Power Factor Correction 3 phase motor

Discussion in 'General Electronics Chat' started by T83, Nov 19, 2008.

  1. T83

    Thread Starter New Member

    Nov 19, 2008
    I would like to clear out something.Let's say I have the following given:
    A 50KW motor,460 Volts,3 phase.It has a power factor of 0.85 and an efficiency of 100%. Ok now let's say we want to raise the power factor to unity, after doing the calculations you will get that we need 30.986 KVAR the required capacitor size.Now here comes my question:
    if I want to calculate the capacitance in Farads,which of the following relations should I use?
    1-KVAR = V^2/Xc
    2-KVAR = (V^2/Xc) x sqrt(3)
    I mean when we calculate Q,we used the following
    Q(KVARs)=V x I x sqrt(3) x sine(phi),so by substituting V x I by V^2/Xc I should get equation 2,right ?

    Thanks to everyone that will contribute.
  2. T83

    Thread Starter New Member

    Nov 19, 2008
    Hello again,

    I worked out the equations & I kind of feel ashamed of asking the previous question.
    Here are my findings:
    1- If it is single phase ---> Q = V^2/Xc where V is the phase voltage

    2- If it is three phase Delta connection ---> Q= (3 x V^2)/Xc where
    V= Vphase = Vline to line

    3- If it is three phase Wye connection ---> Q = (3 x V^2)/Xc where
    V=Vphase & Q = V^2/Xc where V=Vline to line

    Now if I am not mistaking we install a capacitor for each phase of the motor.So basically,if we have 12KVAR then we will install three 4KVAR capacitor ---> Q is divided by 3 ----> Regardless of Wye or Delta connection we need to use Q = V^2 / Xc where V is Vphase,if we want to get Xc so that we can calculate the Farad of the capacitor.

    Hope somebody can review all of this.
    Thank you !