Magnet Wire Failing In PinHole Test

Couple of thoughts;

You have fairly old equipment that does not have heat recovery or pollution reduction. Do your ovens recirculate the air or is it just exhausted out one end of the oven? With a dv of 15 for about 1 mm wire that equates to about 50 feet per minute which is not particularly fast.

If your air only passes through the oven once then you are always drawing 100% fresh air into the oven. Recirculating ovens typically exhaust about 10% of the air which means they draw in about 10% of the air.

This is significant if you have high humidity. Some wire enamels or varnishes are hygroscopic meaning that they will adsorb moisture from the air. This moisture is in form of microscopic droplets of water. Since water does not mix with enamels that are not water based, these droplets are applied to the wire and since water evaporates at a higher temperature than most solvents used in enamel, they become trapped bubbles and, as the wire is heated, the bubbles burst leaving pin holes. Sometimes the next coat of enamel will cover the pin hole(s) and if you get lucky, the wire passes. You can also get pinholes if you get entrapped solvents in the enamel and same thing happens as mentioned with the moisture.

I am not certain how:

a) Pin holes can naturally occur 7 – 10 days after manufacture. I think that they are already there because of the moisture that gets in the enamel.

(b) At the same time I am not sure how months later the wire that failed earlier can now pass. Some enamels continue to cure while sitting on the shelf but that should not cause pin holes to close.

If you had a lot of pin holes that would lead to poor BDV test results.

Think of it like this, you are forcing a high voltage into the wire.

- If the insulation is good and defect free, you can continue to increase the voltage until it is very high and exceeds the ability of the enamel to insulate.

- If you have pin holes that means that there is less enamel insulating the conductor at that point. As you increase the voltage a failure occurs because you have exceed the ability of the defective coating to insulate. This usually occurs at a voltage that is well below the required level. Not an unusual event for wire with pin holes.

I think that in your case you might need to talk to the enamel suppliers. Enamels have evolved a lot and so have machine speeds. A dv of 40-70 and faster for 1 mm wire is not unusual. Enamel suppliers like to provide generic enamels, not enamels to meet every customer’s needs. I suspect that the operating characteristics of your enamels are out of sync with you ovens due to design, age, etc.

Richard

MODERATOR'S NOTE 1

Some information for those of us not in the magnet wire segment of the wire and cable industry:

"dv" is also known as "vxd" is the product of the coating speed in meters per min and the magnet wire nominal diameter in millimeters. Thus the higher the value of "vxd", the more effective the enameling machine.

For example: If the coating speed was 120 meters per minute and the wire nominal diameter was 0.5 millimeters, the "vxd" would be 120 x 0.50 = 60. Now if you wished to know the coating speed on this very same machine for a wire nominal diameter of 0.30 mm the answer would be (vxd/d) = 60/ 0.30 = 200 meters per minute.

"BDV" is an acronym for the breakdown voltage.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

Dear Richard,

Thanks for the fairly detailed response. Let me try and respond the the queries point wise :

One hypothesis that even I was having was that due to the presence of moisture in the atmosphere and due to the hygroscopic nature of enamels; microscopic water droplets could be trapped and escape later causing pin holes. Alternatively solvent trapping could also be one of the causes.

My response to this is that if there was water or solvent trapping the tan delta curve/value should show the same. We were not able to get any clue from the same. I have the graphs of the material and we can send the same to you on your email-id (if provided).

Secondly what is indeed perplexing that there is no deterioration in any of the other properties especially in BDV.

However, if for any reason this is the cause, what should be done – given the limitations of existing system.

My other hypothesis is that due to extremely fine copper dust (which somehow gets bonded on to the copper conductor due to static or some other phenomena!!!!) reacts with the enamel film later.

However, to argue against this would be the point that the wire passes through a pre-annealer in which it is heated up to 400-450 degrees and then goes into a quenching bath - the properties of water quality – I have already mentioned in my first post.

Lastly could poor conductor surface also play a role ? Normally pin hole defects due to poor conductor quality occur immediately after production.

Still perplexed…….
Thanks and regards,
Ranjan

Dear Ranjan,

Copper dust is stuck to the wire by residual drawing lubricant or some other contaminant on the surface of the wire. Make sure your annealer (quenching bath) water is absolutely clean and contains no lubricants of any kind. Likewise make sure your water is changed regularly so that there is no build up of contaminants in that water. This is a very common problem in the wire industry.

How do you manage to get a "bad" surface on your copper wire? Presumably you are using ETP continuous cast copper rod as the feedstock.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

Hello, dv is indeed the diameter (d) in mm multiplied by the speed (v) in meters per minute. Easiest for me for reference is that 18 awg is approximately 1 mm in diameter. Therefore if running with a dv of 15 the speed is 15 meters/minute or about 50 fpm. What is unique about magnet wire is that the dv will vary with every wire size and the insulation. Lower thermal rated wire typically run at faster speeds then higher thermal rated wire - when they have about the same percentage of solids. urethanes run faster than modified polyesters which run faster than polyester whioh runs faster than ML. throw in an overcoat and you change things again. to be continued.

Dear Peter,

The properties of the water in the quenching bath has already been mentioned in my post above and is continuing overflowing.

The poor surface was due to bad surface of the RBD Capstan which has since been rectified.

Regards,
Ranjan