Method required to calculate capacitor for Power Factor compensation 1

[RadLight]

I tried using this method as follows:

1. Calculate the VARs by taking the square root of the difference of the squares of the VA and Watt value.

2. Divide the square of the Volts by the VARs value to obtain the impedance.

3.The capacitor value for pf = 1.00 is supposed to be the reciprocal of the product of 2 , Pi and this impedance.

I used a lower capacitor than the calculate value, on a ballast but sometimes even measured negative power factor values with my meter. I was using a Goosen MetraHit 28S meter. Has anyone a more reliable method for a capacitor?

 

[jghrist]

If you are calculating the capacitance per phase for a three-phase installation, divide the vars by 3. If the capacitors are to be connected wye, use the line-to-ground voltage; if connected delta, use line-to-line voltage.

The capacitance in farads is the reciprocal of 2·pi·frequency·reactance. You forgot the frequency. I'm not sure why you need this step, however, because you normally size capacitors in vars, not farads.

 

[rbulsara]

It appears you are calculating for a single phase ballast. Your steps 1 and 2 are correct. The step 3 is not.

C= 1/(2*pi*f*X)

As jgrisht mentioned you did not include the frequency, a factor of 60 or 50

This capacitor need to be connected 'across' or parallel with the ballast.

 

[RadLight]

Thanks to Jghrist and Rbulsara for pointing out the missing frequency component. I am more familuar with capacitors marked in micro Farads from electronic PCB repairs and have not seen a VAR value. Could this possibly be the marking 47/99 on a 15 uF capacitor marked for 50-60 Hz? Also I am only concerned with single phase lamp ballasts.

 

[jghrist]

You haven't given the voltage, but you can calculate the vars from the equations you have already stated. If V=120V and f=60 Hz, then var=81.4. If V=230V and f=50 Hz, then var=249.3 for a 15 µf capacitor.