Will bad start capacitor blow fuse?
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    Default Will bad start capacitor blow fuse?

    Air compressor went down and is blowing 30A slow-blow fuses. I've checked for a short and there is not any continuity between the line in and motor case. Pump and motor turn freely. It's a big single phase 220v motor. I think, (name plate is hard to read,) it's an 8 or 10 hp motor.

    Can a bad start capacitor be the culprit?

    GM.

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    Definitely, if you have a multi-meter it is easy to check them.

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    Another possibility is a centrifugal switch failure, if if has one for the starting circuit?

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    The centrical switch was replaced about 4 years ago so I suspect it is OK.

    I have a good ohm meter but it doesn't measure microfareds. How would I go about checking them?

    Gm

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    Quote Originally Posted by gmoushon View Post
    The centrical switch was replaced about 4 years ago so I suspect it is OK.

    I have a good ohm meter but it doesn't measure microfareds. How would I go about checking them?

    Gm
    I just Googled this, but it is correct:

    To test the capacitor with a multimeter, set the meter to read in the high ohms range, somewhere above 10k and 1m ohms. Touch the meter leads to the corresponding leads on the capacitor, red to positive and black to negative. The meter should start at zero and then moving slowly toward infinity.

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    Just replace the start capacitor, then you don't have to test it. Watching an ohmmeter response is a ridiculous test for a start capacitor. While you have the cover off look for dirt and bad things. Check the wire connections too. Keep the power turned off and if the power source is out of sight from the motor, lock out the switch or breaker.

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    I should have asked are those fuses blowing immediately or does the motor try to start makes a growling sound and then blows fuses?

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    Quote Originally Posted by Pathogen View Post
    Just replace the start capacitor, then you don't have to test it. Watching an ohmmeter response is a ridiculous test for a start capacitor. While you have the cover off look for dirt and bad things. Check the wire connections too. Keep the power turned off and if the power source is out of sight from the motor, lock out the switch or breaker.
    You just replace things with out testing them and you blast my response?

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    Quote Originally Posted by Dualkit View Post
    You just replace things with out testing them and you blast my response?
    I would always use the multi-meter test first before replacing a capacitor. It's cheap and quick. If it fails on the test, time enough to order another one.

    Remember, this is the Internet where unwarranted contradictions and needless arguments are important elements.

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    Quote Originally Posted by Dualkit View Post
    You just replace things with out testing them and you blast my response?
    Quote Originally Posted by Newman109 View Post
    I would always use the multi-meter test first before replacing a capacitor. It's cheap and quick. If it fails on the test, time enough to order another one.

    Remember, this is the Internet where unwarranted contradictions and needless arguments are important elements.
    There seems to be something about questions regarding electrical components that get answers that don't make sense or flat out wrong. I have actually read people giving step by step instructions that will lead to people shocking themselves. Another one someone step by step gave a procedure that would have erased all the parameters in the machine, this was something good ole P-cancel would probably have cured.

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    Quote Originally Posted by Dualkit View Post
    You just replace things with out testing them and you blast my response?
    The cost of time is way more valuable that the cost of a start cap, if you have to get into the compressor motor removing guards, dog house covers, etc., why not be proactive in the service by doing preventive maintenance.

    I am not slamming your battery voltage level testing of a line voltage capacitor, you keep right on doing it. It's good to practice, not necessarily good practice.

    Apologies

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    Inspect the top of the capacitor. If the top is bulged then contacts have disconnected from the inner leads. This is
    effectively an overload protection if the capacitor is leaky internally or has been subjected to prolonged overloads
    trying to start a stalled motor.

    If the top is bulged, replace it. If not, then remove the capacitor from the circuit electrically: First, disconnect the
    power from the compressor. Short the capacitor with a screwdrive just to be sure, then check to see that a simpson
    meter needle swings when the meter is first connected, after that the needle should eventually reach infinite ohms.

    If the top is not bulged, and it passes the simpson meter test, smart money says the capacitor is not the smoking gun for the
    comressor failure.

