You say "But the voltage on secondary is exactly the same as before." If it was already a 120V to 12V transformer, why did you rewind it?
I salvaged an old transformer and discarded the old wire and kept the core. It was a 220V to 110V or something like that, it didn't really matter to me. I just needed the laminations. And then I wound an entirely new transformer with my own specs.
It's too bad you didn't measure the exciting current, unstack the core, restack it and measure the current with no changes to the windings.
That's exactly what I did. When I first put everything together, I measured the magnetizing current to be 280mA which was what I expected. I then re-stacked it (no changes in windings) and the current went up to almost 500mA. And that is the reason I posted my question.
Then you would have known that any increase in exciting current was due to increased losses in the core alone.
That's what I am not sure about. At first I thought it was due to a short caused by a damaged insulation or something. But like I said, that would have changed the turn ratio and the secondary voltage. So another possibility could be the core losses, more specifically Eddy current losses due to the disturbed insulation between the laminations. But that also sounds strange, as it didn't happen the first time around. On the other hand, every time they are disassembled and re-stacked, some more insulation is lost, so who knows.
If you have another similar transformer, you could try this.
But to get to your current situation, consider this: a transformer in good condition connected to the grid with no load. The core will not heat up very fast. If your rewound transformer's core is heating up, but the windings are not, then it would be reasonable to believe the increased losses are due to loss of insulation between the laminations.
Yeah, when the heat is caused by the copper, i.e. when the wire ampacity is exceeded, then obviously the windings will get hot first and then the core will start heating up as a result, acting as a heat sink. But I think in my case it is the opposite, windings are "cold" but the core is getting warm.
The windings in an unloaded transformer should not noticeably heat up at all except by heat conducted from the core, and that takes quite a bit of time. If the windings are heating up quickly then you have a problem there.
No, windings are not heating up. It is the core that is heating up. The primary is a 22 AWG wire, so it should be able to handle 500mA with no issues.
Make sure the butt joints between each E lam and its mating I lam are tight. I use a rubber mallet to pound the joints tight.
A rubber mallet sounds like good idea. One thing I noticed is the bobbin flanges are bowed outwards slightly (wire pushing on them on the edges) so the oppositely facing E's are not quite touching each other so there is a tiny little air gap between them. But in theory that would just decrease the flux slightly without causing the heat related losses... I know that if I don't tighten the screws well, it will hum, and that's vibration, and vibration is heat. But the way it is currently assembled, it is very quiet.
Commercial laminations are sometimes coated with powdered magnesium silicate to provide insulation between laminations. A lower cost method they use is to form an oxide coating on the laminations by heating them with steam. You might try coating your laminations with something to provide additional insulation; about the only thing I can think of that everyone would have around for this purpose would be flour or corn starch. This might be the best thing for a permanent fix! But it would be an experiment to find out if your problem is indeed increased losses due to insufficient insulation between laminations.
Thank you for your input. I will definitely try to re-coat them. After all, it is either that, or a short somewhere.