10-07-2018, 06:01 PM
(10-07-2018, 10:48 AM)Diabolical Artificer Wrote: I saw one video where they wound the primary and checked every so often with a lightbulb in series I think with said pri until the bulb stopped glowing. For the life of me I can't figure out what they're doing here.
The tricky thing with using unknown cores is that there is no data, no BH curves or type of material used for the core etc.
With the lightbulb in series, they'll be using it as a crude indicator of primary current. If the core is saturating, there'll be gulps of current taken each half-cycle which will light the bulb.
Personally, I'd like to increase the turns 10% beyond the 'knee' of onset of bulb glowing, to give a safety margin for voltage surges etc - you don't want nuisance fuse blows or overload breaker trips.
It isn't a very good method - how would you know what wire gauge to use? No good winding a primary, and find that you have only a 1mm hole in the centre for the secondary! Or alternatively, find that you have such a huge centre hole left that you realise you'll either (a) have to unwind and rewind with a larger size wire, or (b) accept that you'll have much larger primary resistance losses than you need.
As for unknown material, yes quite agree - but most toroidal mains transformers are made with steel that can run at 1.6 tesla quite happily and they are all much of a muchness. Knowing the provenance helps - if it was originally a 400Hz aircraft transformer, it'll use 0.1mm thick steel which is lower loss at this frequency but slightly less efficient on space (measure the size of the cross section and multiply calculated area by 0.92); if it was originally 50 / 60Hz then it'll be 0.3mm steel which has a 95% space factor rather than 92%.
For space-saving applications, exotic materials like cobalt-iron alloy are used which will cope with 2 teslas without saturating. This allows a smaller transformer, because you can get away with fewer turns. But it costs a packet, compared with standard silicon-iron, so for an unknown transformer, it is unlikely.