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Old 24th May 2004, 02:21 PM   #1
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Default Transformer power, how much may i expect?

Old days (sixties), I learned that you only need to take a look in some transformer, to evaluate its maximum current capacity.

They said:

- "When the transformer shape is a square (a=b), and constructed with "E" shape metal blades holded together, try to calculate the center leg área, looking the unit from the top". (scratch is going)

- "You can expect 1 ampere each squared inch, as a MAXIMUM current capacity. In this situation you can expect loose 20 percent AC voltage when 1 ampere flowed into a charge"

This way, a 20 AC volts transformer, 1 squared inch area , will result 16 AC volts when 1 ampere flows into charge... DC will be around 22.5 when in charge... and something around 28VDC when not in charge.

But they told also, this evaluation will work if:

A) Very good (those old days) Magnetic Field Properties of the metal blades used.

B) Perfect Ratio from Primary to Secondary coils.

C)The biggest possible wire sectional área in the secondary

D) And the temperature, when no charge draining current, will be stable in a matter of 10 minutes, and a little bit higher than surround air temperature.

- They said too:

"Always expect losses when evaluate with "just see it" method"

The bigger the transformer, bigger current capacity.

In real world, daily sittuations, this method sometimes give a 40 percent errors, so, can be plus or less 20 percent correction factor.

This sittuation make sense to you?

Can i be more precise, without using complicated formulas?

Those old transformers are still in use in Brazil, people likes those transformers because they are shielded and pretty too. toroidal is not used here, we all know they are better, but not been produced here, do not know why. We have whole needed technologies, from automobiles to modern micro processors, but, do not know why.

Thank you all the attention.

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Old 24th May 2004, 02:48 PM   #2
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Jocko once posted a diagram for estimating the VA rating of
transformers. If you are lucky you might succeed finding it
if searching the forum.

Otherwise, according to wire dimensioning info I have, one
should not go higher than about 3A per sq. mm when using
wire in transformers. Measuring the diameter of the secondary
wires and applying that rule should give a safe max rating
for the current, I suppose. I wouldn't be surprised if many
commecial transformers go above that figure.
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Old 24th May 2004, 08:10 PM   #3
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Default Thank you Christer

May be evolution, better magnetic materials.

I heard somewhere that squared sectional wires are beeing used to ocuupied whole space, without intersticial loosen space between rouden wires, air space.


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Old 24th May 2004, 11:10 PM   #4
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Hi Carlos!
I think these rules might have been suitfull for a certain limited power range and also a certain range of output voltage.

If you go to higher Power, you will get a lower internal resistance,
so a 1kVA Transformer designed for 20V/50A will not drop from
20VAC to 16VAC if loaded with 1A.
If we look at the output voltage it becomes obvious that
a 100VA transformer will be able to handle about 5A if there is
just one secondary winding delivering 20V output voltage.
If you pick a 100VA transformer with a 40V secondary winding, then the current capability will only be 2.5A.

For these simple 50Hz/60Hz transformers the size is a pretty good
indicatpr for the possible power. I think you will find some tables in the WEB about the power rating for any popular shape.

My feeling is that these old rules will still work if you know their limits.
Magnetic cores for 50Hz/60Hz did not change very much during the last 30 years. ...thin plates of iron with some small percentage of silicone... (normally not more than 3-4% silicone, because the material would become to brittle..)
The silicone increases the electrical resistance of the core itself and keeps the eddy currents in the core low. For the same reason the core is not a block but a stack of isolated plates.
Allowed flux density did also not change very much.
Material for the windings is still the same copper.

The most obvious evolution in these transformers is that people
tend to allow higher temperatures..., but I am not aware
of further fundamental news.

Rectangular wires are nice, but more expensive.
Only used in high power or very expensive applications....

So don't worry. I think you are simply wondering about your old rules, because you start to apply them for more different transformers than they for what these rules were intended.
And with this your real results also differ more & more vs the rules.

Christer gave a rule thumb for the allowed current density in the wires. 3A per sq. mm should be fine for transformers in the range
of some hundred W.
The limit of the allowed current density is mainly of thermal nature.
For small sizes you can allow higher current densities than in large transformers, because the outer surface area of the entire transformer (results in certain thermal resitance and ability to dissipate heat) is increasing by the square of the size. But the volume (which is proportional to the generated heat at a certain level current density) grows cubic.....
Also the geometric shape of the design affects the allowed current
density. Typically you can allow higher current densities in toroids than in a similar sized E-core.
And of course if you go for forced oil cooling systems in some
high power transformers then you can also allow very high current densities....

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Old 25th May 2004, 01:29 AM   #5
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Default Thank you Markus

Another great contribution to my knowledge.

Hundred percent understood.


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Old 25th May 2004, 02:37 AM   #6
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Tranformer power is proportional to the weight if you refer to the same type of transformer and the same frequency so you may compare to a catalog or a chart from another company for the same type of tranformer an you will get a fair evaluation.
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Old 25th May 2004, 05:15 PM   #7
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Default Yes audiofan, just checking old ideas

I already realise that will not working anymore...but i tryed to find new "easy way" to evaluate.

And also, of course, let the one that really knows the thing, to explain me the correct thing.

If you now everything, no one will teach you nothing.... we must show our weeknesses because they will run to help us.

If you are proud of your knowledge, will be forgot.

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Old 25th May 2004, 08:24 PM   #8
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Hi Carlos!

...are you sure about your signature?
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