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Thread: Maintenance Costs - PFC Capacitors

  1. #1
    Join Date
    Feb 2003
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    Seattle, WA
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    Maintenance Costs - PFC Capacitors

    It is being suggested that removing the requirement for using power factor correction (PFC) capacitors will save the owner in maintenance costs down the road. I should have thought that these things would not require any maintenance.
    1. What (if any) maintenance would a PFC capacitor bank require?
    2. Is there a reasonable time period such devices can be expected to function without requiring maintenance or replacement?
    3. If there is a device that automatically adds or removes capacitors from the circuit in order to control PF, would that thing require significant periodic maintenance (i.e., enough to make the cost of maintenance worth considering in the decision to include or not include PFC)?
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  2. #2
    Join Date
    Sep 2008
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    UK
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    Quote Originally Posted by charlie b View Post
    It is being suggested that removing the requirement for using power factor correction (PFC) capacitors will save the owner in maintenance costs down the road. I should have thought that these things would not require any maintenance.
    1. What (if any) maintenance would a PFC capacitor bank require?
    2. Is there a reasonable time period such devices can be expected to function without requiring maintenance or replacement?
    3. If there is a device that automatically adds or removes capacitors from the circuit in order to control PF, would that thing require significant periodic maintenance (i.e., enough to make the cost of maintenance worth considering in the decision to include or not include PFC)?
    The maintenance we did was usually during the two week annual shut. Visual inspection, clean, terminal tightness check.
    Yes, there are control circuits that will switch in or out capacitor banks to maintain power factor at, or close to, desired value.
    Si hoc legere scis nimium eruditionis habes.

  3. #3
    Join Date
    Jul 2006
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    San Francisco Bay Area, CA, USA
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    An APFC (Automatic Power Factor Control) system is basically an electronic controller and a bunch of contactors connected to various sizes of PFC caps. The controller measures the displacement PF and decides how many kVARs are needed, then decides which of the various sized caps will get there and turns them on (or off) as needed. Capacitor switching is hard on contactors and there is usually a little dampening resistor across them (see photo, the little wired devices on top of each contactor). That helps, but the contactors eventually wear out and have to be replaced, much quicker if that resistor burns out. Also as caps age, they can fail. Average lifespan is considered to be 7-10 years. So periodically it's recommended to check and track the series resistance to try to determine when they are beginning to outlive their usefulness so that you can do predictive replacement. If they do fail on you, it's usually a big mess.

    Click image for larger version. 

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    Consequences of ignoring it for too long:
    Click image for larger version. 

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    Last edited by Jraef; 06-07-18 at 04:06 PM.
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  4. #4
    Join Date
    Aug 2015
    Location
    Frisco Texas U.S.A.
    Posts
    15
    There are many variables with PF Cap maintenance and usually tied to the power supplied equipment, I.e. UPMs, Static UPS, where the choke transformer losses are suceptable to increases in temperature, room conditions, air flow, etc. Some equipment requires internal caps for PF correction and steady state noise and or surge/sags.

    Larger PF caps on your primary incoming voltage, normally maintained and automatically regulated by the power company and included in your monthly utility costs can also be a single source of failure, especially if only one primary feed.

    The goal is to increase the voltage on your primary feed when the loads on the building increase, conversely, when building loads are low, voltage on primary goes up and needs to automatically adjust down.

    PF correction on your low voltage can be achieved by the use of more three phase motors, rather than a series of single phase motors.

    PF correction was also included years ago in the critical power industry when using a rotary UPS, ( synchronous motors/synchronous generator combination.

    There will always be a need for some type of PF correction.

    Caps/historically used as a low cost alternative to designing the perfectly balanced linier loads that never change. (Not common)

    As long as there's inductive loads, there will be PF caps in some capacity. No Pun!

    Sent from my LGMP260 using Tapatalk

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