Troubleshooting Enamel Coating for High-Amp Transformer Windings

[microfracture]

i am winding a transformer rated for 150A or more, and i can't find wire thick enough, in my neighborhood, to fit the profile.
i did find a shop that sells bare wire at 8awg. so i thought i could put an enameled coating on each winding as i wound it. you dig? is there a commercial "lacquer" i can get at say home depot that would fit my needs?

any help is greatly appreciated.

 

[TurtleMeister]

Why don't you use aluminum or copper strap instead of wire? Then you could use sheets of insulation (such as mylar or mylar tape).

 

[negitron]

You can also wind smaller-gauge wire in a multifilar configuration. It's also easier to wind this way than a single heavy wire. If you insist, however, you can buy some: http://www.planetengineers.com/product.asp?pid=1703

And the answer to your last question is "no."

 

[Bob S]

Do you know what gauge copper wire you need to minimize (optimize) IR drop? Have you calculated skin-effect copper losses for solid conductor? Skin-effect losses in solid copper wire might be another reason to go to individually-enameled stranded wire. By the way, 7-conductor stranded is a magic number of strands, in terms of packing fraction. The next magic number is 19.

 

[negitron]

Assuming this is a power transformer for 50 or 60 Hz, skin-effect losses are entirely negligible at the stated power level, given a skin depth of 8.5 mm at 60 Hz. I²R losses, however, are not.

 

[Bob S]

Here is a wire table stating both current rating and skin depth:
http://www.powerstream.com/Wire_Size.htm
It has two amp ratings for each gauge wire. The second column agrees with my ARRL handbook table. There might be another table somewhere for transformer windings, mainly because there is no convection cooling of the wires at all. All the heat is being conducted through insulation, which is a poor heat conductor. Also, the coil has to conduct a lot of the core (transformer lamination) heat loss.

 

[negitron]

Heh, I have that exact table printed out and tacked up on my bulletin board here at work--I'm looking at it right now. Neither of those two ampacity columns is suitable for transformer windings. The first is for free-air chassis wiring where the wires are in open air and separated. The second is for power transmission including in-wall and conduit runs where convective cooling is minimal or nonexistent. and given a conservative 700 CM/A ampacity As you note, for a transformer, you have layers of insulation to deal with plus the heat of iron losses. You have to calculate your expected iron and copper losses and then use these figures to calculate your winding temperature rise; if you don't, you can easily wind up releasing the magic smoke.