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#13140 - 08/25/02 05:44 PM testing capacitor banks
gramps Offline

Registered: 08/25/02
Posts: 120
Loc: ohio
one of my industrial customers,(a stone quarry), whom i do a lot of work for, recently asked me about taking on another task involving their power factor correction capacitors. looks like they have these Square-D cap banks on every motor, from 20 h.p. and up, in their operation. i've never gotten into these setups before, and i'm not sure whats involved with the evaluation and testing of them. the customer asks that i set up a procedure for having them checked, as well as their proper installation. could develop into another source for wintertime revenue, . all of the equipment here is four years old or less, and he indicated to me that they have suspected problems with some of the cap banks for a long time. i'm not sure what problems he might be referring to. i hate to pass this one up,(work is work, ya know). any help from you guys on this forum would be appreciated.
thanx, ron

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#13141 - 08/25/02 07:51 PM Re: testing capacitor banks
electric-ed Offline

Registered: 07/08/02
Posts: 184
Loc: Canada
Power Factor correction capacitors are normally rated in Kilovars. (kvar)
I assume this is a three phase installation. You say each motor has it's own capacitor bank. This could be three single phase capacitors or one three phase unit. They should be connected similar to the 3 phase example in the sketch below. (This is a very basic schematic with no protective or control devices shown.)

Tests -
Current should be equal in all three leads.
Measure the volts and amps and use the formula (in the sketch) to calculate the kvars. Should be as high or higher than the namplate rating.
To determine if the correct capacitor is installed is more complicated. For more information, see the sample calculation on my website at

Hope this helps,

#13142 - 08/26/02 02:36 AM Re: testing capacitor banks
gramps Offline

Registered: 08/25/02
Posts: 120
Loc: ohio
thanx, electrical-ed, for your quick response. yes, these are all 3-phase applications. most of them also have Benshaw soft-starts on them. btw, great website!

thanx, ron

#13143 - 08/26/02 06:29 AM Re: testing capacitor banks
JBD Offline

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Three phase capacitors are easy to test. You should be drawing equal current in each leg of the bank (this is similar to testing a transformer).

Most manufacture of electronic soft-starts recommend extreme caution when using capacitors with their SCR controlled equipment. Typically you can expect a large number of capacitor cell failure (often violently). Check with the soft-start manufacturer.


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