Power Transformer Temp

[wdcw]

I have a small power transformer (Primary 240v -Secondary 240V & 6.3V)

I needed to make sure that the small transformer will not get to hot & burn out for the small amp I want to build, so I tested it.

Could you have a read of how I tested & worked the transformer temperature out to make sure I have done it correctly.
I am new at this & want to make sure.


Firstly I ran a circuit simulation & the current draw fully loaded on the secondary winding in the simulation is 200mA with a bridge rectifier WITHOUT THE HEATER CURRENT ADDED.

My initial thoughts were that the transformer was to small to run the heaters as well so I will use another small transformer for the heaters.



With the aid of a Variac & a variable resistance I set things up to test the temperature rise in the transformer with 250v on the Primary winding--200mA current draw on the secondary winding.

I used 250V instead of 240V on the primary side to over do it a little.

I tested the loaded transformer for three hours & the math & results are as follows.

To-Initial Temp = 16.2 Deg Celcius--Ambient
Ro-Initial wire resistance = 50.6 Ohms
a- Temp Coefficient of Copper = 0.00393/C deg.
R- Hot wire Resistance after 3Hrs = 59 Ohms

T = R-Ro/a*Ro+To
= 59-50.6 / 0.00393 * 50.6 + 16.2
= 58.44 Deg Celcius

I then did the math for a 30 degree Celcius ambient day as it was only 16.2 deg C here at the moment.
The figures suggest that the only difference in temperature rise is the difference between the 16.2 degree ambient & the 30 degree ambient temperature which of course is 13.6 degrees Celcius making the transformer temperature approx 72.deg Celcius on a hot day.

Is this correct, have I tackled this properly?

I have read that up to 100 deg Celcius is ok for a power transformer, is this correct?

Cheers.

[Tony]

patrick turner has a very educational site: powersupplies

it is not only the copper losses to consider, the iron used has a certain loss in watts per kg...

the surface area of your traffo also come into play.....

[DualTriode]

Hello,
Some insights, simplifications and rules of thumb (from my opinion)
1. Amplifier manufactures try to get the most performance for the money.
2. Transformers may be limited by voltage drop (regulation) or temperature gain. Small VA transformers the limiting factor tends to be regulation.
3. The VA rating of a transformer is based on an AC load. Operate a transformer into a rectified DC load and the VA rating no longer applies or as I think of it a derating needs to apply.
4. For a rectified DC loaded transformer I apply a minimum a 50% reduction to the name plate VA.
5. For protection of me, the cats and grand children 140 F is my maximum operating temperature. Fluke makes a nice hand held infrared thermometer.
6. To avoid coupling nasty rectifier (from DC filament supply) spikes and noise into the B+ Install separate filament and B+ transformers.
DT
All just for fun!

[wdcw]

Thanks Guy's

I understand what your saying but am a little confused about the use of an infrared thermometer. This only gives an external core temperature measurement & not the wire temp or the core temp in the centre of the transformer which is hotter than the external sections.

When I did my test on the transformer as above the surface temp of the iron core was only 44deg Celcius.
But by calculating the wire resistance cold & hot as above gives a true temperature measurement of the centre core & the wire which was 58.44 deg Celcius, quite a bit hotter than the external temp of 44deg Celcius.
The 44 deg surface temp was measured with a thermocouple.

If the external core temp was 140 deg F--60deg C then the centre wire & core temp would be at a greater temperature.

That is why I thought it necessary to calculate the wire & centre core temp using resistance.
Doesn't this give a more accurate account of what the temperature is inside the transformer which is where it counts?


Cheers

[Conrad Hoffman]

I believe you've done it correctly, though I haven't worked through this in years. It works for anything with copper windings, speakers, motors etc. What I don't know is how hot the core should be allowed to run. I've seen poorly designed audio power transformers run too hot to touch, which suggests very high internal temps. 100C under worst case conditions seems reasonable but I'm just some joker on the internet. Possibly you could get internal temperature specs from a manufacturer. These people are very good with custom designs and might give you a number if you asked.

[DualTriode]

Originally Posted by wdcw

Thanks Guy's

I understand what your saying but am a little confused about the use of an infrared thermometer. This only gives an external core temperature measurement & not the wire temp or the core temp in the centre of the transformer which is hotter than the external sections.

