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Capacitor on generator drives me insane

  1. Jun 23, 2011 #1
    I run a 4.2 kw chinese diesel gene which has a 15 ùF capacitor on it. This capacitor keeps blowing up regularly. It cost 18Au$ to replace each time and the city is 100km away... Obviously it does 'nt like my big 2.4 kw grinder or my MIG welder (though the 10hp engine and 4.2 kw gene can handle it ). What should I do?? Can I place additional capacitors in parallel on the circuit? I understand this works like a shock absorber. Please don't tell me to get a bigger generator... Thanks for considering my question, I am not on the grid and depend on that gene.
  2. jcsd
  3. Jun 23, 2011 #2
    I think, if it is inductive spikes blowing up the capacitor, it would help more to put capacitors in series to reduce the voltage across them than in parallel, but your capacitance would drop and you might not get the current surges your equipment needs, so you'd need 2 pairs of capacitors in series in parallel, or you could buy a capacitor with a bigger voltage rating, but I have no experience with this so this is just a guess.
  4. Jun 23, 2011 #3
    Thanks, Dragoon but the rating on the capacitor says 400 V and I am dealing here with monophase 230 V. What about the 15uF (microfarads) I am not too sure what it means and how its value is worked out for a given situation. I am more familiar with the basics of resistance, voltage and intensity however capacitors are a bit of a head scratcher to me, sorry...
    What you say about the capacitor array (2 in serie X 2 in parallel) makes sense, I feel like trying this...
    Last edited: Jun 23, 2011
  5. Jun 23, 2011 #4


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    Why not just use a Capacitor with a much higher voltage rating? It may cost more but it would be a fit-and-forget. If you want to make it up with two or more in series / parallel then it is a good idea to put a small load (say a low value mains light bulb - which would be a few kOhms) in parallel with each parallel pair of capacitors. This will ensure an equal share of any voltage drop across all of the capacitors.

    btw, I imagine that the capacitor value is to suit the expected Power Factor. The PF describes the difference in Phase between current and voltage when you have a non-resistive circuit. Motors tend to be Inductive so a C will reduce / eliminate the circuit Reactance.
  6. Jun 23, 2011 #5
    Don't forget that 230 VAC has a peak voltage of 324 volts. You don't have a lot of margin with a 400 V capacitor. I concur that using a 15 uF at a higher voltage rating is probably the best solution.

    How stable is the 230 volts on your generator? How much does it vary under varying loads? Does it ever go above 230 volts?
  7. Jun 23, 2011 #6
    Capacitor ratings are tricky.
    230 volt RMS has a peak of 325 volt. 400 volt peak rated capacitor does not leave a large safety factor when operating on 230 volt RMS.

    Is replacement capacitor rated for AC or DC. AC rating is usually MUCH lower than DC rating.

    Some capacitors are rated for motor start use. These capacitors can only have voltage applied for a short period of time.
  8. Jun 23, 2011 #7
    Thanks guys for your replies. I can see a bit more clearly now.
    Indeed the voltage varies and can be more than 240V
    The replacement capa is probably off the shelf motor start capa.
    How about electrolytic capacitors? What is the benefit compared to a dry capacitor?
    I will look for a higher rated capacitor in 15uF. Not easy in North Queensland. Any internet address in Australia?
  9. Jun 23, 2011 #8


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    Electrolytics are for dc and lower voltage applications. Just put a higher operating Voltage 15uF in. If necessary, order from abroad. It can't cost too much. Job done.
  10. Jun 23, 2011 #9
    The ability of a capacitor varies greatly with it's construction. Most common motor capacitors are constructed with paper and oil as the dielectric. This is a cheap but poor construction.
    Polypropylene capacitors are much more robust.

  11. Jun 23, 2011 #10
    Leave it out. Your grinder and MIG welder will be fine.

    When the Chinese generator finally goes, rectify your original mistake and buy American, Japanese or German generators.
  12. Jun 26, 2011 #11
    Indeed that's what a mate of mine did: He chucked off the chinese generator but kept the original diesel engine (these are ok) and fitted an italian generator and had no worries since (his conked out completely...)
    Thanks Fellows I know what to do now. I realize this forum is really great and deals with many subjects of interest. Glad to be amongst you! Cheers
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