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sbjoshi
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Join Date: May 2012
Posts: 2

Capacitor For PF Improvement

05/30/2012 2:10 AM

At a construction site I have motors rangin in 200 KW to 30KW (squirrrel cage induction Motors). If I try to improve PF by installing capacitors directly at motor terminals, will there be any issue of switching? will my current s/G (DOL) will remain usefull? Also in case of soft starter started motor what shall be the additional precaution? is there some thumb rule for capacity of such cap bank with motor rating.
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PWSlack
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#1

Re: capacitor for PF improvement

05/30/2012 3:17 AM

Start by examining the economic reason why capacitors are installed.

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PWSlack
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#2

Re: capacitor for PF improvement

05/30/2012 3:52 AM

The best place to install capacitors is at the downstream end of the feeder cable before the motor starting equipment, as it is the feeder cables that are economic to protect. That way, there is no complication with motor starting equipment.

If in doubt, contact a reputable supplier of power factor correction equipment and work with them towards purchsing a correctly-designed, installed and warranted system.

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imagey12
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#3
In reply to #2

Re: capacitor for PF improvement

05/30/2012 4:39 AM

If I try to improve PF by installing capacitors ...http://imageediting.outsourcing-services-india.com

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sbjoshi
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#4
In reply to #2

Re: capacitor for PF improvement

05/30/2012 5:39 AM

But putting before Motor s/g will need some equipment for isolation, I want to save that cost. Further, when motor is not running, PF may go in capacitive zone. We work arround 12 Hrs. a day. During working Hrs. also some of the Motors will not be running.
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PWSlack
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#5
In reply to #4

Re: capacitor for PF improvement

05/30/2012 6:24 AM

Oh, look. An argument.

<unsubscribes>

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JRaef
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#6

Re: Capacitor For PF Improvement

05/30/2012 10:36 AM

The best place to locate PFC caps is as close to the motor as possible, and any reputable PFCC mfr will provide you with a selection chart based on your motor size and its existing PF. The only issue on DOL starters is with the settings on your motor thermal overload protection, and contact your supplier for that as to whether or not to compensate for the presence of the PFCCs.

With soft starters it is a different matter. You CANNOT have capacitors down stream of the SCRs in the soft starter, it becomes a race to see which one fails first. You must use a separate contactor to switch in the PFCCs after the soft starter goes into bypass (assuming such) or at the very least, only after and if the SCRs are gated fully on.

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rodeh
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#7

Re: Capacitor For PF Improvement

05/30/2012 6:20 PM

Hi,

Since your load is mainly DOL with frequent switching, it is best you invest in PF management at the bus level. For each switch board you may install PF correction with switched stages to allow for optimal PF management. This is on the assumotion that your site is fairly large.

If the site is not fairly large then you may use PF correction panel at the main switchboard with CTs installed at the mian feeder , thus providing global compensation.

You can not install a dedicated capacitor for SS driven load , hence it will be more reasonable to use either options above.

You may contact the nearest Schneider Electric branch in your city and they will be abel to help size and supply an optimal solution to your site.

cheers
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JRaef
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#8
In reply to #7

Re: Capacitor For PF Improvement

05/30/2012 8:18 PM

Aside from being a blantant advertisement, this is actually a very poor recommendation. Bulk power factor correction is the worst approach, not the best. it is responsible for many many field failures of equipment, especially anything with electronic power devices such as VFDs and Solid State Starters or heating Controllers.

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rodeh
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#9
In reply to #8

Re: Capacitor For PF Improvement

05/30/2012 8:59 PM

Hi,

Bulk PFC was never my intention since i mentoined switched stages using a PFC controller. PFC controllers can measure PF changes then determine the size of the step required. Zig-Zag controllers operate with dead band allowing them to obsorb rapid changes in reactive energy when an induction motor is energized. This is a standard product which has been available for long time and can be purchased from any supplier. Since the loads are mainly DOL type ,thus THDV% &THDI% will not have any adverse effect on the capacitors. The fact VFDs are not present on site then standard PFC equipemnt shall be used.

Hope this will clear any confusion.
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imagey12 (1); JRaef (2); PWSlack (3); rodeh (2); sbjoshi (1)

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