When I did my test on the transformer as above the surface temp of the iron core was only 44deg Celcius.
But by calculating the wire resistance cold & hot as above gives a true temperature measurement of the centre core & the wire which was 58.44 deg Celcius, quite a bit hotter than the external temp of 44deg Celcius.
The 44 deg surface temp was measured with a thermocouple.

If the external core temp was 140 deg F--60deg C then the centre wire & core temp would be at a greater temperature.

That is why I thought it necessary to calculate the wire & centre core temp using resistance.
Doesn't this give a more accurate account of what the temperature is inside the transformer which is where it counts?


Cheers

Hello,
Heat like water goes downhill. Sure the inside is hotter than the inside, but you cannot measure that. The manufacture most likely will wrap a thermalcouple in the windings of a sample like a motor manufacture winds sensors for thermal overload. You can guestimate (calculate) if you like. My reason for 140 F is burn protection. It takes more than a few seconds to burn flesh at 140 F, enough time to give a warning before you blister.
For the small transformers used for our audio applications they are voltage limited before they are temperature limited. That means as current goes up the voltage will drop below nameplate long before they are hot enough to be damaged by heat.
DT
All just for fun!

[wdcw]

Thanks again for the replies,

DualTriode,

I must read your comments wrong

Heat never goes downhill, Heat rises & in an overloaded Transformer it will get to the stage of a meltdown if you let it.

Yes of course it's difficult to measure the internal core temp of a transformer but you can calculate it closely, within reason, that's how they design transformers, by calculation like everything else is designed.

By reading the external temperature of a Transformer tells us nothing about what the temp is internally where it counts.
I just read an article by an engineer who said that the temperature rise in a transformer has nothing to do with the external temperature at all.

He made a good statement & said you can liken it to a light bulb where the external surface maybe be touchable but the filament may be 2000 deg inside.

For this reason I would like to learn more about the transformers as they are the heart & soul our beloved Tube amps.

I believe it's not much good building something if you don't know how it works, especially considering that the power transformer is the first step in any amp build.

Surely there is someone here who can shed some light on this subject, I can't believe that everybody builds Tube amps & can't get past the first step?

Any help would be appreciated.

Cheers

[DualTriode]

Hello wdcw,
We are looking at your how hot will a transformer get question differently.
A transformer is overloaded when you exceed the name plate VA, VA is volts times amps, watts in a perfect world. In a lab apply a load that does not exceed the VA rating of a given transformer the internal temperature will rise and the heat will transfer “downhill” to the exterior. Given standard test conditions the manufacture will know very closely what all the operating conditions will be including; inside temperature and surface temperature.
Apply goofy loads like switch mode power supplies or bridge rectifiers and all bets as far as VA rating are off. That is why I recommend derating the name plate VA to 50%. Look up transformer “K” ratings for fun.
For our audio transformers with a steady load if you can put your hand on the on the outside nothing is cooking on the inside. Yes it will be hotter in there.
First step:
Add up all your currents multiply by the output voltage (rectified DC) of the transformer multiply times 2 and that is a safe bet for the minimum VA of the required transformer.
DT
All just for fun!

[Hi-Q]

My own Ecel spreadsheet program gives the following:-

Resistance Cold = 50.6
Resistance Hot = 59
% Rise (Ideal Maximum 20%) = 16.60079051
Temp increase in Deg C = 42.24119724
(Ideal Maximum 50 Deg C)
Maximum Resistance allowed = 60.5429

So you may be OK with your transformer.
Les

[wdcw]

Thanks for the replies once again,

Gday DT,

Yes I see what your saying, I have been trying to get a better understanding as I have a lot of older power transformers that have no Name Plates or manufacturers stamps.
I can only go by the actual core dimensions to get a rough idea on the VA rating of each, hence the reason for wanting to test each one first for a given load.
Best to be safe than sorry & it's a good learning process.
Thanks for the help!

Gday Les,

Yes that's what I have been trying to get a handle on, the maximum resistance allowed as this is a very informative figure.
I have made a very simillar spreadsheet but I was unable to decide exactly what figure to put on the maximum resistance allowed, I thought maybe a maximum of 30% increase would be ok?.

From the figures I got from testing & your spreadsheet figures it looks like this transformer may me just ok, the thing that worried me was that the ambient temp was only 16.2 Deg C when tested.
I am concerened that with higher ambient temperatures say 30deg C that this transformer may start to get to hot as the resistance will of course increase with a thermal increase.

I guess the best way to find out for sure is to warm the sucker up to 30deg or so & run the tests again to make sure.

Thanks again for the help everybody.

Cheers