[ARC5] [Milsurplus] Old Dynamotors- Don't be a DooFuss Like Me....

Mon Apr 1 19:45:44 EDT 2013

Thermal inertia, yeah, it's real.

I had a sobering experience on Field Day several years ago, 2008.  I always take a vintage rig, and that time was using a WW2 HRO powered from the rack-type supply used with an RAS.

There was something very funky about the juice coming from the Chinkee-Chinee-made generator the club was using.  Either it was not a sine wave, or perhaps was producing two different waveforms simultaneously which were "adding" to produce too much peak voltage (my Weston meter -- RMS of course -- read 118 volts), or maybe some IDIOT had decided that reducing the generator RPM would reduce gas consumption but didn't affect the voltage (such is the mind set of the Kodeless n Klueless), no thought whatsoever about the frequency -- whatever was going on, I started noticing that my receiver was getting unstable (it NEVER had, before) and that the power supply was hot enough to cook an omelet on the top cover.  So was the power supply in my LM freq meter.  And the transmitter!

I shut down my rig at 9 pm, and never turned it back on.  At 7 am the next morning, the RAS/HRO power supply -- specifically the transformer -- was still very noticeably warm.

It took me a week after Field Day to get up the nerve to try the power supply on my work bench.  I brought it up slow with a variac, and it never even hiccupped.  Nor did it ever get warm.

Had that not been a mil-spec transformer, it would have fried for sure.

73
Mike
W4DSE

--- On Mon, 4/1/13, J. Forster <jfor at quikus.com> wrote:

> From: J. Forster <jfor at quikus.com>
> Subject: Re: [ARC5] [Milsurplus] Old Dynamotors- Don't be a DooFuss Like Me....
> To: "Dennis Monticelli" <dennis.monticelli at gmail.com>
> Cc: "Military Radio Collectors Association" <mrca at mailman.qth.net>, "milsurplus" <milsurplus at mailman.qth.net>, "ARC-5 List" <arc5 at mailman.qth.net>
> Date: Monday, April 1, 2013, 5:53 PM
> Yes, but you should limit the power
> to a few to a few tens of watts. If
> you put too much power in, you may not notice it until too
> late.
> Transformer have a lot of thermal inertia and DC heats them
> from inside.
> 
> You can do this with many dynos also. Just pull the input
> side brushes, so
> only the shunt field is powered..
> 
> -John
> 
> ==============
> 
> 
> > Another trick that works is to apply DC current to the
> primary in a
> > sufficient amount to heat the interior via the temp
> rise of the windings.
> >  Put several watts into it and let it sit for a
> day or more.  Proportion
> > the current to the size of the transformer.
> >
> > Dennis  AE6C
> >
> >
> > On Mon, Apr 1, 2013 at 7:09 AM, J. Forster <jfor at quikus.com>
> wrote:
> >
> >> For years, I've always baked out stuff with
> non-hermetic magnetics.
> >>
> >> I put the thing on a few bricks, put a couple of
> 100 Watt incandescent
> >> lamps underneath it, and a big cardboard box with
> some holes in the top
> >> over it, and let it sit for a week or more.
> >>
> >> You want the thing to get up to about 130F for
> several days, at least.
> >> Longer is better, IMO.
> >>
> >> YMMV,
> >>
> >> -John
> >>
> >> ==============
> >>
> >>
> >> > On Mon, Apr 1, 2013 at 12:50 AM, David Stinson
> <arc5 at ix.netcom.com>
> >> wrote:
> >> >
> >> >>
> >> >>
> >> >> I thought about putting the whole thing
> sans end bells into a
> >> 200-degree
> >> >> oven for a couple of  hours to dry it
> out
> >> >>
> >> >
> >> > This is  a problem with big chunks of
> iron in old transmitters, too.
> >> Many
> >> > do not realize how similar to a sponge the
> laminations and windings
> >> are.
> >> > And old transformer left sitting in a damp
> cellar or storage shed is
> >> an
> >> > accident waiting to happen if you just haul it
> out, hook it up, and
> >> apply
> >> > voltage. Might happen in 5 seconds, 5 hours,
> or 5 days, 9.9999 times
> >> out
> >> > of
> >> > 10 it'll zorch.
> >> >
> >> > The oven works great as is provides a means of
> getting rid of the
> >> moisture
> >> > without introducing more. On bigger pieces it
> can take a day or two,
> >> maybe
> >> > more, to feel safe enough to put power to it.
> And there's never a
> >> > guarantee, of course. But recovery rate is
> far, far better when dried
> >> out
> >> > than when not. The trick is to heat it up long
> enough for the heat to
> >> > reach
> >> > the core and drive out the moisture. It's all
> about the mass. Smaller
> >> =
> >> > faster; hours vs days.
> >> >
> >> >
> >> > A good reminder as many of my dynamotors are
> out in the garage here in
> >> > humid NC. Though lately humidity has been
> amazingly low.
> >> >
> >> > ~ Todd,  KA1KAQ/4
> >> >
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