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Old 26th October 2006, 01:38 PM   #1
AndrewT is offline AndrewT  Scotland
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Question transformer primary windings Q

Hi,
I am unwrapping an AVEL LINDBERG transformer ready for adding some secondary windings. It is about 500V to 800VA judging by size and weight.
The tappings are
0-100-110-120 + 0-100-110-120 for the primary
and
0-24 secondary, but the secondary is tri-fillar wound.
so it is effectively
0-24 + 0-24 + 0-24.
I plan to add a 0-24 winding to give me a pair of dual secondaries.

But a big surprise as the insulation came off:-

some of the primary has been wound OUTSIDE the secondary.

There are not many primary turns outside, probably just the voltage adjustment windings.
It appears that the two neutral ends are definitely outside.

The question
Could AVEL have also wound the other live 10V tapping on the outside as well?

Is this pratice common on multi-tapped transformers, or is this an AVEL quirk?

Any ideas why and how I should tread?
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Old 26th October 2006, 02:11 PM   #2
Did it Himself
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Interesting Andrew, when I made transformers we never made any ones like that.

Personally I am not a fan of simply adding windings to get additional secondaries. This is because the core is already pretty full usually. Also I prefer bifilar windings as it achieves a more symmetrical supply, not just in terms of voltage but also impedance.

How are you going to determine how many turns you need for the secondary?

If you insist on using this item (which may be attractive to you if you got it for free/cheap and are on a budget) then I would leave all the existing windings alone - just remove the melinex or tape - and just wind over the top. As there is primary on the outside I would use some tape before adding the new winding.

Have you done this kind of thing before? I have to say hand winding toroids is not my idea of fun! Good luck.
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Old 26th October 2006, 02:32 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi Ritchie,
thanks for the caution, it's that kind of guidance I am looking for.
Yes, I have adjusted transformer windings before.
Yes, it's a hassle and takes time and organisation to end up being neat and safe (no damaged enamel).

But, I have never come across primary (or just some of the primary) on the outside.

I could unsolder the common taps and try to fathom out what's happening, but there may be someone out there that can save me a bit (or a lot) of deeper investigation. There's a lot of wires to label if I start unsoldering and removing the coloured input wires (19 in total).

Putting on an extra winding is easy once I have the turns number and wire length. Just prewind a long bobbin and go for it. A few extra mohms in one winding to a cap input supply will not bother me or the amp.

I will put on a loose wound 10 turns and measure the open circuit voltage, then just ratio up to the 25.2Vac o.c. voltage of the originals. I can always add or remove a single turn if the final voltage is slightly out. Pity it is tri-fillar, bi-fillar or quad-fillar would have saved quite a bit of work.
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Old 26th October 2006, 03:40 PM   #4
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As the transformer has already I assume been certified safe to use and you need the full amount of primary anyway, I would not mess about with unsoldering things. What would you achieve by this anyway?

Your method to determine turns is what I would do.
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Old 27th October 2006, 05:36 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
seems looking at my AVEL (not navel) gets me thinking even more.

The primary windings are bilfillar (yellow and pink).

The lower enamelled wire is alongside the upper enamelled wire.

When connected to a 100/10/120Vac supply these two windings are at about the same voltage from end to end.

BUT

when connected to 200/210/220/230/240Vac the adjacent wires are about 120Vac apart.

That means two layers of undamaged enamel separate 120Vac.
What if one or both enamels are or become damaged?

Is this normal practice in a twin primary toroid?
Quote:
assume been certified (safe to use)
I'm beginning to think you were also referring to the designer.
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Old 28th October 2006, 12:03 AM   #6
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Default transformer construction

Interesting to hear your comments about transformer construction. I can't explain why they would interleave the two winding so oddly. In some constructions, I'd say minimizing leakage inductance could be a reason, but that isn't necessary here.

And as far as the voltage between adjacent windings goes, that does not sound good to me. I did some investigating of "enamel" insulation a year or two ago, and the manufacturer reported a breakdown around 2 KV if I remember correctly. I was surprised it was so high. I've made a lot of transformers , and I wouldn't put 100 v between two enamel wires that lay next to each other.

Do you happen to have experience with AnTek transformers and their construction? I'm thinking of ripping one apart to develop confidence in the constuction quality. In particular, I'm wondering if they do any potting, and how they insulate the core from the bottom layer of the coils
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Old 28th October 2006, 11:51 AM   #7
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I wouldn't worry too much about the proximity, it's not ideal but neither is it all that bad. The wires will not be moving so no damage should occur. They may be using grade 2 coated wire which has pretty thick enamel.

The whole transformer sounds a bit weird to me, but at the end of the day, if it's got the proper safety markings it must be OK.
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