transformer rewinding

[troyo]

hi want wind a 70-0-70 5000w i have an old 5000w stabilizer transfromer with a core area of 80 cm square .what size of wire(mm) would u recomend for this and the turns to be used

[johansen]

i'll plug the numbers into my spreadsheet later (i'm at work) but i doubt that is a 5 kw transformer.
Its probably good for around 2 kw, because most voltage stabilizers are configured like an auto transformer with a capacitor and a magnetic shunt.

what are the dimensions? do you intend to rewind the primary as well? if not what is it wound with and how much window area does it take up?

[AndrewT]

core area: 80cm square = ?
do you mean 80cm by 80cm?
or
do you mean 80square cm? i.e.~10cm by ~8cm

If the latter 80sqcm then that is ~ 12sqin.
A disputed formula for arriving at VA rating is
VA ~= 30 * [sqin]^2
For your 80sqcm VA ~= 30* 12.4*12.4 ~4.7kVA

Is the magnetic circuit completely closed, i.e. no deliberate air gap?

[johansen]

my reply was in haste.

80 square cm of core gets you 42 turns needed for the primary at 1.3T.
this means you can fit 9 parallel strands of 10 awg wire assuming that's a loss-less cut.
for the secondary 8 parallel strands should fit. 9x46 turns (window area of 125mm by 42mm)

If the iron consumes 5 watts per kilogram it will dissipate 162 watts.
for the copper to match that you have to pull 12KW from the transformer.

In real life thermal limits will be reached at probably half this because you won't be able to fit 9 parallel primary coils of 10 awg unless you use an equivalent copper strip for the primary and secondary. the excition levels may have to be cut below 1.3T if 160 watts no load is too high. but 1.2T is probably good and that will cut the optimum efficiency point down to around 9.5kw.
if you cut it down to 1.2T then you need 46 turns for the primary and that means you can only fit 8 strands of 10 awg and 7 times 54 turns for the secondary.

keep in mind those are theoretical figures that are difficult to achieve in real life, but can be exceeded by using copper strip.

[Tony]

Originally Posted by troyo

hi want wind a 70-0-70 5000w i have an old 5000w stabilizer transfromer with a core area of 80 cm square .what size of wire(mm) would u recomend for this and the turns to be used

what is you line frequency, mains voltage, secondary voltage?