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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Core area is not what dictates power, the real limiting factor is how much wire you can fit. It gives a ballpark figure but that does not mean you can't get 50% more or less power. E.g. the core area on my welder transformer translates to something like 1kVA, but it has a large window and by calculating the windings, the result was something like 4kVA continuous (from copper cross-section).
IIRC MOTs use something like 1.7T, so for a homemade transformer which will not be used for hours and doesn't matter if it gets hot, I'd probably use something like 1.3T.
Registered Member #222
Joined: Mon Feb 20 2006, 05:49PM
Location:
Posts: 96
Such a large transformer will be very expensive in just the raw materials alone.
If you want to make it smaller, depending on what you want to use it for, you could build a higher frequency inverter, say 400Hz or higher, and be able to scale down the size to something more affordable.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Dr. Kilovolt wrote ...
Core area is not what dictates power, the real limiting factor is how much wire you can fit. It gives a ballpark figure but that does not mean you can't get 50% more or less power. E.g. the core area on my welder transformer translates to something like 1kVA, but it has a large window and by calculating the windings, the result was something like 4kVA continuous (from copper cross-section).
IIRC MOTs use something like 1.7T, so for a homemade transformer which will not be used for hours and doesn't matter if it gets hot, I'd probably use something like 1.3T.
Intermittent operation wasn't mentioned in the original specification, but in any case, a bigger transformer can only operate safely at a lower power density because the heat dissipating surface area increases less than the heat-producing volume.
To go further, we would need to have the core material data sheet, and the window geometry, amongst other things.
In any case, I don't quite understand how if 100 Euros is too much for an NST, how a large power transformer of the sort talked about here could be built for anything like so small a cost - unless, I suppose, by re-cycling an existing core.
Afterhtought: unless the budget be large enough to pay for bespoke core manufacture, then the designer needs must in any case fall back on standard core sizes.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You can often pick up old transformers at a scrap metal yard, and get the core out. Here is a 2kVA one I did a while ago:
This was a large plate transformer that had a shorted turn. It was made with two 1kVA C-core sets. I planned to make a huge power and output transformer for a tube amp out of it, but I managed to score a 300 watt OPT for 90 dollars on Ebay one day.
Finally, let's say you deliver pizzas at 10 dollars an hour, and it takes you 20 hours to salvage a core and make a transformer. That's $200 worth of your time, so you can credit that $200 against any transformer you might want to buy. And you still havent bought the copper wire and kraft paper, etc.
Or, on a more positive outlook, you could imagine that you took a free transformer design class and got to take home your transformer at the end
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Steve McConner wrote ...
Finally, let's say you deliver pizzas at 10 dollars an hour, and it takes you 20 hours to salvage a core and make a transformer. That's $200 worth of your time, so you can credit that $200 against any transformer you might want to buy. And you still havent bought the copper wire and kraft paper, etc.
But you're missing the main thing, where's the fun of bulding it yourself? Another good thing is that you can design it exactly to your specs, and you're the one who built it so know what to expect, how much abuse it takes etc.
Registered Member #1887
Joined: Sun Dec 28 2008, 02:19PM
Location: most na soÄi,slo
Posts: 8
ok transfomer will be used for powering a tesla coil I have alredy buld a transformaer of 2.4kVA for powering my mesurment transformers (they need 100V on the primary) it costs me 60€ (core from the scrap metal yard) for this transformer i plan about 130€ for everything this isn't so much becau it's 4kVA.
I'll post data of the core as soon sa i get it. I know for 1.2T becau a friend told me thet i will get thet core.
i dont know how do you calculate the core area but in school they teach us thet the core area is=curry out of power (i hope it is written corect)
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
If you have another transformer of suitable power rating, you can disassemble the core and assemble it back with an air gap in it. The gap shall be adjusted to set the desired current.
You then connect this modified transformer's primary in series with your HV transformer.
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current limitation
Another good thing is that you can design it exactly to your specs, and you're the one who built it so know what to expect, how much abuse it takes etc.