

richard