[ARC5] Slightly OT: Transformers

Thu Nov 28 15:10:24 EST 2013

And one never knows the transformer history if it is a recent find so there 
may also be moisture inside.

Small ones can be baked in a toaster or larger oven.

For "big ones" I short the secondary with an ampmeter in series and with a 
metered Variac on the primary slowly bring it up to guesstimated half the 
kVA and let it cook and drive the moisture out. Both windings are fused 
since I have left them run for several days to a week at times. My 225# 
6000/7500VCT 1946 monster was done that way after spending about 40 years in 
an old mill building with broken windows and then a shed. After a week 
cooking in the garage it was slowly brought to full voltage while watching 
for anything funny, and then left that way another 8 hours while metering 
the magnetizing current. It was then brought up to temperature again and 
then dipped into a large container filled with transformer varnish, hoisted 
out and left there to drip and dry.

The next step was using the same engine hoist to lift onto a heavy duty 4 
wheel dolly, towing to the house bulkhead with a lawn tractor, and then 
using the hoist to lower down into the basement while sliding on a couple of 
2 X 12's. Its been running fine for about 4 years now powering a 1500W AM 
amp and modulator. Ive also cooked large modulation transformers using the 
same process as well as 2 modern plate transformers that spent a day under 
water when a basement flooded and we lost power and the generator wouldnt 
start.

Carl

----- Original Message ----- 
From: Dennis Monticelli
To: J. Forster
Cc: Geoff ; ARC-5 Maillist
Sent: Thursday, November 28, 2013 11:47 AM
Subject: Re: [ARC5] Slightly OT: Transformers

It seems to me that one could measure the cold resistance of a winding (say 
the primary), then put the transformer under load, and then remove the load 
and take a fresh resistance reading right away.  The temp rise of the Cu is 
what you are trying to limit and that is easily calculated via the tempco of 
Cu which is about +0.39% per degree C.  The max temp rise of the winding 
should be kept below 150C.  This assumes the insulation is in decent shape 
and can handle 200C which modern transformer do.  You may wish to de-rate 
for old iron.

Dennis AE6C

On Thu, Nov 28, 2013 at 8:19 AM, J. Forster <jfor at quikus.com> wrote:

Steel quality sets max flux density.

Circular mils is cross-section of copper in the windings, hence copper loss.

Generally transformers are designed so:\

Copper loss = core loss.

-John
=================

> Ratings vary from 500 to 1000 circular mills per amp depending upon the
> quality of the steel used. The 500 rating goes back to the 30's and was
> repeated for decades in the ARRL Handbook but in WW2 and later in consumer
> electronics materials improved.
>
> There was a posting somewhere of estimating the VA of a transformer by
> measuring the core but I cant find it right now.
>
> Carl
>
> ----- Original Message -----
> From: "Fuqua, Bill L" <wlfuqu00 at uky.edu>
> To: "Eric Mynes" <kc8wzb at gmail.com>; <ARC5 at mailman.qth.net>
> Sent: Wednesday, November 27, 2013 11:16 PM
> Subject: Re: [ARC5] Slightly OT: Transformers
>
>
>>  In the late 60's or 70's there was a multipage article in Electronics
>> Magazine on power  transformers
>> and how you can determine the rating of a transformer up to kW range.
>> The
>> magazine is gone now
>> but perhaps there is a URL that has the PDF of those magazines.
>> 73
>> Bill wa4lav
>>
>> ________________________________________
>> From: arc5-bounces at mailman.qth.net [arc5-bounces at mailman.qth.net] on
>> behalf of Eric Mynes [kc8wzb at gmail.com]
>> Sent: Wednesday, November 27, 2013 6:10 PM
>> To: ARC5 at mailman.qth.net
>> Subject: [ARC5] Slightly OT: Transformers
>>
>> Greetings all,
>>
>> I have some questions about transformers and this seems to be the only
>> group I'm in now that deals with tube rigs.  I hope you don't mind
>> elmering
>> me a bit by answering a few questions.  I mention slightly off topic
>> because one day (hopefully soon) I'll start building a power supply to
>> use
>> with my ARC 5 rigs, but for now, I just want to be able to power some
>> basic
>> homebrew tube transmitters.
>>
>> So, my questions go along the lines of...  I have several transformers
>> that
>> have followed me home from hamfests or someone has given me.  But most
>> are
>> not labeled in any fashion as to what they are rated for.  As stated
>> above,
>> I'd like to build a small power supply that can run some homebrew tube
>> rigs.  I might actually use some of the thousands of tubes that followed
>> me
>> home from hamfests instead of leaving them piled in boxes with no use.
>> I've tried googling any number or marking on the transformers trying to
>> find a manufacturer or specifications and haven't had any luck.  How can
>> I
>> test these safely as to not hurt myself or the transformer so that I
>> find
>> out at least the voltages I should expect out of them?  Right now, I'm
>> not
>> too concerned about finding how many amps they will provide as much as
>> now
>> much voltage is on the end of the wire.  The number of amps question
>> will
>> come soon though.
>>
>> Currently I have a transformer on my bench that has the following
>> markings
>> 110v004h82
>> 352052
>>
>> It appears to have 3 windings.  A black set (I presume is the primary),
>> a
>> red set with CT, and a green set with CT.
>>
>> I'm kind of skittish at just applying 110v to the black wires and seeing
>> what the others have for voltages.  So, I figure why not ask some hams
>> that
>> might have a better clue than me.
>>
>> Please respond to me off the list as I don't want to waste too much of
>> the
>> list's bandwidth on something that might be determined as an elementary
>> question that we all should know.  I'll be quick to say that I'm a very
>> much at an elementary stage on using HV and know enough to ask questions
>> before just trying things.
>>
>> Thank you for the bandwidth and Happy Thanksgiving to all
>>
>> 73,
>> Eric
>> k8ohz
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