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Old 30th Jan 2021, 6:17 pm   #1
Bazz4CQJ
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Default Getting a VA Rating from a Transformer’s Mass

There was a recent thread asking for advice about getting the VA rating of a transformer, and a comment was made about VA being related to mass.

I wondered how accurate that might be, especially as applied to transformers from different eras, different grades of quality, some low voltage and some combined LT and HT transformers.

So I weighed 8 transformers, which had good VA data on them, and the results are attached. Blue data point are for low voltage only (<20V), red data are for combined LT+HT transformers. I’ve never seen such a plot before, and it does seem to be that the correlation, allowing for all the "incidental" variables involved, is pretty good.

B
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Old 30th Jan 2021, 6:35 pm   #2
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Default Re: Getting a VA Rating from a Transformer’s Mass

Interesting point Bazz, and I'm certain that I'm not the only one here who's often looked at a second-hand/anonymous NOS mains transformer and thought, now what can I reasonably get from this? I suppose that there will be outliers in expensive specialist types with exotic magnetics and particularly cheap'n'nasty ones that run hot but low duty-cycle for ephemeral consumer electronics but most run-of-the-mill types presumably run in a middle ground. After all, ISTR even that copper and steel have similar densities (copper typically 10% higher), so even an individual design bias toward one or the other won't throw things too far off track.

As Mr. Scott would say, ye cannae change the laws of physics....

Colin
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Old 30th Jan 2021, 6:52 pm   #3
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Default Re: Getting a VA Rating from a Transformer’s Mass

Fascinating.... and interestingly linear!

I wonder what effect 'modern' high-temperature-rated winding insulation would have when compared to older enamels? I've dealt with some modern transformers which were rated for continuous-duty at a core temperature of 150 Centigrade, which would definitely be life-shortening for the enamel on a 60-year-old transformer.

[150 Centigrade isn't that hot when you think about a power-transformer living in a fully-enclosed amplifier case with a couple of KT66 and a 5U4G a few inches away]

Equally, open-frame vs fully-enclosed, and potted vs unpotted or oil-filled must have significant influence.
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Old 30th Jan 2021, 7:02 pm   #4
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Default Re: Getting a VA Rating from a Transformer’s Mass

Thanks Bazz for taking the time, much appreciated. In the past I have used my own rule of thumb of V difference between no load and loaded voltage being a drop of 10% to give ESTIMATED rating. When I can (mobility being the hurdle at the moment!) I will have a dig around some txfmrs I have and post the figures. Some are RS so should be accurate on V & I ratings.
No doubt, Ed Dinning will provide some useful input to this thread.
Chokes can be another problem to estimate as well.
Your chart is I imagine ignoring items like "Admiralty Pattern" items that also served as anchors, usually so massively over-rated.

There was an offered post recently of a transformer said to weigh 1.4kg that the ratings plate showed a combined output of almost 400VA.
https://www.vintage-radio.net/forum/...d.php?t=174989
Although lowish voltages, this is a very long way off from your graph for some reason?

Rob
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Old 30th Jan 2021, 8:08 pm   #5
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Arrow Re: Getting a VA Rating from a Transformer’s Mass

As a rule of thumb, I've always used VA = 26*A², where A is the cross-sectional area of the core in sq. in.

Al.
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Old 30th Jan 2021, 8:19 pm   #6
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Default Re: Getting a VA Rating from a Transformer’s Mass

Thanks you for doing this, intersting but (in good old hindsight) not unexpected, transformer manufactures don't waste money! I bet 60Hz ones are a bit lighter and audio where they may go down to 20Hz a bit heavier (well for good ones anyway).
 
Old 30th Jan 2021, 11:03 pm   #7
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Default Re: Getting a VA Rating from a Transformer’s Mass

Well, it kicked off because I was searching the junk box for a 12V transformer to give me ~2A, and there was one there but with no markings on.

Among the low volts transformers in the survey there's an RS one which I think would be 60's or 70's, a couple of newish ones from the far east and the smallest two are both recent encapsulated ones for PCB use. The two LH/HT transformers could be 50's or 60's, and I did not expect them to fall in to the same data set.

I too was quite surprised there isn't more scatter, but they were chosen simply on the basis that they were sitting in a drawer and not mounted to a chassis and they were all very clearly marked, so really quite random.

I might add my 25kg transformer at a later date; that's the heaviest one in stock (something like 40V at 25A I think), from a PSU found in a skip... it just needed a new internal fuse .

B
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Old 31st Jan 2021, 3:31 am   #8
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Default Re: Getting a VA Rating from a Transformer’s Mass

The one linked to in post#4 is a "C" core design.
Torroid will also differ.
They should all follow the weight/VA relationship if they are the same core type.
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Old 31st Jan 2021, 6:05 am   #9
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Default Re: Getting a VA Rating from a Transformer’s Mass

There is a section on exactly this for SMPS magnetics in one of Linear Technology inc's early applications notes. It includes a photo of a supermarket checkout labelled "A local inductor power assessment center"

It was only partially jocular, the author was quite serious about problems with the clueless selecting inductors purely on inductance, finding the smallest in that value, and then complaining that the SMPS chip didn't work.

