Why can't transformer winding company do "interleaved" winding?

Hello,
We have just sent off a Flyback 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 bobbin:
**broken link removed**

Pictur of ETD44 vertical former
**broken link removed**

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?

Perhaps you can specify the splitted winding as two separate windings and connect them externally, either on bobbin pin or "in flight".

I can imagine that winding the wire intermediately on a mandrel isn't considered an acceptable production method. Sounds more like basement workshop than industry production.

I can imagine that winding the wire intermediately on a mandrel isn't considered an acceptable production method. Sounds more like basement workshop than industry production.

Click to expand...

In the past, agw.co.uk have done it like this for me.

Perhaps you can specify the splitted winding as two separate windings and connect them externally, either on bobbin pin or "in flight".

Click to expand...

thanks, you have hit the nail on the head again....we originally had the first half of the primary terminating to a separate pin, and then the second half of the primary taking off from this same pin, but the winding company complained that since a the "wire" is actually three strands of 0.5mm enamelled copper wire, they reckon there's no room for having the six (overall) wire strands coming out from the one pin.

The winding company have actually said they will try and connect the two halves of the primary by bringing them both out on top of the auxiliary coil which is on top. I am not sure how successful they will be in doing this, especially considering that it might mean lots of leakage inductance, as well as much leakage variability between transformers....also, it will be awkward for them to keep the 3000VAC isolation as the first primary half has to somehow get passed the secondary.

This is a new thread here, but is posted here as its related to the above..

We wish to make a margin wound flyback transformer using enamelled copper wire and an ETD34 core.
Its 17 secondary turns on one layer. This is sandwiched between two layers of 17 primary turns. (so primary is 34 turns total.)
We need the full 3.5kv of isolation voltage between primary and secondary as its an offline flyback

We obviously need to wind the 34 primary turns as two separate layers and join them at a pin.

How much do you think this will deleteriously affect leakage inductance in comparison with the method of winding the first layer of primary, then winding the second primary layer temporarily on a manderel, then winding the secondary layer, then winding the primary layer that’s been waiting temporarily on the manderel over the top of the secondary?

I had audio transformers made by Hammond and they interleaved the windings for excellent high frequency response. The small extra cost was worth it because cheaper ordinary transformers produced poor high frequency response.

Thanks, which of the above two methods of interleaving gives the lowest leakage?......using the manderel, or terminating each primary half to an intermediary pin.?

Depends on the wiring details. The question is just pointless.

With equivalent wire in each case.
As you know, slight differences in wiring etc change the leakage slightly, which is a worthwhile thing to investigate, as
in a flyback , we want low leakage as possible.

With the method with the intermediary termination to the intermediary pin, the end of each of the primary layers has to be dragged back across the wound coil, back to the former pin, which sounds like not being good for leakage, but with the manderel method, the second primary layer continues directly from where the first primary layer left off.

I think you can agree that these two very different interleaving methods will not give similar values of leakage?

Question is, which is best, giving low leakage.?