transformer thermals

In article <5h5kifpi9csk1ps3r90q0ht7srn3qf1ucj@4ax.com>, 
jlarkin@highlandsniptechnology.com says...
> 
> We can sense the primary current of this transformer, with a shunt and
> an isolated delta-sigma ADC. The FPGA squares the samples and filters,
> so we can pick that up and square root to get RMS current. The tranny
> is rated for 240 VA, which would be 9.6 amps RMS in the primary.
> 
> So I ran it for a few hours with 10 amps DC in the primary. Temp rise
> was about 26C in free air. I think people design transformers for
> equal copper loss in the primary and secondary, so temp rise would
> double when loaded in the system. But wafting a little air over it
> cuts the rise in half or so, so I'm back to something like 25 c rise
> in real life, where there will be lots of air. So we'll set the
> software shutdown at 12 amps maybe.
> 
> https://www.dropbox.com/s/ylm8dc1e14dwv7y/P900_Xfmr_Thermal.jpg?raw=1
> 
> https://www.dropbox.com/s/5b55ybfoq2pkp9v/P900_Xfmr_Thermal_2.jpg?raw=1

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.

On Wed, 05 Aug 2020 13:21:31 -0700, boB <boB@K7IQ.com> wrote:

>On Tue, 04 Aug 2020 19:35:39 -0700, jlarkin@highlandsniptechnology.com
>wrote:
>
>>
>>We can sense the primary current of this transformer, with a shunt and
>>an isolated delta-sigma ADC. The FPGA squares the samples and filters,
>>so we can pick that up and square root to get RMS current. The tranny
>>is rated for 240 VA, which would be 9.6 amps RMS in the primary.
>>
>>So I ran it for a few hours with 10 amps DC in the primary. Temp rise
>>was about 26C in free air. I think people design transformers for
>>equal copper loss in the primary and secondary, so temp rise would
>>double when loaded in the system. But wafting a little air over it
>>cuts the rise in half or so, so I'm back to something like 25 c rise
>>in real life, where there will be lots of air. So we'll set the
>>software shutdown at 12 amps maybe.
>>
>>https://www.dropbox.com/s/ylm8dc1e14dwv7y/P900_Xfmr_Thermal.jpg?raw=1
>>
>>https://www.dropbox.com/s/5b55ybfoq2pkp9v/P900_Xfmr_Thermal_2.jpg?raw=1
>
>
>
>Why aren't  you measuring  this transformer using AC ? 

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

>
> It is going to be hotter with AC.

Hardly any.

>
>DC will certainly give you a best case reference though.
>

jlarkin@highlandsniptechnology.com wrote:

> I think people design transformers for
> equal copper loss in the primary and secondary

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

boB wrote:

----------
> 
> 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.
>

** 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 

On Wed, 5 Aug 2020 23:32:02 +0200, Piotr Wyderski
<peter.pan@neverland.mil> wrote:

>jlarkin@highlandsniptechnology.com wrote:
>
>> I think people design transformers for
>> equal copper loss in the primary and secondary
>
>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

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.

On Wed, 5 Aug 2020 16:33:48 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

>boB wrote:
>
>----------
>> 
>> 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.
>>
>
>** 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 

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.

https://www.amazon.com/gp/product/B00080G0BK/ref=ppx_yo_dt_b_search_asin_image?ie=UTF8&psc=1

Really cool gadget.

 Piotr Wyderski wrote:

===============================
> > 
> > I think people design transformers for
> > equal copper loss in the primary and secondary
> 
> 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.
>

** 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 

On Wednesday, August 5, 2020 at 1:45:36 PM UTC-7, John Larkin wrote:
> On Wed, 05 Aug 2020 13:21:31 -0700, boB <boB@K7IQ.com> wrote:
> 
> >On Tue, 04 Aug 2020 19:35:39 -0700, jlarkin@highlandsniptechnology.com
> >wrote:
> >
> >>
> >>We can sense the primary current of this transformer, with a shunt and
> >>an isolated delta-sigma ADC....
> >>So I ran it for a few hours with 10 amps DC in the primary. Temp rise
> >>was about 26C in free air.

> >Why aren't  you measuring  this transformer using AC ? 
> 
> Because there's a 10-amp DC supply on the bench.
> 
> >
> > It is going to be hotter with AC.
> 
> Hardly any.

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...

whit3rd is Witless wrote:

-------------------------
> 
> > > It is going to be hotter with AC.
> > 
> > Hardly any.
> 
> Iron lossses due to eddy currents and hysteresis aren't insignificant.  
>

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


> It this for variable frequency use?
> Those iron losses are frequency-dependent.
> it might take a bunch of testing to characterize.

** 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 ? 

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

** So what ? 


....  Phil 

On Wed, 5 Aug 2020 18:01:49 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

>On Wednesday, August 5, 2020 at 1:45:36 PM UTC-7, John Larkin wrote:
>> On Wed, 05 Aug 2020 13:21:31 -0700, boB <boB@K7IQ.com> wrote:
>> 
>> >On Tue, 04 Aug 2020 19:35:39 -0700, jlarkin@highlandsniptechnology.com
>> >wrote:
>> >
>> >>
>> >>We can sense the primary current of this transformer, with a shunt and
>> >>an isolated delta-sigma ADC....
>> >>So I ran it for a few hours with 10 amps DC in the primary. Temp rise
>> >>was about 26C in free air.
>
>> >Why aren't  you measuring  this transformer using AC ? 
>> 
>> Because there's a 10-amp DC supply on the bench.
>> 
>> >
>> > It is going to be hotter with AC.
>> 
>> Hardly any.
>
>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.

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