Determining Unknown Transformer's Current Capability

[Crumb]

Hello;

I harvested some nice flat double bobbin transformers from a power
supply. These are 32 VRMS and about 6" x 5" x 2.5" in size. There was
two transformers in the supply and it was fused at 125VAC @ 5 amps
(2.5 amps per tranny).

I would asume that the current capability was over spec'ed and fusing
at only 2.5 amps @ 125 VRMS per transformer is again, lower than the
actual VA of the transformer.

Is there a way to determine the current capability of these?

Thanks

 

[Rich Grise]

Hello;

I harvested some nice flat double bobbin transformers from a power
supply. These are 32 VRMS and about 6" x 5" x 2.5" in size. There was
two transformers in the supply and it was fused at 125VAC @ 5 amps
(2.5 amps per tranny).

I would asume that the current capability was over spec'ed and fusing
at only 2.5 amps @ 125 VRMS per transformer is again, lower than the
actual VA of the transformer.

Is there a way to determine the current capability of these?

Thanks

Weigh them. You could go through all kinds of tests and crap, but
bottom line is you'll generally come out to X VA per pound - someone
here did a whole tranny thing recently, maybe he'll chime in with
what it weighed and what his final VA were, then you can just apply
that factor. (whatever X is - I'd guess it doesn't vary by more than,
say, 2 to 1 over a range of transformers any of us is likely to be
handling)

Or, you could set them on a bench with a load, and watch the temp.
rise.

Have Fun!
Rich

 

[John Fields]

Hello;

I harvested some nice flat double bobbin transformers from a power
supply. These are 32 VRMS and about 6" x 5" x 2.5" in size. There was
two transformers in the supply and it was fused at 125VAC @ 5 amps
(2.5 amps per tranny).

I would asume that the current capability was over spec'ed and fusing
at only 2.5 amps @ 125 VRMS per transformer is again, lower than the
actual VA of the transformer.

---
Shouldn't be... Why would you want to not be able to use some of the
transformer's capability by underfusing it? More than likely the fuse
is rated at about twice what the transformer's primary current will be
when its secondary is fully loaded, so you're prob'ly looking at about
100 - 150VA transformers.
---

Is there a way to determine the current capability of these?

---
Yup; load 'em down and see how hot they get. Around 75°C lamination
temp with an ambient of 25°C is probably as hot as you want them to
get if you don't know how they're made. If they've got a UL listing
on them that might get you closer.
---

 

[John Larkin]

Weigh them. You could go through all kinds of tests and crap, but
bottom line is you'll generally come out to X VA per pound - someone
here did a whole tranny thing recently,

Me? I plotted a bunch of different trannies on graph paper and got,
fairly consistantly, about 15 VA/pound, 30 VA/lb for torroids.

Or, you could set them on a bench with a load, and watch the temp.
rise.

That's the best way. If you can't touch it, or if it starts to smell
like varnish, that's it.

John

 

[Rich Grise]

Me? I plotted a bunch of different trannies on graph paper and got,
fairly consistantly, about 15 VA/pound, 30 VA/lb for torroids.

Cool! And even my range WAG panned out! This definitely calls for a
brewski!

That's the best way. If you can't touch it, or if it starts to smell
like varnish, that's it.

"When you smell it, turn it off"? ;-)

Cheers!
Rich

 

[Tam/WB2TT]

---
Shouldn't be... Why would you want to not be able to use some of the
transformer's capability by underfusing it? More than likely the fuse
is rated at about twice what the transformer's primary current will be
when its secondary is fully loaded, so you're prob'ly looking at about
100 - 150VA transformers.
---


---
Yup; load 'em down and see how hot they get. Around 75°C lamination
temp with an ambient of 25°C is probably as hot as you want them to
get if you don't know how they're made. If they've got a UL listing
on them that might get you closer.

Since you have 2 transformers, a load test would be easy to do. Connect the
transformers so that you go 125:32 - 32:125, and try some lamps of 100 to
300W for a load. If you can't hold your hand on the transformer, it's too
hot. If you measure the wire diameter of the secondary winding, that should
give you a clew as to current capacity; work it backwards from some assumed
circular mils/amp.

Tam

 

[Terry Pinnell]

Yup; load 'em down and see how hot they get. Around 75°C lamination
temp with an ambient of 25°C is probably as hot as you want them to
get if you don't know how they're made.

In my thread 'Estimating transfomer current rating?' a couple of weeks
ago, in message
I found 53°C 'too hot to hold'. That was the recommended criterion
from several contributors. For future reference, are you sure 75°C
isn't too hot?

 

[legg]

In my thread 'Estimating transfomer current rating?' a couple of weeks
ago, in message
I found 53¡C 'too hot to hold'. That was the recommended criterion
from several contributors. For future reference, are you sure 75¡C
isn't too hot?

Safety agencies generally restrict absolute spot surface temperatures
of 'hand-hold' mechanical details to 55degC (ie delta35 in 20degree
lab; delta5 in 50deg ambient) in metal and 80degC (ie delta60 in
20degree lab) in plastic. 'Touchable' surfaces are permitted an extra
ten degrees.

Transformers are generally evaluated differently, as they are not
expected to be held or touched in normal use. A common commercial
safety spot temperature limit is 85degC. (35degree rise permitted in
50 degC ambient - produces 55deg spot temperatures in ~20degree lab
environment).

Thermometers are not outragiously expensive nowadays, and their use
can remove some ambiguity from evaluation of this sort. Confusion
between temperature 'rises' and absolute temperatures, and
device-specific standards can occur spontaneously, however.

RL

 

[Crumb]

Me? I plotted a bunch of different trannies on graph paper and got,
fairly consistantly, about 15 VA/pound, 30 VA/lb for torroids.

John

John;

is that primary or secondary VA? I would assume some secondary loss
due to inefficiencies et al. Where do these come into play?

Thanks

 

[John Larkin]

John;

is that primary or secondary VA? I would assume some secondary loss
due to inefficiencies et al. Where do these come into play?

Thanks

That's mfr's rated power output into a resistive load, just plotting
power vs weight for a bunch of catalog parts.

John

 

[BobGardner]

I found 53°C 'too hot to hold'. That was the recommended criterion

from several contributors.

===========================
My buddy Buzz used to say "If it doesn't leave skin, it isn't too hot"

 

[John Larkin]

===========================
My buddy Buzz used to say "If it doesn't leave skin, it isn't too hot"

If I can hold my finger on it for 10 seconds, it's 50C. 1 second, it's
60C.

John

 

[Winfield Hill]

John Larkin wrote...

If I can hold my finger on it for 10 seconds, it's 50C. 1 second,
it's 60C.

Yep, the finger tissue's long thermal time contant at work. But
in this test, what if the temperature is 70C, 80C, 90C or more?
What then? Hint: the nose knows. ... Whew!

 

[Crumb]

Me? I plotted a bunch of different trannies on graph paper and got,
fairly consistantly, about 15 VA/pound, 30 VA/lb for torroids.

Hello John;

I did some research on another transformer and found this:

http://pctransformer.com/web2/rectifier_power_trans_.htm

It appears that once a transformer gets so big, the VA/pound
increases. Check it out.

Thanks

 

[John Larkin]

Hello John;

I did some research on another transformer and found this:

http://pctransformer.com/web2/rectifier_power_trans_.htm

It appears that once a transformer gets so big, the VA/pound
increases. Check it out.

Thanks

Those transformers have 130C insulation, so maybe they can be pushed
hard. I just plotted some catalog items and got a fairly straight
line, roughly 15 va/lb, but the data points are fairly scattered. Good
laminations and good (high-temp) insulation can push the numbers up.
I'll post the plot on Tuesday, to a.b.s.e.

I'd suspect that the ratio should go down for big transformers, since
it's volume of iron and copper versus surface area for cooling. But
I'm just guessing here.

John