transformer thermals

...and the metal plate is supposed to simulate a chassis mounting?

A vibration/stress mounting will involve rubber gaskets, which will actually increase Rth, and reduce surface area.

If you're using fans, you're using fans.

RL

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** Better way to monitor temp rise is via the copper resistance - as recommended in EU and other standards.

Rise in C = 256 x ( Rhot/Rcold -1 )

A cool breeze can easily double the VA rating of a toroidal tranny.

.... Phil

It keeps from burning the bench if the transformer smokes.

Think so? It should reduce theta. Most anything conducts heat better than air.

That's profound. Let me think that over.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

Still air conducts; moving air convects. Almost anything solid convects poorly compared to air.

Moving heat is NOT a story of conduction alone.

What about the heat generated by losses in the secondary?

You can even see on the label (looks sort of like a Noratel) the input is

266VA and output is rated 240VA. So depending on power factor at full load, it could be upto 26 watts of loss, although I doubt a toroid that size would have such a poor efficiency. The iron losses will surely be less than 50% on a toroid. Maybe the maker can tell you the ratio?

Cydrome Leader is Funny:

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** Doubles the temp rise - as JL just claimed. ** PF has no effect - VA is all that matters, effectively just the RMS current. Iron losses are a watt or so per kg or iron. I mag is tiny.

So 20 watts copper loss, 6 watts for iron.

Regulation about 8%.

Toriods are very simple.

.... Phil

This transformer was made for us by Amgis. I specified it so I know the ratios: 1 : 1.4 : 1.4 : 1.4 : 1.4. We have a relay board that switches the secondaries to get four output voltage ranges.

Unloaded, the AC operating primary current is essentially zero, so I don't think core loss is significant.

As an alternator simulator, voltage increases with frequency, which keeps Imag low on the low end.

It's a weird application. We specialize in weird.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

Is that why you reply to Rickit?

Why aren't you measuring this transformer using AC ?

It is going to be hotter with AC.

DC will certainly give you a best case reference though.

That reminds me of when I was a schoolboy my Dad got me inolved in rewinding a transformer to get 20V AC from 240V mains. I don't remember what the original secondary rating was.

This was ostensibly to run some 20V decorative candle bulbs for pretend safe candles for the upcoming Christmas. Only when presents were opened I discovered that the real reason was to power a 20V Meccano electric motor (which I still have ~65 years later.)

Anyway, the relevance was that we put back only ~ half of the laminations before trying it so see whether our calculation of the turns-per-volt was correct. When we got 20V AC I went rushing off to tell my Mum of our success (I was only junior!). Only to be called back by my Dad shouting that the transformer was smoking...

After a pow-wow, we decided that maybe all the laminations were needed to keep the losses within bounds, and finished re-building the core. That transfomer gave years of service subsequently, and was not the only one I built or rebuilt. I even used my Meccano to build a coil-winding machine!

Mike.

Because there's a 10-amp DC supply on the bench.

Hardly any.

BTW, what is the source of this and similar rules of thumb (e.g. equal copper and core losses)? Logic says that one should always design for minimal total losses, given the economic constraints.

Best regards, Piotr

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** The standard test involves shorting the secondary with an amp meter and applying enough AC volts to the primary to get the rated current.

This will, after a time, establish the temp rise and copper losses.

DC only heats one winding and needs an expensive supply.

JL likes to do things the hard way.

.... Phil

I don't think transformers are usually designed for equal core and copper losses. This one runs cold at full AC voltage but no load.

They are designed for equal primary and secondary copper losses at full load, I think.

Cooling depends on surface area. You get more surface area by adding more copper, and that relationship is not linear (the volume-surface thing, like mice and elephants) so core loss might require a lot of expensive copper.

The 10-amp DC supply was right there on the bench, plugged in even. We may have an old Variac somewhere in the basement, but I haven't seen it in years.

The DC measurement told me what I wanted to know. It wasn't hard.

The fan has a USB cable for power, and I didn't have a USB power supply, but the oscilloscope has a memory stick connector on the front, so I used it to run the fan. The scope was also right there, plugged in too.

Really cool gadget.

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** The source is simple calculus that finds the minimum or maximum in a curve. It's also kinda obvious that if the primary and secondary run at different temps then you have one heating the other. Same goes for core and windings.

The issue with tape wound toriodals is their very sharp saturation curves - forcing the designer to keep away from that condition. Hence their low contribution to heat from the core.

Commercial toroidals are made using a clever machine passing wire through the centre hole - this sets a limit on the amount of copper that can be used.

I know of one amplifier designer who at my suggestion had the winder use his machine for the primary and wind the secondary by hand in order to fill the hole almost completely. This almost doubled the amount of copper used.

Along with a bit of fan cooling, the result was a 3kW rated amplfier that used a 1kW size transformer core.

..... Phil

Iron lossses due to eddy currents and hysteresis aren't insignificant. It this for variable frequency use? Those iron losses are frequency-dependent. it might take a bunch of testing to characterize.

Early switchmode power supplies occasionally heatsinked the transformer cores...

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** They are with GOSS tape wound into a toroid.

** No it wont.

JL's tranny is rated for 50/60Hz and will work fine with lower losses at any higher frequency. Core magnetisation goes DOWN with rising frequency for a given primary voltage.

Never seen an audio output tranny in your entire life ?

** So what ?

.... Phil

200 Hz to 4K. This simulates a 6-pole PM alternator on a jet engine. Primary current is very low when the transformer is unloaded.

It's a good transformer.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard