Results 1 to 13 of 13
  1. #1
    Join Date
    Jun 2006
    Posts
    308

    capacitor" too hi or low enough"

    a question was ask not to long ago about capacitor sizing what is the rule of thumbif one does not have the excect capacitor the unit takes,comprasor/blower/fan motor how low ir how hi of a capacitor can i use and still be safe to the unit, for exsample the condanser fan motor it takes a 5.7 uf but i noly have a 7.5 would it be oky, can i go higher but not lower and how hi or low can i use ?

  2. #2
    Join Date
    Feb 2004
    Location
    Medford Oregon
    Posts
    807
    on start capacitors 10% variation won't be a problem. But on RUN capacitors
    the mfd should not vary (can go up on voltage but mfd should be same) Get Copeland's electrical handbook and learn how to combine Capacitors in parralel and series so you can always provide proper mfd:

    examples: 2ea 30 mfd 440v
    in series 30 mfd
    in parralel 15 mfd

    a 5 + 10 in parallel = 15 mfd (mfd adds)

    I need a 15 mfd at 440 v
    all I got are 30 mfd at 330
    30 + 30 in parallel = 15 mfd at 660 volts (volts add)

    pages A7,A8,A9 of Copeland's handbook give details on this

  3. #3
    Join Date
    Feb 2004
    Location
    Medford Oregon
    Posts
    807
    sorry
    2 each 30 mfd in parralel = 60 mfd (mfd's add - I got bifocals)

  4. #4
    Join Date
    Feb 2004
    Location
    Medford Oregon
    Posts
    807
    Sorry, my typing didn't follow my brain today

    here is an excerpt from Copeland's handbook

    Capacitor Voltage
    The voltage rating of a capacitor indicates the highest nominal
    voltage at which it is designed to operate. Use of a capacitor
    at voltages below its rating will do no harm. Run capacitors must
    not be subject to voltages exceeding 10 percent of the nominal
    rating, and start capacitors must not be subjected to voltages
    exceeding 30 percent of the nominal rating. The voltage to which
    a capacitor is subjected is not line voltage, but is a much higher
    potential (often called back electromotive force or back EMF) which
    is generated in the start winding. On a typical 230 volt motor,
    the generated voltage may be as high as 400 volts and is determined
    by the start winding characteristics, the compressor speed, and
    the applied voltage. PARALLEL AND SERIES CAPACITORS Capacitors,
    either start or run, can be connected in series or parallel to
    provide the desired characteristics if the voltage and MFD are
    properly selected. When two capacitors having the same MFD rating
    are connected in series, the resulting total capacitance will be
    one half the rated capacitance of a single capacitor.

    The formula for determining capacitance (MFD) when capacitors are
    connected in a series is as follows:
    1/MFD = 1/MFD + 1/MFD

    For example, if a 20 MFD and a 30 MFD capacitor are connected in series,
    the resultant capacitance will be:
    1/MFD = 1/20 + 1/30
    Get common denominator
    1/MFD = 3/60 + 2/60
    1/12 = 5/60 = 3/60 + 2/60
    MFD = 12


    the voltage rating of similar capacitors connected in series is equal
    to the sum of the voltage of the two capacitors. However, since the
    voltage across individual capacitors in series will vary with the rating
    of the capacitor, for emer¬gency field replacements, it is recommended that
    only ca¬pacitors of like voltage and capacitance be connected in se¬ries to
    avoid the possibility of damage due to voltage be¬yond the capacitor limits.
    When capacitors are connected in parallel, their MFD rating is equal to the
    sum of the individual capacitances. The voltage rating is equal to the lowest
    voltage rating of the in¬dividual capacitors.
    MFD + MFD + MFD
    20 = 15 + 5

    It is possible to use any combination of single, series, or parallel start
    capacitors, with single or parallel run capacitors (run capacitors are seldom
    used in series).

    just try doing a few till you get the hang of it, and very seldom will it be
    that you can't provide proper mfd's

  5. #5
    Join Date
    Sep 2006
    Location
    Winter Springs, Fl
    Posts
    1,755
    rocket, is the copeland book something that can be downloaded from their website or does it have to be ordered from copeland? If it can be downloaded, do you have a link?
    If you can't learn to do something well, learn to enjoy doing it poorly.

    The HVAC-Talk Educational Forums is the best place on the net for your HVAC/R info!!!

    VETO PRO PAC - The Official Tool Bag of HVAC-Talk.com

  6. #6
    Join Date
    Feb 2004
    Location
    Medford Oregon
    Posts
    807
    not available for download (about 150 pages)
    you can get at any Copeland Wholesaler, though has $7.00 price, a good customer should be able to get it free. (I did)
    "Electrical Handbook 6400"

  7. #7
    Join Date
    Jul 2000
    Location
    Louisiana , USA
    Posts
    3,280
    This is Turtle

    Here is the old time rules when all Cap.s was retangle only.

    Up or Down 10% of the uf ----- Up or Down to the next size of the uf.

    Here is the uf sizes 2.5 , 5 , 7.5 , 10 , 12.5 , 15 , 20 , 25 , 30 , 35 , 40 , 45 , 50 , 55 , 60 , 70 , 80 , 90 .

    TURTLE

  8. #8
    Join Date
    Feb 2004
    Location
    Medford Oregon
    Posts
    807
    Turtles rule is fine for START capacitors

    it is wrong for RUN Capacitors, RUN capacitors need to be same mfd
    wrong mfd can seriously affect performance and rpm of motor, don't do it!

  9. #9
    Join Date
    Feb 2004
    Location
    Medford Oregon
    Posts
    807
    questions:
    how long is start cap in the circuit while compressor is running (hint less than 5 seconds)

    how long is run cap in the circuit while compressor is running (hint all the time)

    define power factor (as related to a run capacitor)

    add on limits per Tecumseh on substituting run capacitors
    10 to 20 mfd (max + 2-1/2 mfd)
    20 to 50 mfd (max + 5 mfd)
    over 50 mfd (max + 10 mfd)
    above from Tecumseh handbook table 3-8

    add on limits per Copeland on substituting run capacitors
    0 mfd

    Answering the first 3 questions indicates why RUN cap mfd should not vary

  10. #10
    Join Date
    Oct 2007
    Location
    ATLANTA
    Posts
    120
    Quote Originally Posted by rocket View Post
    Sorry, my typing didn't follow my brain today

    here is an excerpt from Copeland's handbook

    Capacitor Voltage
    The voltage rating of a capacitor indicates the highest nominal
    voltage at which it is designed to operate. Use of a capacitor
    at voltages below its rating will do no harm. Run capacitors must
    not be subject to voltages exceeding 10 percent of the nominal
    rating, and start capacitors must not be subjected to voltages
    exceeding 30 percent of the nominal rating. The voltage to which
    a capacitor is subjected is not line voltage, but is a much higher
    potential (often called back electromotive force or back EMF) which
    is generated in the start winding. On a typical 230 volt motor,
    the generated voltage may be as high as 400 volts and is determined
    by the start winding characteristics, the compressor speed, and
    the applied voltage. PARALLEL AND SERIES CAPACITORS Capacitors,
    either start or run, can be connected in series or parallel to
    provide the desired characteristics if the voltage and MFD are
    properly selected. When two capacitors having the same MFD rating
    are connected in series, the resulting total capacitance will be
    one half the rated capacitance of a single capacitor.

    The formula for determining capacitance (MFD) when capacitors are
    connected in a series is as follows:
    1/MFD = 1/MFD + 1/MFD

    For example, if a 20 MFD and a 30 MFD capacitor are connected in series,
    the resultant capacitance will be:
    1/MFD = 1/20 + 1/30
    Get common denominator
    1/MFD = 3/60 + 2/60
    1/12 = 5/60 = 3/60 + 2/60
    MFD = 12


    the voltage rating of similar capacitors connected in series is equal
    to the sum of the voltage of the two capacitors. However, since the
    voltage across individual capacitors in series will vary with the rating
    of the capacitor, for emer¬gency field replacements, it is recommended that
    only ca¬pacitors of like voltage and capacitance be connected in se¬ries to
    avoid the possibility of damage due to voltage be¬yond the capacitor limits.
    When capacitors are connected in parallel, their MFD rating is equal to the
    sum of the individual capacitances. The voltage rating is equal to the lowest
    voltage rating of the in¬dividual capacitors.
    MFD + MFD + MFD
    20 = 15 + 5

    It is possible to use any combination of single, series, or parallel start
    capacitors, with single or parallel run capacitors (run capacitors are seldom
    used in series).

    just try doing a few till you get the hang of it, and very seldom will it be
    that you can't provide proper mfd's

    Hi

    Im confused on the example below. Can someone help me to understand this? A 20mfd and a 30mfd equals 12mfd????? Also is there an illustration of how these are connected in series??????

    The formula for determining capacitance (MFD) when capacitors are
    connected in a series is as follows:
    1/MFD = 1/MFD + 1/MFD

    For example, if a 20 MFD and a 30 MFD capacitor are connected in series,
    the resultant capacitance will be:
    1/MFD = 1/20 + 1/30
    Get common denominator
    1/MFD = 3/60 + 2/60
    1/12 = 5/60 = 3/60 + 2/60
    MFD = 12
    Last edited by AtoZhvac; 09-29-2008 at 12:41 AM. Reason: Added question

  11. #11
    Join Date
    Dec 2000
    Location
    phoenix, arizona
    Posts
    1,125

    Does voltage make a difference on capacity requirement?

    Quote Originally Posted by rocket View Post
    questions:
    how long is start cap in the circuit while compressor is running (hint less than 5 seconds)

    how long is run cap in the circuit while compressor is running (hint all the time)

    define power factor (as related to a run capacitor)

    add on limits per Tecumseh on substituting run capacitors
    10 to 20 mfd (max + 2-1/2 mfd)
    20 to 50 mfd (max + 5 mfd)
    over 50 mfd (max + 10 mfd)
    above from Tecumseh handbook table 3-8

    add on limits per Copeland on substituting run capacitors
    0 mfd

    Answering the first 3 questions indicates why RUN cap mfd should not vary
    Let me add this to the confusion; What if L1-L2 = 208VAC, What if L1-L2 =230VAc, What if L1-L2 = 245Vac. Do you need to adjust capacitance?
    Decide what you want, decide what you are willing to exchange for it. Establish your priorities and go to work." H.L. Hunt

    "In order that people may be happy in their work, these three things are needed: They must be fit for it. They must not do too much of it. And they must have a sense of success in it." John uskin

  12. #12
    Join Date
    Nov 2003
    Location
    Montreal, Qc.
    Posts
    780
    You guys should sign up for professional memberships.

    Here are some good threads on capacitance in the pro section.


    Sign up for pro status here
    http://hvac-talk.com/vbb/showthread.php?threadid=116113

  13. #13
    Join Date
    Jul 2007
    Location
    regina saskatchewan
    Posts
    257
    As someone mentioned above,10% diff. of mf rating on start caps is ok.Run cap mf rating should be the same as original.VAC ratings must be the same or greater on capacitors.For example:a 35mf 370 vac cap can be replaced with a 35mf 440 vac cap.
    Artificial Intelligence is no match for Natural Stupidity

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event