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Noob plays with transformers
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soldar:

--- Quote from: rebelrider.mike on November 14, 2019, 09:11:22 pm ---Well, I'm no further trying to predict how much power an unknown transformer core can handle.
--- End quote ---

I know you are having fun and I don't want to spoil it but "an unknown transformer core" can "handle" no definite amount of power because the core does not "handle" power. I understand what you are trying to get to but you are looking at the problem from the wrong angle.

The entire transformer handles power and you cannot really put a theoretical limit on what a core can handle and it depends also on what you mean by "handle". 

A separate primary and secondary are very different from an auto-transformer and the power they can handle will be different.

But, the way I look at this problem is the other way around. It is not "how much power can the core handle?" because I look at it more as the conductors being the ones that handle the power so I ask more "how much power can the conductors I can fit in that core handle?"

When designing transformers I would start out with a core and bobbin chosen almost by eye and then design the coils. If the coils did not fit in the bobbin I would switch to the next standard size up and start over again.

As you already know, Al wire will handle less power than copper wire. If you could get a wire with much less resistance than copper the same core could handle much more power.

So, you have a core, you need to determine how many turns you need, then what diameter wire will fit with those turns and the copper resistance will be what mostly determines the power it can handle.

So, you have a core and bobbin and want to build a 1:1 transformer. You can either measure the characteristics of the core or just assume a safe, low value.

If you give us the dimensions of core and bobbin I can take a stab at it.

If you can measure the properties of the core that would be even better. Assemble the core with, say 100 or 200 turns and measure inductance. at mains frequency by putting in series with a light bulb.

You want to determine how many turns you need to have a suitably small magnetizing current. Once you determine that you just put that number of turns with wire as thick as will fit.
Cliff Matthews:
Careful with bolt isolation. I had some 500VA hot bolt issues and got it sorted-out with Kapton tape. Good luck!
exe:
I think what limits power handling is rise of temperature. As long as a tranny remains within the temperature limit it should be fine. Afaik Rod Elliot wrote that he could get more from trannies by forced cooling: https://sound-au.com/xfmr2.htm?

> Fan cooling can increase the effective VA rating of a transformer significantly, but does not improve regulation.  Large power distribution transformers are almost always oil cooled, and they are now starting to use vegetable oils because they are less inclined to catch on fire, and pose minimal environmental impact should there be a coolant leak or other major fault.


Having said that, by looking at the size of your tranny, it should have a better regulation, more than 80, imho (you can see some figures for similar-sized trannies to get a rough idea how much it can take). So, this makes me thinking you pushed it well beyond what it was designed for.

Also, due to high mass the temperature may raise quite slowly. This may give a false impression that it can sustain current load. Don't get fooled by this, in a real device the tranny is unlikely to be in open air, so cooling will be much worse. Also, I suggest doing performance testing for at least half an hour with closely monitoring the temperature. Trannies are good at handling short overloads, but it in the long run they may tend to overheat if constantly abused.
The Electrician:
The power "handling" capability of a silicon steel transformer such as you are working with is given by the area product WaAc.

See this reference: https://coefs.uncc.edu/mnoras/files/2013/03/Transformer-and-Inductor-Design-Handbook_Chapter_5.pdf

and: https://www.mag-inc.com/getattachment/Design/Design-Guides/Transformer-Design-with-Magnetics-Ferrite-Cores/TransformerDesignWithFerrites2013.pdf?lang=en-US
Zero999:

--- Quote from: rebelrider.mike on November 14, 2019, 09:11:22 pm ---So here are the results:
It was loud, but didn't trip the breaker.
121V in vs. 119V out, without load.
Under load, the input was 119V, 3.58A, 426W.
Output was 115.5V, 2A, 231W.


With Watts in / Watts out * 100 to measure efficiency, I've got a whopping 54.2% Impressive, no? No.

--- End quote ---
Well if you've gut the core size, it might be saturating slightly which will increase losses. If the voltage drop is an issue, then wind extra turns on the secondary, but accept the off-load voltage will be a little on the high side. The output voltage of transformers is normally specified at full load, so the off-load voltage will be significantly higher, but you might want to go for 5% higher than desired voltage off-load and 95% of the desired voltage on-load.

One thing to note is that auto-transformers can be much smaller than isolating transformers, given the same power rating. An auto-transformer with a 2:1 ratio will have half the copper and iron of an isolation transformer of the same power rating. If your transformer is rated to 1kVA, then it'll be rated to no more than 500VA, when re-wound as an isolating transformer. Indeed, it's possible to convert an isolating transformer into a 2:1 auto-transformer, with double the power rating, just by connecting the windings in series. Think about a 1:1 120V:120V transformer, with the secondary winding connected in series with the primary winding. Assuming the phasing is correct, the voltage on the secondary will be double the primary voltage. This is how auto-transformers work.
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