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    Quote Originally Posted by Pathogen View Post
    The cost of time is way more valuable that the cost of a start cap, if you have to get into the compressor motor removing guards, dog house covers, etc., why not be proactive in the service by doing preventive maintenance.

    I am not slamming your battery voltage level testing of a line voltage capacitor, you keep right on doing it. It's good to practice, not necessarily good practice.

    Apologies
    It's good practice to test electrical components before replacing them.

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    Quote Originally Posted by Newman109 View Post
    It's good practice to test electrical components before replacing them.
    Especially when new in the box doesn't always mean good these days. I have gotten brand new toggle switches, push button switches, and circuit breakers that did not work. You could throw a perfectly good item away and install something faulty.

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    Quote Originally Posted by Pathogen View Post
    The cost of time is way more valuable that the cost of a start cap, if you have to get into the compressor motor removing guards, dog house covers, etc., why not be proactive in the service by doing preventive maintenance.

    I am not slamming your battery voltage level testing of a line voltage capacitor, you keep right on doing it. It's good to practice, not necessarily good practice.

    Apologies
    My caps are not under guards and sitting right on top of the motor, my meter is in the top of my tool box 25 feet from the compressor. I bet I could get them checked and the covers back on before you could hit the internet and get an order placed and paid for.

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    The multimeter test will show if the cap is effectively open or shorted. If it acts "capacitor-ish", then you've got nuthin, because the multimeter test described will not tell you if your 108-130 uF capacitor is actually only 50 uF due to some problem. If you have the meter, and it's no big issue, then it might be worth doing as a "sanity check".

    many multimeters do have an actual capacitance check range, though. There are faults that will not show up in that test, however.

    If the circuit is acting like the cap is open, but the cap gives a reasonable response, then you know the real issue of the day is elsewhere. But you still do not know if the cap is in-spec. I have a nice little capacitance meter that came from "SUPCO", which does a good measurement, but not if the cap has one of those faults that can fool the meter..

    It is worth just replacing in many cases, simply to avoid future hassles. But if you are not sure the new one is a good one, that changes the suggestion.

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    Quote Originally Posted by gmoushon View Post
    The centrical switch was replaced about 4 years ago so I suspect it is OK.

    I have a good ohm meter but it doesn't measure microfareds. How would I go about checking them?

    Gm
    if the centrifugal switch was "replaced about 4 years ago" , why then would you "suspect it is OK"? sounds to me, that makes it highly suspect.

    if you do not have a cap test function on your multimeter, take it out of the circuit, and test it with a resistance setting on your meter that results in a 4-5 sec. slope to infinity, reverse the leads and observe a similar slope. compare to a reference cap of similar MFD rating. that will only give you a test at the voltage of the ohmmeter, but it is a jumping off point.

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    Of corse “!warning!” Disconnect power using proper lock out tag out procedure and short out capacitor(s) before any test!!!

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    Quote Originally Posted by cyanidekid View Post
    if the centrifugal switch was "replaced about 4 years ago" , why then would you "suspect it is OK"? sounds to me, that makes it highly suspect.
    "Bingo"

    And new start cap. And- if also present - new run cap and new snubber.

    It is ALL about your labour - not being at work on OTHER things, if nothing else, never mind the billing rate - and the challenge to other things not progressing from your air supply DOWNTIME!

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    Quote Originally Posted by JST View Post
    The multimeter test will show if the cap is effectively open or shorted. If it acts "capacitor-ish", then you've got nuthin, because the multimeter test described will not tell you if your 108-130 uF capacitor is actually only 50 uF due to some problem. If you have the meter, and it's no big issue, then it might be worth doing as a "sanity check".

    many multimeters do have an actual capacitance check range, though. There are faults that will not show up in that test, however.

    If the circuit is acting like the cap is open, but the cap gives a reasonable response, then you know the real issue of the day is elsewhere. But you still do not know if the cap is in-spec. I have a nice little capacitance meter that came from "SUPCO", which does a good measurement, but not if the cap has one of those faults that can fool the meter..

    It is worth just replacing in many cases, simply to avoid future hassles. But if you are not sure the new one is a good one, that changes the suggestion.
    I realize the ohm test isn't perfect, but in most cases will be good enough.


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