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Power Factor Correction


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We have factor control system employing 4 capicators of 40KVAR each, in our pharmacuetical plant we have mostly AC motors ranging frm 1KW to 40KW, what I see on the pf controller that pf indicatios at times go into negative range, i believe that this may be due to capacitance on line is more than required. Electricity bill shows average pf 0.72 for the month.

my questions are 1. shall a combination of 5, 10, 20, 40, 40 KVAR be all right, so that controller can bring required capacitor on line, in that case shall there be a sequence to be followed like mentioned above or controller can bring anyone needed on line. 2. Our main distribution line current goes upto 300A, currently we have 200/5 CT giving feed back to pf controller or should I be using 400/5 current transformer 3.What happens if capicitance on line is more than needed, is negative indication of pf on the controller related to this.

4. How do we calculate KVAR of the inductive load, by knowing KW, pf, I and V

I may be asking basic questions because my trade is Mecahnical Engineering. I think with your help I can have an understanding of the system.

Best regards


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Hello hamidpia


If you apply too much correction, the power factor goes through 1.0 and then falls again with a leading power factor.

This is most likely to happen when you have steps that are too large. For example, if you have motor load that calls for 20 KVAR and you connect 40KVAR, you will switch from being under corrected to overcorrected and in this case, the power factor will be the same, but leading rather than lagging. Some controllers will show this as a negative power factor.


With some controllers, you can have unequal banks, however this is controller specific. It is common to use 1 - 1 - 2 - 2 - 2 etc.

The other potential problem, is to ensure that there is sufficient correction with all banks connected, to correct the maximum load. It is common to provide insufficient correction for maximum load.


The most important time for correction, is when the load is highest. Under light load conditions, there is no specific need for accurate protection. If you have a dedicated supply transformer, it is best to not apply protection when the transformer load is less than 20%.

If your electricity bill shows a power factor of 0.72, that is probably the power factor at maximum load and suggests that you need to add more correction.


If you know the existing power factor and the existing KW, you can calculate the required power factor correction.

I suggest that you down load the electrical calculations software at http://www.LMPhotonics.com/busbar32.zip as this will enable you to make these calculations.


Best regards,

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