That author was Jim Williams, one of the legends of Silicon Valley.

David
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Old 31st Jan 2021, 8:33 am   #10
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Default Re: Getting a VA Rating from a Transformer’s Mass

There's another related way, measure the tfmr, then lookup similar offerings from a tfmr manufacturer. This applies to toroid cores, EG if you have an unknown toroid core, weigh it, double it, then have a look in a tfmr catalogue.

Andy.
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Old 31st Jan 2021, 3:59 pm   #11
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Default Re: Getting a VA Rating from a Transformer’s Mass

Hi Robinshack

Just an observation. I notice you say that "Admiralty Pattern" transformers are over-rated, do you mean under rated? In my experience they can be run continuously at well over the marked rating and just get slightly warm, particularly those made in the 40ies & 50ies. These were usually made to a very high standard and were both impregnated and enveloped. This was before they moved to"c" cores and oil filled potting. [but even these could be used as anchors or ballast!]

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Old 21st Feb 2021, 4:53 pm   #12
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Arrow Re: Getting a VA Rating from a Transformer’s Mass

Bazz: the red and blue lines of your graph are very nearly on the same straight line. Assuming that line is indeed the same straight line, a few minutes arithmetic gives the relationship between VA and mass as approx. 0.037 VA per gm.
That could be a useful rule of thumb when trying to select a xfmr. for a given need.

Al.
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Old 21st Feb 2021, 5:25 pm   #13
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Default Re: Getting a VA Rating from a Transformer’s Mass

Hi Al,
I plotted the group of small LT transformers (mostly quite modern ones) on a separate line from the old HT/LT transformers expecting them to be significantly different, but as you can see the two lines are pretty close and could reasonably be plotted as a single group.

The line over the (blue) LT data in the linear best fit, and provides that equation y=24.6x + 40, where y is mass and x is VA, so the reciprocal of 24.6 is 0.041, pretty close to your 0.037 figure. For very small transformers, it might be best to use the equation of the line as it's not a line that passes through the origin.

I did take notice of you suggestion about relating core cross section to VA, and had a quick try with that, but I wasn't seeing a clear relationship.

B
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Old 22nd Feb 2021, 12:24 pm   #14
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Default Re: Getting a VA Rating from a Transformer’s Mass

All the chokes, mains and output transformers I have made were designed using a small book called "Coil Design and Construction Manual. Power transformer design starts at Chapter 4 page 31 and there are various tables to work out the core section and the recommended turns per volt.
Please see attached PDF file which has been expanded to fill an A4 sheet of paper to make it easier to read for me.
Regards Stan.
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Old 22nd Feb 2021, 1:24 pm   #15
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Default Re: Getting a VA Rating from a Transformer’s Mass

Orakle42, yes, my mistake, I did mean under rated.

Bazz, at last I am getting more mobile and just brought three of my old Radiospares ones in to weigh.
The published ratings are somewhat different to your graph.
I wonder if the lower Radiospares published va total is due to being under rated?
The Standard @ 2300g is a long way from your graph and the Economy is closer.
BTW, an older Economy in semi shrouded form is 10g lighter than my newer open frame one.

Here are the published ratings.
The" Economy" open frame version @ 1627g
250 0 250 @ 75mA (18.75va),
6.3 @ 2A (12.6va)
and 0 - 5 @2A - 6.3 @1A (10va OR 6.3va)
Max 40.35va

The "Standard" semi shrouded with mains tap plug 2300g
250 0 250 @100mA 25va
6.3 @ 2.5A 15.75va
0 - 5 @2A -6.3 @ 1A (10 va OR 6.3va)
Max va 50.75

Rob
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Old 22nd Feb 2021, 2:17 pm   #16
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Default Re: Getting a VA Rating from a Transformer’s Mass

I measured 3 different manufacturer toroidal ones lying around:

120VA 1.3Kg
160VA 1.53Kg
240VA 2.0Kg

Again seem to be linear but with a different constant to yours.
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Old 25th Feb 2021, 8:35 pm   #17
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Default Re: Getting a VA Rating from a Transformer’s Mass

Quote:
Originally Posted by Bazz4CQJ View Post
There was a recent thread asking for advice about getting the VA rating of a transformer, and a comment was made about VA being related to mass.

Without going OT - ain't this weight, rather than mass? I'm on planet earth.

I note you know a world more about these two things - working as you have in how to get things off the ground and keep them there - than I ever will, but I'm curious about your choice of terminology in this instance.
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