Measure capacitance on HVAC capacitor

Discussion in 'General Electronics Chat' started by gisdude, Sep 5, 2014.

  1. gisdude

    Thread Starter Member

    Oct 30, 2008
    16
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    Hi all. Recently my HVAC went belly up and I kinda figured it was the capacitor (I replaced it about 6 years ago). So I removed it and checked the terminals. There are 3 labeled: 1. "C" - common, 2. "Fan", and 3. "Herm". The name plate of the bad cap was 40MF(MICROFARAD) +/- 7.5. The readings on all three terminals was ZERO. Ok. So I get a new cap at the appliance store, with same specs, I tested all three terms and guess what? It reads ZERO across all three terms! I installed the new cap and everything is working fine, but I'm wondering if I'm using the CAPACITOR function wrong with my meter? I've attached a pic of the meter and the settings of the capacitor values. I tried ALL the settings.

    I'm pretty sure it's operator error, but I'd like to know for future...

    Thx,
     
  2. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    A lot of DMMs with capacitance ranges can't measure much past 20uF. Check your users manual and see what the maximum readable capacitance is for your particular meter.
     
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  3. #12

    Expert

    Nov 30, 2010
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    You know the capacitor terminals are on the left of the meter? Of course you do.

    HVAC caps don't read correctly at low voltages. You have to connect them to a power line and measure the current, then do the math to find their capacitance.
    Xc = 1/(2Pi F C)
    and E/I = R (or Xc)

    Here in America, I boiled that down to C = amps/(377 x volts)
    An amp off a 120 volt line would be 22 uf.
     
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  4. gisdude

    Thread Starter Member

    Oct 30, 2008
    16
    0
    ok...well I have a lame meter. When I took the old cap into the appliance shop, the clerk measured "0L", which we both interpreted to be a failed cap. He used a clamp on meter with alligator leads. Maybe I should invest in a new meter?
     
  5. #12

    Expert

    Nov 30, 2010
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    Try it on some electronic capacitors first. A lot of perfectly good meters simply don't work on HVAC caps. That's why I HAD to devise a method to measure them.
     
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  6. Fuse_Burner

    New Member

    Dec 9, 2013
    7
    1
    I wouldn't blame the meter, you are trying to measure a 40μf cap with a meter that maxes at 20μf.
     
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  7. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Exactly what I was thinking about and referring to earlier.
     
  8. inwo

    Well-Known Member

    Nov 7, 2013
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    It should have read the fan side. Usually lower value.
    Although I'm not sure if the +- 7.5 was correct, or 7.5 was the fan value.???
     
  9. #12

    Expert

    Nov 30, 2010
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    7.5 uf is the fan capacitor in this dual section can.
    The statement about all three terminals reading wrong might suggest gisdude was measuring from each of the three terminals to the can, which should always be an open circuit. The capacitors are from herm to common and fan to common. Any capacitance to the can is merely unfortunate because it constitutes an AC current leak to bond (earth ground).
     
  10. gisdude

    Thread Starter Member

    Oct 30, 2008
    16
    0
    @ #12. I measured from C-Herm AS WELL as C-Fan. Still no go.
    I'll have to try your method.

    @fuse burner - You made me realize that just now!
     
  11. MrChips

    Moderator

    Oct 2, 2009
    12,440
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    Just to reiterate what has been said already:

    1) Your meter does not measure above 20uF.

    2) You cannot use the meter test leads to measure capacitance. The connections for measuring capacitance are the two slots on the left of the range switch.
     
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  12. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    gisdude
    Welcome to the DIY air conditioner maintenance. The run and start capacitors have made the HVAC people millions of dollars over the years. It is good to see more people learning the tricks of the trade. I keep a stock of run/start capacitors along with a fan motor ready for my house AC. You should do the same.
     
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  13. #12

    Expert

    Nov 30, 2010
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    Umm...if you want to cover that angle, most people need 2 fan motors because the outside motor doesn't match the inside motor. Of course, you could get all smart and figure out which motor will work in both positions It doesn't matter much if the horsepower is a little high. A 1/3 HP motor on a 1/4 HP fan blade just won't use its rated full load amps. Watch them RPMs while you're at it. HP required goes up by RPM cubed! A 1750 RPM motor in a 1050 RPM position will exceed its amp rating immediately.

    ps, buy a matching capacitor with each motor so you aren't hunting for some queer dual section cap on a Saturday at 4 P.M.
     
    Last edited: Sep 6, 2014
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