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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Dr. Slack wrote ...
XravenorX wrote ...
Closed core it is, then. Would an EI core work with the primary wound on the middle leg, and the secondaries on the outer legs?
Yes, it would work in principle, but there is one important consideration
The two half-windings on the outer legs *must* be balanced in flux and hence voltage, which means they must be connected in parallel. This stops you using it in what I presume was your preferred centre tapped scondary output connection.
Dr. Slack is right.
I should have said this as well XaveroneX, the connection of the output matters, and i presume youll attach the core to a ground potential on the lower side, to avoid floating problems, the end-to-end and end-to-core insulation matters for the primary and secondary, you can bulk up on the insulation so much that you lose the abilty to wind enough turns if your not careful.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Since you do have access to access to some metal, but don't know its magnetic properties you might test some of it in a small core. The setup consists of an oscilloscope with X-Y inputs and a voltage and current probe. Since what you need is loss in watts/kilogram, a calibrated B-H curve would (bit of number crunching) be a good starting place as well as saturation.
Another approach is to wind i small test core, energize it at 240 volts, and use a Kill-A-Watt meter, good for about 12 amps to tell you what the iron loss is once you've subtracted the copper loss.
Some small diameter iron wire in a bundle makes an interesting core, and you can mess with it by annealing it, and in doing so, it will attain an oxide layer which will reduce eddy losses.
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Ok, I'm getting a better understanding now.
What material could be used for insulating the laminations from each other without adding too much bulk to the core? I'd also intended to weld the core together, but if the laminations need a high resistance, welding the core would essentially cancel out any benefits of insulated layers, correct?
I don't mind the idea of parallel wiring the secondaries and I have enough wire for the coils, but it seems the core is going to be very expensive to construct, since the thinnest steel sheet I have is 3 mm, so I'll have to buy all the core material at retail price :(
If I could only get my hands on a small pole pig or PT I'd be happy, but I haven't been able to find either out here in Australia :(
Maybe when I'm old and rich I can justify spending $2000 on sheet metal...
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Core laminations do not have applied insulation coatings, they are treated to have an oxide layer. We routinely fire motor stators to burn away varnishes that were used in the manufacturing of the new motor, when the motor is rebuilt and re-dipped. The original oxide lamination surface remains. For the few KVA you are building there are other than pole pigs out there. A 10 KVA, indoor-use transformer is a common item, and they do burn out and are scrapped. Start hunting down electrical contractors who do building maintenance, or better still scrap metal dealers who buy up this scrap. You will have to pay for what copper, or even aluminum and iron they contain. Hunting oil filled polepigs is hard because the utility knows where they all are. But dry transformers are a different story.
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
radiotech wrote ...
Core laminations do not have applied insulation coatings, they are treated to have an oxide layer. We routinely fire motor stators to burn away varnishes that were used in the manufacturing of the new motor, when the motor is rebuilt and re-dipped. The original oxide lamination surface remains. For the few KVA you are building there are other than pole pigs out there. A 10 KVA, indoor-use transformer is a common item, and they do burn out and are scrapped. Start hunting down electrical contractors who do building maintenance, or better still scrap metal dealers who buy up this scrap. You will have to pay for what copper, or even aluminum and iron they contain. Hunting oil filled polepigs is hard because the utility knows where they all are. But dry transformers are a different story.
Yeah, if I could get my hands on a suitable core, then I don't believe I'll have much trouble with the coils.
Sounds like it will also turn out a lot cheaper to buy a scrapped transformer than the raw steel for a core.
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
In your diagram in your first post, the text states that you will use "liquid electrical tape" to insulate the windings. DON'T DO THAT if you plan to immerse the windings in dielectric transformer oil! I can tell you from my own experience that "liquid electrical tape" turns to a gooey mess when you get transformer oil on it for any length of time. You would be better off finding some transformer insulating varnish that was formulated to remain chemically stable while in contact with transformer oil for long periods of time. I cannot recommend a specific brand or type, so you will need to do your own research.
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I need help with a transformer design
