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Member

Join Date: Jan 2014
Posts: 9

01/27/2014 2:41 AM

Dear All,

We need to install 125uF at 380Volts, three phase delta connected capacitor bank parallel to 37KW Motor,

1. The capacitor bank has to produce 125uF at 380Volts, the equivalent KVAr rating from the formula KVar=2*pi*f*C*V2 is 5.6 KVar.

2. Due to the unavailability of the capacitor bank rated at 380Volts,we proposed the Kvar rating of 7.5Kvar at 440 Volts.

3. On applying 380Volts to the capacitor rating of (7.5Kvar,440Volts), the capacitor has to produce the Kvar rating of 5.6Kvar by the equation , Actual Kvar=Nameplate Kvar x (Applied Voltage)2/(Rated Voltage)2.

What will be the capacitance output of the capacitor bank at this condition? Will the capacitance also reduce with the reduction in Kvar?

Pathfinder Tags: capacitor farad KVAR Voltage
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Guru

Join Date: Jan 2007
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#1

### Re: Capacitor Bank Farad Check

01/27/2014 5:03 AM

Homework?

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#2

### Re: Capacitor Bank Farad Check

01/27/2014 8:41 AM

I don't know the answer, either.

I always start here: Wikipedia, the free encyclopedia

Guru

Join Date: Feb 2011
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#3

### Re: Capacitor Bank Farad Check

01/27/2014 9:25 AM

if this isn't homework it's scary to think you're actually consulting an internet forum to double check your information. real scary actually just tell your boss I flipped a coin and said the 440 is fine in place of the 370

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Guru

Join Date: Oct 2009
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#4

### Re: Capacitor Bank Farad Check

01/27/2014 11:16 AM

Dear Sreekant, Your formula in (1) says it all. Doesn't it?

If kVAr = 2 x pi x f x C x V2, then, obviously the kVAr increase at 440V, is due to the increased voltage only, with all other parameters remaining constant.

Yes, it is as simple as that.

Member

Join Date: Jan 2014
Posts: 9
#7

### Re: Capacitor Bank Farad Check

01/28/2014 5:28 AM

Thank you sir.

Infact i already know this, as my client was arguing with me that the capacitance also reduces/increases with the changes in voltage.I just want to verify that and had doubt if there would be any other factors?

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#5

### Re: Capacitor Bank Farad Check

01/27/2014 3:06 PM

Mainly a capacitor is built of 2 metallic plates and a dielectric-solid or electrolyte- between. See:

http://www.elna.co.jp/en/capacitor/alumi/principle.html

Capacitance=ε*S/d S- plate area
d-distance between plates. ε=permittivity [dielectric
constant].

Usually ε does not depend on voltage [electric field] but depends on frequency [mainly
on high frequency]and temperature. Briefly, this capacitance will remain constant.

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#6

### Re: Capacitor Bank Farad Check

01/28/2014 1:18 AM

the rated voltage of the proposed capacitor = 440V

KVAR at 440V = 7.5

System voltage = 380 which is 86.3 % of the rated voltage 440 of the capacitor

KVAR at 380V = 7.5x0.863x0.863 = 5.585 KVAr which you are looking for.

Are you satisfied now?

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