Why can't transformer winding company wind transformer as we wish?

Hello,
We have just sent off a Flyback SMPS transformer manufacturing spec to a winding company. They have come back and told us that they cannot wind it as we wish.
All turns, primary and secondary, are made up of 3 strands of 0.5mm enamelled copper wire. (we are doing margin winding, by using 3mm margin tape round the edges of the former)

ETD44 vertical former: (bobbin)
https://www.infantron.net/milesplatts/pdf/fd9660.pdf

Picture of ETD44 vertical former (bobbin)
https://www.milesplatts.co.uk/Product-Info/fd9660.php

We have one layer of secondary (13 turns) , which is "sandwiched" between the two primary halves, which are in series (26 primary turns altogether).
Therefore we instructed them to wind the first primary layer, then wind the second primary layer temporarily on a manderel. Then wind the secondary, then wind the other half of the primary (which had been on the manderel) on top of the secondary.
The winding company say they cannot do this winding of half of the primary on a manderel. Do you know why?....i have had this done by previous transformer manufacturers before. The winding company say that the enammeled copper wire would get scratched by winding it on a manderel, but previous winding companies have had no trouble with this.
Do you know why they cannot wind it on a manderel as explained?

Must be different mandrels. Are these slotted with sharp edges?

well, I thought that mandrels were available without sharp edges for this purpose. I am sure you agree that the act of having one winding as two interleaving halves is widespread, so I am surprised that they are sounding as if its something really far out that I am asking for....mind you, I can see the point that keeping the isolation when your bringing the primary up past the secondary for the "top" of the sandwich is going to potentially violate the isolation voltage.

Is this what you are doing?
https://www.fairchildsemi.com/application-notes/AN/AN-4140.pdf

Thanks Tony, that is exactly what we are doing, (though we have one layer of secondary).

Flyback said:

The winding company say they cannot do this winding of half of the primary on a manderel. Do you know why?

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I don't; but obviously they do . Have you asked them?

Yes I asked them and they said the enamelled copper wire could get scratched on the mandrel. I would have thought you can get mandrels that don't scratch the wire.

Flyback said:

i have had this done by previous transformer manufacturers before.

Click to expand...

So i wouldn't worry about it to much. It's probably just due to the experience of that company. i.e they've tried to do this before and had insulation issues so now they wont do it. But it seems that other people have been able to do this.

At the end of the day if that's the winding you want then it's up to them to give you a solution - if they can't do it with a mandrel then ask them what would enable them to do it.

BTW whats the wire spec? could be that you're previous suppliers have use thicker enamel to avoid problems.
Alternatively could you wind the two primary layers separately and then solder a connection post winding.

Its this wire...
https://www.farnell.com/datasheets/1876784.pdf

Alternatively could you wind the two primary layers separately and then solder a connection post winding.

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I know what you mean by that but it means soldering on top of the transformer winding themselves, which wouldn't be easy, especially doing it in a way that will stillpass the leakage spec of <1% of primary inductance.

Well you could go to a grade 3 enameled wire.
Have you considered putting a layer of kapton tape between primary and secondary. If you did that it wouldn't matter (as much) if the enamel was scratched as it wouldn't short primary and secondary. although you could still get some partial discharge or localized heating which would all depend on the frequency used.

As I understand it, you want the second primary to be wound onto the mandrel so that, when it is later wound on top of the secondary, there is no splice between the two primaries. I can recognize the apparent simplicity of doing it that way.
But, I can also see the complexity of doing it that way in production using an automatic winding system.

The coil winding machine will need two, independently controllable spindles, or they will have to remove the bobbin when winding onto the mandrel.

When winding the secondary, the mandrel will fly around or be anchored in some way to the bobbin. Remember, it cannot be held stationary or the wire from the 1st primary will be twisted to the point of breakage.

Now, to wind the second primary, the mandrel needs to be held but still leave it so that it can spin while the wire is spooled of from it.
I have skipped over the intermediary steps of winding your margin and layer tapes, but those will need to be done and will be more complex with the mandrel connected during parts of the total process.

I'm sure that what you want to do is possible, but may not be practical on standard automatic coil winding equipment without extensive manual intervention.

to be honest, I am pretty sure that most if not all mains SMPS transformers are wound by hand....at least those that don't use triple insulated wire....it doesn't have to be a manderel, the wire can just be hanging out somewhere, its only about a metre of wire (by "wire" I mean 3 strands of 0.5mm enamelled copper wire).

Flyback said:

to be honest, I am pretty sure that most if not all mains SMPS transformers are wound by hand....at least those that don't use triple insulated wire....it doesn't have to be a manderel, the wire can just be hanging out somewhere, its only about a metre of wire (by "wire" I mean 3 strands of 0.5mm enamelled copper wire).

Click to expand...

Well, I'm not sure exactly what you mean by "wound by hand," but this video matches what I have seen in the dozen or so transformer companies I have visited.
While there is a lot of hand work, I wouldn't call it hand wound.

Notice how fast the machine spins about 15 seconds into the video. Can you imagine what it would be like to have three, one meter long strands of wire hanging off the end of the bobbin? Or maybe I should say, whipping in the wind and getting tangled up on everything.

thanks, but they were terminating each coil to a pin...and there wire was single strand and fine so that's no problem for a machine.......we tried to do that but it was too much bulk on that pin when the two halves of the primary were terminated to the same pin. Ours has to be done by hand.