:On Thu, 28 Aug 2008 02:25:13 GMT, Ross Herbert : put finger to keyboard and composed: : :>On Wed, 27 Aug 2008 08:17:39 -0700 (PDT), "dersh.z" wrote: :>
:>:> On Tue, 26 Aug 2008 08:54:50 -0700 (PDT), "dersh.z" :>wrote: :>:>
:>:> :Hi Everyone, first time post... Mr. Goldwasser suggested I run this :>:> :question by this group... :>:> : :>:On Aug 27, 8:45 am, Ross Herbert 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. :>
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:>You will have to contact them to get details of purchasing. :>
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: :It seems to me that choosing a better spec cap is still only delaying :the inevitable. IMHO a better approach would be to choose a 9.1uF cap, :polypropylene film or otherwise. This assumes that the capacitance :required for resonance at 60Hz is 9.4uF. As it is now, the circuit is :slightly detuned above its resonant frequency. AFAICS this means that :when the 9.7uF cap inevitably degrades, the circuit drifts
*toward* :resonance, in which case the capacitor's current and voltage both :
*increase*, resulting in further degradation and even more drift :toward resonance. If, however, the circuit were to be detuned on the :other side of resonance, then any degradation would result in a drift :
*away* from resonance, with a
*reduction* in current and voltage, and :this would in turn would slow the rate of degradation of the cap. : :- Franc Zabkar
I doubt that "losing capacitance" is the only symptom of the capacitor failing in this circuit - it's just that we don't know what else is happening to the capacitor in the application. The only data available for the GE97F9002 cap doesn't give details of rated or peak current or dV/dT rating. All we know is that it is specced as a motor run capacitor. The normal use for this capacitor type is in series connection with a motor winding having considerable inductive reactance across an ac supply where the peak inrush current period is limited and infrequent in nature, and normal operating current is not likely to be anywhere near 6A. The OP is endeavouring to use it in a circuit where it receives frequent high peak inrush current and a large inductive kick when the circuit is broken, at each swing of the pendulum. I doubt that these conditions are ideal for maximising the life of this type of capacitor. While the original GE capacitors might have done the job for 10 years in the OP's first coil design, these later incarnations made by a different process by Regal Benoit may not be as good as the originals - ie. they are built down to a price.
While using a better capacitor might not be the most elegant solution in engineering terms, if it works without having to change the current coil design then it is still an effective solution. My guess is that the GE97F9002 cap doesn't like the constant hammering of pulsed ac operation where the inrush current is surely quite large and the current at resonance is still rather high for a capacitor which is meant to be permanently connected across an ac supply in series with a higher impedance winding than the 0.75H coil, and having a relatively small current swing. The alternative cap I suggested does come with the necessary data and is designed to handle high currents and with a large dV/dT rating. Surely this is a good starting point.
Even if we were to find exactly what the failure mechanism for the GE cap is, it would most likely turn out that a better specced capacitor is the only solution anyway, so why not anticipate this necessity. It might give the electronics perfectionist a warm glow to know exactly why a component fails but in practical terms the end user only cares whether the item works and that it keeps on working.
If the circuit performance doesn't deteriorate over time with the use of a better specced cap it will have proved by empirical method to be an effective solution. If the OP later decides to design a better coil then this cap will be even better suited since it will possibly have to endure even lower stress.