Transformer Rewinding

[mayhemsm]

Can anyone tell me if I currently have a transformer with:

Primary: 120v
Secondary: 40v

I want to rewind the secondary to output 80V with a max current of 20A. According to an AWG chart I'm looking at I need to use 8awg wire to support 20A, I dont have any 8awg coated wire to rewind with, can I rewind it with 2 secondary coils at 80v with a max of 10A each (with 12awg), and then run the coil's in paralell to provide my 80V @ 20A?

[Al_The_Man]

Is the transformer VA rating sufficient at present?
You will need a Transformer with a min of around 1.6KVA.
Which means the present winding would be rated at about 40amp at 40volts.
If the rating is sufficient, you could wire it as you suggested.
Al.

[mayhemsm]

Thanks Al,

I'm not sure what the current rating is. Ill have to investigate further to see if I can find out. Assuming the transformer is rated lower than desired, what would happen if I ran it at the desired VA, over heat? possible 'melt down'...

[Al_The_Man]

Overheat is the first symptom. Unfortunately there does not appear to be any defined standard as to transformer VA rating, it is up to the individual manufacturer.
If you go too high in current, saturation will then occur, and this is usually accompanied with burn out.
Al.

[vger]

Physical size is an indicator of the power rating of a transformer. For 1.6KVA you are probably looking at a transformer that weighs in at about 25 to 30 lbs. The size of the core itself is really the limiting factor due to magnetic saturation as Al said. If it's an EI core (laminations shaped like a capital E and I) then you will probably find that the secondary is wound underneath the primary. You will have to get the right number of turns of wire on the secondary for the voltage you want. If your primary has 1200 turns then the secondary will have to have 800 turns to get the proper ratio.

If you need 80 V at 20 amps output, then your input current at 120 volts will be about 14 amps and the primary will have to have wire that is sized accordingly.

At any rate, it'll be a big transformer. Maybe 6" X 8" X 4" total (just an educated guess)

One handy source I can think of for a transformer core that might fit your needs would be an old battery charger. You would need one that would deliver a little better than 100 amps at 12 V.

Best of luck
Steve

[mayhemsm]

Thanks guys for all the information, it really helps. I think my current transformer wont be capable so I'll be trying some local places in search of a better one.