[ARC5] Slightly OT: Transformers

J. Forster jfor at quikus.com
Thu Nov 28 18:18:41 EST 2013 

Generally, this level of design effort is only appropriate if you are
building a large number of units and are trying to save every penny.
Unless you have skids of identical UFO parts, it's not worth the trouble.

In response to the OP, I'd get a rough Wattage from the weight.

Measure the output voltages, per my previous post.

If the LV winding wire comes out, measure the OD, and estimate the current
using 500 +/- cm/Amp.

Multiply the LV vAC * current from wire size to get Watts/winding.

Add up the LV Watts; subtract from guessed capacity from weight.

Divide difference Watts / HV voltage ===> allowable HV current.

Then load the transformer per above, turn on power, and see if it gets too
hot to touch after a couple of hours. Remember, the bigger the
transformer, the longer it takes to come to equilibrium.

YMMV,

-John

======================



> Yes, it is certainly non-uniform but insulation of wire and windings is
> the
> biggest problem.  Cu temp tells you more about that.   I didn't read about
> this anywhere, just thunk it up.  Nice to know it is accepted practice.
>
> Dennis AE6C
>
>
> On Thu, Nov 28, 2013 at 9:16 AM, J. Forster <jfor at quikus.com> wrote:
>
>> You certainly can use the Cu as a thermometer, but the temperature
>> inside
>> a transformer is non-uniform.
>>
>> -John
>>
>> ================
>>
>>
>>
>> > 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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>> >> >>
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>> >> >>
>> >> >> -----
>> >> >> No virus found in this message.
>> >> >> Checked by AVG - www.avg.com
>> >> >> Version: 10.0.1432 / Virus Database: 3629/6372 - Release Date:
>> >> 11/27/13
>> >> >>
>> >> >
>> >> > ______________________________________________________________
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>> >> >
>> >> >
>> >>
>> >>
>> >> ______________________________________________________________
>> >> ARC5 mailing list
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>> >>
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>> >> Please help support this email list: http://www.qsl.net/donate.html
>> >>
>> >
>>
>>
>>
>

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