
The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it. 

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#1




3 phase power factor calculations, capacitors
I used the power factor information here to calculate the power factor and calculate the capacitor size required to correct the power factor. It cut my electric bill by 20%!
So, now I want to help a friend who has 3 phase on his shop. Can anyone tell me how to practically calculate the capacitor size needed when using 3 phase? I assume it only requires 2 caps, one between each of the three line wires. Is that correct? 
#2




Quote:
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#3




mik3 is correct 3 capacitors. phase AB, phase BC, and phase AC.

#4




Ok, but how do I calculate the capacitor size for 3 phase using a clamp meter and the power meter?

#5




DB
but why in Delta why not star connection of Capacitors?

#6




Just out of curiosity, what value of capacitors did you install to save you 20% of your electrical bill, and what loads are you running to get such a poor power factor?

#7




Quote:
The three phase system loading may not be balanced across all phases and the individual phase power factors may be quite different. You might have to individually compensate each phase. If the loads vary appreciably then the power factor compensation may have a detrimental effect by actually overcompensating the load. Remember you can only compensate a lagging power factor with a capacitor  which is usually the case anyway. I'm guessing you knew that anyway. There are also issues with adding capacitors to your installation which requires due consideration to safety and ciruit protection. Presumably you have allowed for this in your friend's case. In principle you can compensate each of the 3 phases individually as you did in your single phase installation. You can place the capacitors between each phase line conductor and the neutral  if neutral is connected in the service. You can use lower voltage rating for the capacitors in this mode but the currents will be higher than with a linetoline connection mode  there's always a tradeoff. Using the power meter and ammeter you can determine the required compensating capacitor (linetoneutral connection) for each phase as follows. 1. Measure the phase power P 2. Measure the phase current I 3. Calculate the phase reactive power Q = √((V*I)^2P^2) where V is the known phasetoneutral voltage. 4. The required compensating (for unity power factor) capacitance C=Q/(ω*V^2) where ω=2**f and C is in Farads 5. Capacitor ratings  Vmin=linetoneutral voltage and Imin=ωCV Amps For instance if P=1kW, V=110V, I = 12A and f=60Hz Q=√((110*12)^21000^2) = 861.6VAR C=861.6/(2**60*110^2)=1.89e4F or 189uF (rated for 110V and 7.84 A continuous duty). This was a 'bad' case with an original power factor of 0.76  so you end up with a mighty large capacitor to get back to unity power factor. 
#8




As a supplementry comment I'm also puzzled that your electric power provider charges you for reactive power. In my country domestic consumers are only charged for the real power they consume  and only fairly substantial customer installations attract a benefit for installing PFC at their mains connection point. Hence my interest in GetDeviceInfo's comment / question. Your reactive power load must have been fairly high for the conductor I^2R losses in your dwelling to have been so significant. I'm assuming (perhaps incorrectly?) that your are a domestic consumer.

#9




Quote:
The remaining part of my last post is still relevant and your further comments re the details of your installation improvements are certainly of interest. 
Tags 
calculations, calculationsm, capacitors, factor, phase, power 
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