RATING POWER TRANSFORMERS QUESTION

[ampfab]

I found this in another thread for approximating power transformer current supply:

For a 120 volt AC supply the VA rating and primary resistance is as
follows.
30 VA = 30 to 40 ohms
50 VA = 13 to 16 ohms
80 VA = 7 to 9 ohms
120 VA = 5 to 6 ohms
160 VA = 2.5 to 3.5 ohms
225 VA = 1.8 to 2.2 ohms
300 VA = 1.0 to 1.3 ohms
500 VA = 0.45 to 0.55 ohms
Simply multiply all ohmage values by two for a 230 / 240 volt supply. Derate to 65%, which is probably reasonable and to allow 15VA for the filament windings.

The VA rating is for the primary, but after you subtract the allowance for the filament supply, you should have a good idea about the HT secondary. I hope this helps.

Could someone walk me through the equation? not sure correctly comprehend the math involved.
my transformer has a 550 ct secondary and measures 1.7 ohms across the primary coil. 120v power supply
please pardon my ignorance.
I think this was originally posted by Phil S.
thanks

[Reeltarded]

200 VA

It's a table of "close enough".

[ampfab]

Yeah, how would that translate to ma output?
I got about 200ma, if I'm doing it correctly.

[Reeltarded]

225 VA = 1.8 to 2.2 ohms

^^^

Going by known ratings and this table (thankfully supplied by PS, Mr. wonderful!) I rounded to the most likely rating. The rating is 200mA.

Who needs maths when you have smart friends.

btw, I am a theoretical arithmetician. According to my calculations roots aren't mean or square. What the heck..

[ampfab]

another stupid question,
a walk through the math,
-off the chart I chose the 225VA =1.8 to 2.2 ohms, because I measured 1.7 ohms.
-derated to 65% by multiplying 225VA x .65= 146VA
-subtracted 15VA for heater windings from 146VA = 130VA
-to convert VA to mA, 130VA/550v=.236A, (550v being my secondary voltage)
is this right??
sorry to beat a dead horse.
andy

[Reeltarded]

I'm no good at math and my signature is not directed at you.

(hugs)

[katopan]

That's correct for the maths. The other thing I'd want to check is that the core size is large enough compared to other similar rated PTs. Dimensional drawings on vendor sites are a good reference, like ClassicTone, Edcor or others.

I like the test of loading the PT up until the secondary voltages drop to 90% of their unloaded voltage. That gives you an upper limit to current draw.

[ampfab]

I found a copy of "Rating Unknown Power Transformers" by H.Q. Duguid from Electornics World magazine, july 1964. it's referenced in other posts. this is a nicer quality than i can find anywhere else online. Thanks to M. Holley at SWTPC.com

[katopan]

Very cool! Thanks for posting that.

[JazzGuitarGimp]

I found a copy of "Rating Unknown Power Transformers" by H.Q. Duguid from Electornics World magazine, july 1964. it's referenced in other posts. this is a nicer quality than i can find anywhere else online. Thanks to M. Holley at SWTPC.com

Wow! I was not aware South West Technical Products Corp has a presence on the web. I built a few of their kits back in the mid 70's. Their guitar preamp was cool and i used it along with a Fender Power Speaker all through high school.

[jazbo8]

Elliot has a few useful worksheets for transformer analysis >> http://sound.westhost.com/articles/xfmr3.htm, try 2.2 - Transformer Analysis 2 (Martin Czech), it seems to be pretty accurate.

[Phil_S]

FWIW, I can't take any credit. This was posted in another forum years ago when I asked the question. I've been using it ever since. I can't vouch for it's accuracy or origin, but it seems to be useful at least as a rule of thumb. I'm not sure I'd be willing to rely on it without some empirical testing for gathering more data. I'm just sharing free stuff. It's worth what you paid for it!

[R.G.]

The rules of thumb are good, and you should use them, but understand that they can't be perfect. This is because the power/current rating of a transformer is based on how hot the insides can get without damaging the internal insulation.

It happens to work out that for most but not all consumer moderate-power transformers, the materials and designs converge on a similar power-per-unit-weight. This is the underlying but usually unstated assumption in the calculators. The calculators at ESP do take into account the internal resistance and voltage sag for calculations, but I didn't see much about relating wire resistance to ma rating.

Transformer internal insulation "systems" come in several temperature ratings. The common ones for commercial/consumer transformers is Class A/105C and Class B/130C. These are the highest average temps the hot spot inside the middle of the trannie can stand on a continuous basis without shorting eventually. But there are systems up to at least Class H/180C and even beyond for special transformers.

A transformer will still work as a transformer even if you can fry eggs and bacon on the outside of it as long as the internal turns of wire do not get shorted. This works up to the curie point of iron, about 770F. The insulation won't go that high, usually.

It is true that higher-resistance windings do produce more heat from I-squared-R losses, but additional heat comes from the eddy current losses in the core material. Those depend on how hard the iron is being driven by the input AC voltage, what the iron metallic content is, and how thin the laminations are to foil eddy currents.

Again, economics steps in and makes most common commercial transformers be 4% (or so) silicon alloy iron, and today most iron is grain oriented and a modest, but not terribly thin lamination. So the designs converge to economically feasible materials.

Again, use the rules of thumb and calculators for - well, rules of thumb. But be aware that the real limit on a transformer is thermal, and the real test of whether it works in an application is how hot it gets inside.

And that's a *real* rule of thumb - well, OK, rule of finger. It happens as a happy accident that most humans won't hold their fingertip on a metal surface that's over 130F, and that happens to be about the *external* temperature of a common/commercial transformer with Class A or Class B insulation at an internal temp of about 105C. So if you can hold your fingertip on the surface when the transformer is fully warmed up and running at full power, then it's pretty sure to not be overloaded, and is suitable for the job at hand.

There are some more accurate guess-timates that can be made, but this one is really useful.

[JMFahey]

And that's a *real* rule of thumb - well, OK, rule of finger. It happens as a happy accident that most humans won't hold their fingertip on a metal surface that's over 130F, and that happens to be about the *external* temperature of a common/commercial transformer with Class A or Class B insulation at an internal temp of about 105C. So if you can hold your fingertip on the surface when the transformer is fully warmed up and running at full power, then it's pretty sure to not be overloaded, and is suitable for the job at hand.

There are some more accurate guess-timates that can be made, but this one is really useful.

You bet and thanks for posting

And it's not a happy accident: 130F amounts to 55C .
Considering that meat gets cooked at 60C which means protein is coagulated and no life is possible, no matter what, and the process is not reversible, a couple Million years of Evolution has given us a very effective thermal alarm known as pain somewhat below that.

Of course, what matters is actual temperature at the thermal sensor, the nerve ending inside the finger, so surface temperature can be slightly higher.

Even red hot takes 2 or 3 seconds to be felt , you first hear the "hisssssss" before jumping in pain, because there's a small time constant / delay between burning skin and actually reaching the under surface nerve.

That's why touching a transformer (or a heat sink) you can only pronounce it "safe" if you can hold your finger there for, say, 10 or 20 seconds at least; just lightly brushing or tapping with your finger for a second is not long enough.

[Phil_S]

I've got one inexpensive DMM that came with a temperature probe and has a temperature setting. I much prefer that to my finger.

If breakdown of the insulating varnish is the tipping point, you can get well above temperatures you can touch and still not melt it. I'd say you want to be less than about 90% of the boiling point of water at the core. I suppose that translates to something less at the outer edge of the lams. My only point is that I think folks are being very conservative here.