Equivalent capacitor for an inductor to make PF=1

  1. For some reason I had to apply a capacitor in parallel to an AC circuit which has reduced the power factor as a result some power generating AC equipments are complaining (overload).

    Now I need to apply an inductor in parallel to fix that. For that I need a formula which gives me the equivalent inductance for some capacitance.

    Thanks for the solution!
     
  2. jcsd
  3. The equation for Pf-correcting capacitance is derived by plugging the impedance of a capacitor into the equation for complex power. The Pf-correcting inductance can be calculated in the same way:

    ZL=jωL
    S={Vrms2}/{ZL}

    substituting the first into the second:

    QL={Vrms2}/{ωL}

    L={Vrms2}/{ωQL}
     
  4. Whatever you did by placing the capacitor in will be undone by placing an inductor in parallel.
     
  5. Baluncore

    Baluncore 2,708
    Science Advisor

    If you now have capacitance in parallel with the load, and the PF is over compensated, then it is far better to reduce the capacitance than to add an inductor.

    Your aim should be to select a capacitance that reduces the total current to a minimum, that will give a PF close to 1.00

    I keep a set of various AC capacitors with a current meter, I can then select the appropriate value by measuring the total current with different capacitance values.
     
  6. jim hardy

    jim hardy 4,768
    Science Advisor
    Gold Member
    2014 Award

    Listen to Baluncore.
    Beware of creating circuits that resonate at line frequency. They can cause destructive levels of current and voltage.
     
  7. Baluncore

    Baluncore 2,708
    Science Advisor

    You may find that you need to add inductance in series with the load, rather than in parallel as used with capacitor correction.

    One problem that is often seen with power supplies that employ rectifier diodes charging a DC storage capacitor, is that the current flows only during voltage peaks. No matter how much capacitance is used in parallel to provide sinusoidal PF compensation, it cannot correct for the high frequency harmonics of the current pulse peaks.

    To correct PF in that situation you need a PF corrected DC power supply that has a different input topology.

    The addition of an inductor between the input rectifier and storage capacitor can make a difference to some existing systems, but that is usually more expensive than the newer switching PF corrected supplies.
     
  8. I may remove that capacitor, but only if I figure out what's an STR.
     
  9. From all inductors I'm seeing online, their voltage rating is now given.
     
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