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Power Factor Correction Capacitor

Discussion in 'Electronic Design' started by [email protected], Sep 15, 2006.

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  1. Guest

    Hello- I am running an AC refrigeration compressor on a sailboat using
    an inverter. By determining the DC watts and the AC VA i figure a
    power factor of about .5 . I would like to correct this power factor,
    and have calculated that I need a 150 uF capacitor to do so. My
    question is, what kind of capacitor should I get with that capacitance
    that will handle 120 V AC? I appreciate the help. John
     
  2. Why?
     
  3. You need an AC rated capacitor, oil filled or polymer film capacitor
    or bank of paralleled capacitors.

    But be warned that your inverter may not like having a power factor
    capacitor connected across it.
    Example of oil filled type:
    http://cgi.ebay.com/28-5-UF-330-VAC...oryZ4662QQssPageNameZWD1VQQrdZ1QQcmdZViewItem
    Example of film capacitors:
    http://dkc3.digikey.com/PDF/T063/1303.pdf
     
  4. Guest

    As has been pointed out it would serve no useful purpose and may cause
    you additional problems. By the way, how did you arrive at the 150uF
    figure?
     
  5. Guest

    Thank you all for your help. First of all the reason I wanted to
    correct the problem was because I have both a DC Ammeter and an AC
    ammeter on the electrical panel, as well as an AmpHour battery monitor,
    and its just nice for them all to correspond if at all possible. But
    if there's a chance that it will hurt things to add a capacitor I
    definitely won't do it. The only thing I really care about is the DC
    Ammeter and the DC battery monitor. My question is are these DC
    readings still accurate as far as true amount of amps being drawn from
    the battery banks even when the inverter is putting out way more power
    AC? Right now to run the compressor I get about 45 amps DC and 9 amps
    AC... so should the 45 amps DC be accurate? Thanks again for your help.
     
  6. The problem is that the inverter probably outputs a waveform with some
    fast rising and falling edges, rather than a smooth sine wave. Even
    if it did produce a perfect sine wave, there is a fast edge every time
    you turn it on, or connect the capacitor. The current through a
    capacitor is proportional to the rate of change of voltage across it.
    Fast edges imply large currents.

    I have seen power factor capacitors added by having small inductances
    in series, to limit the peak current, but it is tricky to keep the LC
    resonance from being excited by any harmonic in the system (which
    would also produce large current at that harmonic frequency).
    The DC meter should represent an accurate measure of load on the
    battery. AC power factor current represents energy borrowed and
    returned each half cycle, so it heats the inverter up a bit, as it
    sloshes back and forth through it, adding to its losses, but it
    doesn't change the average DC load much. It just adds ripple to it.
    But a DC meter should average that ripple and give an accurate average
    answer.
    It is an accurate measure of battery drain. It might drop a few
    percent if you could eliminate the power factor current, because the
    inverter would be wasting a little less power because the borrowed and
    returned energy would be sloshing back and forth between motor and
    capacitor, instead of between motor and battery, through the inverter.
     
  7. Old rule: "If it ain't broke, don't `fix' it". It's good advice.
     
  8. vasile

    vasile Guest

    Did you measure it ? There is tool called "cos fi" meter.
    But it does measure correct only sinusoidal shapes (as your AC
    voltmeters and ammeters) There is a coefficient of 4.44 to 4 (shape
    coefficient) between sinusoidal and rectangular waves.
    About power factor, if you have a regular energimeter in your house,
    and such a small cos fi, there is no compensation required because
    there will be no effect in the money you'll pay for your energy...

    If you gave an active energimeter and also a reactive energimeter in
    your house (I have doubts you do) then it worth the trouble of mounting
    capacitors...

    greetings,
    Vasile


    I would like to correct this power factor,
     
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