Re: Pendulum capacitor failing...

In article <jcmdb4h16i7qef87f8mvm30qp6p8ovisr5@xxxxxxx>,
greenpjs <greenpjs@xxxxxxxxxx> wrote:

On Thu, 28 Aug 2008 02:25:13 GMT, Ross Herbert
<rherber1@xxxxxxxxxxxxxx> wrote:

On Wed, 27 Aug 2008 08:17:39 -0700 (PDT), "dersh.z" <dersh.z@xxxxxxxxx>
wrote:

:> On Tue, 26 Aug 2008 08:54:50 -0700 (PDT), "dersh.z" <ders...@xxxxxxxxx>
wrote:
:>
:> :Hi Everyone, first time post... Mr. Goldwasser suggested I run this
:> :question by this group...
:> :
:On Aug 27, 8:45 am, Ross Herbert <rherb...@xxxxxxxxxxxxxx> wrote:
: Assuming there is very little series resistance (other than the dc
:resistance of
:> the inductor) you are left with a series resonant circuit which is
:> resonant
at
:> approx 58Hz for the values given (L = 0.75H and C = 10uF). I calculate
:> the
:> current through the circuit at more than 6A so it might be dissipating a
:> fair
:> amount of heat depending upon duty cycle. The pulsed nature of the drive
voltage
:> might also contribute to premature failure.
:>
:> Normally, a motor run capacitor is connected across both legs of the ac
supply
:> in series with an auxiliary winding in an electric motor in a similar
:> fashion
to
:> your circuit but I would hazard a guess that the current through the
:> motor
:> winding would be much less than in your arrangement with 0.75H.
:>
:> Does the capacitor get hot?
:
:Thanks Ross, et.al,
:
:The capacitor does get warm but not hot, the coil does get quite hot
:tho without a cooling fan.
:
:Your calculations are what I get as well, I found this thing swings
:quite nicely when tuned to aprox. 58-59Hz. The old setup (1960's
:design) swung about 3ft max @220VAC, I can now get 8ft+ @115VAC...
:but, I' m having the cap failures at a much higher rate than the old
:setup. I might have changed the original caps every 10years or so.
:
:Both setups had the cap in series with the coil, the only major change
:in my setup is the coil - the old coil lost it's smoke, couldn't get
:it back in... so I had a new coil made. The old coil was hand wound
:with a solid steel core, this new coil was machine wound (epoxied,
:etc...) and I used 1/16" (no flux) welding rods tightly bundled into
:an aprox. 1.25" core diameter (I also have a few brass tubes creating
:air gaps in the core for forced air cooling from below - will cook
:without the forced air cooling tubes).
:
:I have been thinking Ceramic but am unsure as to which I should use in
:this sort of an application. I generally don't do the "industrial"
:stuff, I'm more of a component level tech - these motor caps are not
:something I normally deal with.
:
:Thanks to all for the repies, very much appreciated.


Stick to polypropylene filmcaps instead of ceramic. I would suggest a better
capacitor to use is one made by American Capacitor. The VW2M106K (1000V)
which
has rated RMS current of 19A and a peak of 2910A with high dV/dT.
http://www.americancapacitor.com/PDF/VData.pdf

You will have to contact them to get details of purchasing.
http://www.americancapacitor.com/index.htm

Hi all,
I am afraid I can't help the original poster, but I find this thread
very interesting. There is a pendulum at a local museum and I always
wondered how it worked. I assume that an electromagnet is used to
replace the small amount of energy lost during each swing of the
pendulum, but how is it actually connected and where is the
electromagnet? What does having an resonant circuit tuned to
slightly below the power line frequency have to do it? I guess I'm
asking for someone to explain the theory. I don't suppose there's a
"how stuff works" article on the subject, but I'll go check that out
now.

As the OP said, the circuit is tuned "off resonance" when the pendulum
is not close. Then when it approaches, its proximity alters the
resonance in such a way that there is more attraction while it is
approaching, and slightly less while it is receding. The net result is a
transfer of momentum to the pendulum. Sort of like the way a "slingshot"
orbit works.

Isaac
.

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