Unwound the wire off the primary and know the exact length,L. Also the bobbin dimensions of course. The wire I have around, is a slightly smaller diameter ,d2, original d1. Would the rewind length , for the approx same number of turns,be simply L * (d2^2)/(d1 ^2)ie shorter by the ratio of areas or some other consideration like packing factor change, assuming current carrying capacity is sufficient. I know I should have counted off the turns but I assumed there would be a "welded" mess of metal in the middle, and would have to rely on weight of the wire.
Could you back-calculate the number of primary turns from the secondary (which I presume is still in place)?
Wind on 10 primary turns but don't bother to replace the core plates, then energise the secondary to some proportion of its working voltage from a signal generator at a high enough frequency that the lack of an iron core doesn't matter. From the voltage appearing across the temporary primary you can calculate what nominal value of turns per volt it would have when the secondary is running at its rated voltage.
From that, you can calculate the nominal number of turns required in the primary. To allow for losses, you will then need to reduce the actual number of turns by a percentage which depends on the type and size of transformer. For units around 10 to 20 VA, the reduction could be as much as 10% (especially if you are using slightly more lossy wire than the original), but a 250VA transformer may need less than 5% reduction.
To avoid having to dismantle the whole thing and rewind it yet again, err on the side of too many turns (which is what will happen anyway if you use the correct length of slightly undersized wire). That way you may be able to slip off a few turns until you get the correct number. Another alternative is to leave a few tapping points and insulate-off the ones you find you don't need.
--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
What he said...but... My experience has been that I can never hand wind the turns as tightly as the machine. Wire not tightly wound can take up considerably more space than you calculate.
At least the replacement wire is thinner and so more space to sloppy. I will use a manual coil winder but I never seem to get the traverse rate just right , but better than by hand
As that procedure has some suck-it-and-see I think I'll calculate via area ratios and add arbitrarily 20%. Wind that whole length on , counting turns, and then try with some medium frequency drive, sans iron, and then count off the error. Even if that +20% is too little there will be space to do a well insulated splice of some more turns
It turned out ,for same voltage out and for my ,in effect, manual winding, the same length of wire required , going down from .32mm to .28mm. Using just the Es minus 1, and no Is , magnetising not complete and could only run the variac up to 70 per cent or so, but gave the voltage ratio at 50Hz. Next time I'll probably add the Is as well, as its easier inserting the Is than the Es. Tip for final E insertion, force a razor blade in between the interim last E and the bobbin, then insert the last E between the razor, bent a bit, and the in place E.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.