From: John De Armond
Subject: Re: New Q about Leaving converter plugged in
Date: Tue, 12 Oct 1999 02:06:18 EDT
Newsgroups: rec.outdoors.rv-travel

Ben Franklin VI wrote:

> Leaving it on is ok.  Probably will last longest if left on.  Inductors have
> nasty trait concerning current and voltage.  Switch them on and off you get
> current surges and voltage spikes -- a self destructing behaviour.

Yup.

> The noisy transformer may have a loose lamination.  There may be another piece
> of sheet steel near the unit that is vibrating in sympathy with the magnetic
> field of the transformer.
> 
> For noise reduction you could try mounting the unit on rubber taking care to
> keep it solidly restrained.  Also build a box around it with holes for
> ventilation -- check into the hospital for a few days [any malady you can
> survive will do  :)  ]  --- when you leave ask for the sponge rubber waffle
> mattress pad from your bed -- use this to line the box -- the rough side goes
> toward the noise.

There is another solution to this problem, one that addresses the
root cause - something loose and vibrating inside the transformer. 
This solutions assumes you're comfortable enough working inside
electrical things to remove the transformer.  Assuming you are, read
on.

Most of these noise problems originate from the mfrs getting cheap
and not varnishing the finished transformer.  The varnish adds
insulation and it bonds all surfaces together to prevent them from
vibrating.  A final dip and bake costs extra and so they don't do
it.  You can.

Locate your nearest friendly local electric motor repair shop.  Take
the transformer to the motor repair shop and ask them to dip it in
their varnish tank.  Make sure they leave it in the tank until all
the bubbles quit.  This usually takes 10-15 minutes.  Tell them NOT
to dip the leads.  Normally they know not to do this (makes the
insulation brittle) but just in case... If your transformer has
screw terminals, coat them (and anywhere else you don't want
varnish) with a heavy coating of grease.  This will keep the varnish
at bay.  Be sure to have a cardboard box lined with a garbage bag
with you.  This varnish cures by baking.  It will never dry without
baking.  It's REAL sticky when uncured.  CAUTION:  The baking
process is fairly stinky.  If you think that will cause domestic
problems, let the motor shop bake the transformer.  This will be a
come-back-tomorrow affair since the baking takes several hours and
they'll probably not want to fire off their oven for one little
transformer.  If you want to do it at home, simply put the
transformer in the oven, put a foil pan below to catch the drippings
and bake for 2-3 hours at 275-300 deg F.  There will be lots of
solvent odors but no smoke (unless the varnish drops on the heating
element) so the odor won't linger and a vent hood will quickly pull
it out.

Another source of noise, sometimes significant, is nearby sheetmetal
being magnetically stimulated by the leakage flux from the
transformer.  This is different from the acoustic stimulation Ben
mentioned.  Leakage flux is a sign of a poorly designed
transformer.  The solution to the noise is the same - dampen the
sheet metal so it won't vibrate.  Two methods I use are dampener
sheeting and aerosol automotive undercoating.  The dampening
sheeting is made by 3M and is used by body men on sheet metal to
make it not resonate.  One of the tricks used to give car doors that
nice "thunk" when you slam them.  The stuff comes adhesive-backed
and can be cut with ordinary scissors.  Just stick it to the inside
of whichever metal panel is vibrating.  Available from auto parts
stores that cater to body men.  Undercoating does the same thing but
it more inconvenient and messy.  OTOH, it is available almost
anywhere car accessories are sold.

If you're like me and HATE the sound of buzzing appliances, you
probably won't think these solutions to be TOO radical :-)  Nothing
less than the total absence of 60hz buzz is acceptable to me.  This
solution works well too for those cheap but buzzy mercury vapor
security lights one can buy at Wal-Mart, etc.

John