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Reactors for capacitor banks


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Hello Neutron


The reactors in series with the capacitors are commonly called detuning reactors and are sized to set the resonant frequency away from harmonic frequencies. There are a number of different frequencies to which the resonant frequency can be tuned. Of course, it is dependent on the line frequency as well.

Commonly, detuning reactors are rated in percentage where the reactance of the inductor is compared to the reactance of the capacitor. Typical inductor reactances are 5%,7% or 11%.

The higher the percentage of the inductive reactance, the greater the reduction in the effective capacitive reactance.


Best regards,

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Thanks for the answer, but my experience in electrical has never had anything to do with reactors ... therefore I am rather dumb about them.


As I stated before:


each phase of 1 stage has 600uf of capacitors.....this shows as about 46 amps per phase.....do you know how I would determine the actual reactor needed for this? Remember...I know nothing about this kind of thing LOL

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Hello Neutron


You can calculate the reactance of capacitors as 1/(2 x pi x f x C) where f is the frequency of the supply and C is the capacitance.


You can calculate the reactance of an inductor as 2 x pi x f x L where f is the frequency and L is the inductance.


You have not specified the foltage or frequency so I can not easily give you a definative answer.

You have not specified whether that capacitance (600uF) per stage is the total capacitance, or the capacitance per phase, or whether the capacitors are star or delta connected.


For the purposes of demonstration, I will assume that you are in USA, that your voltage is 440 volts and your frequency is 60 Hz.


In that case, the effective reactance per phase is equal to 440/rt3 /46 (V/I) and is therefore 5.52 ohms per phase and the correction is 440 x rt3 x 46 VAR = 35,056VAR = 35KVAR


If we chose a 7% reactor, then the series reactance would be 7% of the capacitive reactance at line frequency, or 0.386 ohms.


This converts to an inductance of 0.386/(2 x pi x f) = 0.386 / (2 x pi x 60)

= 0.001H = 1mH per phase.


Best regards,

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  • 1 month later...

Hi just stumbled onto your site. Full of info and I must admit feeling even more stumped after reading through a few bits.

My question has to do with capacitors. I run a site which has power correction installed. We have recently been approached by a company who are trying to sell us a system.

They have told us that because of our motor load in the buildings are so high that using capacitors on each motor will give us up to 14% savings on our energy bill.

Sounds way to good to be true. I can't find any info,anywhere that tells me what kind of capacitors thet are but i have seen one site where it was suggested capacitors can be used in this way but again no info on what they are or where to find them.


Mark G.


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Hello Mark


There are a number of companies/people out there promissing to save energy by power factor correction.

The bottom line is that there will be a reduction in KVA and KVAR used from the supply if you currently have a poor displacement power factor. This is usually associated with connected motor load.

The reduction in KVAR and KVA can reduce your energy bill, but only iff you are paying a surcharge based on KVA, KVAR or Power factor. The addition of power factor correction will not have any significant impact on KWHr usage.

If you already have power factor correction on site, and it is working correctly, and it is sized correctly, there will be no advantage in adding the extra capacitors.

The first thing to look at is your electricity bill and see what you are actually paying for. If the bill is primarily for KWHr (units), then I would do nothing.

If you post your details, we may be able to comment further.

Best regards,


ps. - if you register on the forum, you can be advised of replies to your questions. There are no registration costs.

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