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  1. #16
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    Check that the belt is not running on the bottom of the pulley. This will be the case if the bottom of the pulley is brightly polished.

    For a vee belt to work under full load it must only touch the pulleys with its sides. If it is running on the bottom the belt is worn out and needs replacing. If the replaced belt still runs on the bottom, (usually of the motor pulley,) then the pulley is worn out and needs to be replaced as well.

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  3. #17
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    Aug 2009
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    Thans dan it has a new belt on it

    I just bought the capacitor should be here monday/tuesday

  4. #18
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    Somewhere it got lost in straslation i wanted to pick up the ebay capacitor and it got sent out in the mail but thats ok it wouldnt be here untill thursday next week so i found another ebay seller and picked one up today

    Some farting around going to jaycar than to rs online than back to jaycar i was trying to work out which terminals connectors would work on 450volt jaycar was unsure but rs online looked it up theirs were good for 600volts but i only needed 2 they sold them by the 100 pack so the guy at rs online said to go back to jaycar as i did

    I just installed the new capacitor and powered her up she ran faultlessly i hit the shut off and lowered the pressure 4 times happy to say a $19 capacitor saved me spending $500 on a new air compressor

    Thanks for all the help, going to sped my cash on a new hose reel, water catch and maybe a digital pressure gauge plus some new fittings

  5. #19
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    For anyone interested in checking their capacitors the bloke today checked mine before selling it to me, he used the 200ohms setting my old capacitor i only got a reading on 2000ohms and it was reading 180+

    Its a 50uf or 50 micro farads so testing should show a good reading of 50+- 5ohms

  6. #20
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    Quote Originally Posted by gazza2009au View Post
    For anyone interested in checking their capacitors the bloke today checked mine before selling it to me, he used the 200ohms setting my old capacitor i only got a reading on 2000ohms and it was reading 180+

    Its a 50uf or 50 micro farads so testing should show a good reading of 50+- 5ohms
    Can't see how this works.
    The resistance of capacitor should be MEGA Ohms, I just measured a bunch and got from 3 MΩ upwards.
    If it is as say kΩ or less it's stuffed or nearly stuffed.

    Capacitance (the amount of electrical Charge held in store per Volt applied to an object, units in Farads) and Resistance (the ability of an object to conduct a charge - Units Ohms) are not related, and a resistance meter should not be used to measure capacitance. Some multimeters have a Capacitance measuring mode but that is often restricted to less than 100 µF. There is a way of measuring higher capacitances with such meters by combining known capacitors together with the one you are testing in series.

    Lets say you have capacitor that you want to test that is supposed to be 150 µF but your meter tops out at 50µF
    If you have a 20 µF cap (check exactly that it is that), and then combine it in series with the one you are testing (Cx) and measure the capacitance of the two (Ctotal) with the meter.
    Ctotal should always be smaller than the smaller of Cx and the 20 µF cap and should follow the 1/Ctotal = 1/20 + 1 /Cx rule

    Lets say you measure Ctotal to be 17.5 µF .

    Solving for Cx 1/17.5=1/20+ 1/Cx gives 1/Cx = 1/17.5 - 1/20 = 0.0714 when inverted gives 140 µF

  7. #21
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    The problem with motor capacitors is not that they become leaky, which is all that a resistance reading will discover, but that with time they loose capacitance i.e. become too small for the job.

    The leaky ones just get hot and explode their guts out.

  8. #22
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    Quote Originally Posted by Bohdan View Post
    The problem with motor capacitors is not that they become leaky, which is all that a resistance reading will discover, but that with time they loose capacitance i.e. become too small for the job.

    The leaky ones just get hot and explode their guts out.
    I believe its called "dielectric breakdown" - its the ability of the dielectric material between the capacitor plates to sustain an electric field at a given V, but as you say many multimeters cannot measure the degree to which it has broken down, it may be from 10's of megohms to just megohms. Over time this can lead to small leaks which will rapidly grow to result in the blowout.

  9. #23
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    I was just using 2 multimeters the guy at the factory used a meter with a 200ohm setting with alligator clamps on the cables

  10. #24
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    Quote Originally Posted by gazza2009au View Post
    I was just using 2 multimeters the guy at the factory used a meter with a 200ohm setting with alligator clamps on the cables
    Curious - are you sure he wasn't using a Capacitance setting?

    Anyway I'm really pleased you got it sorted - Happy sanding!

  11. #25
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    I'm not sure Bob it just looked like a regular multimeter to me i dont know a lot about them

    And yeah i am absolutely cheering the compressor got sorted saved a few bucks only bad thing is the new capacitor is to big to fit inside the housing but only slightly

  12. #26
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    I have checked large capacitors with the ohm reading before. You put the leads on on way and as the capacitor charges the reading goes up. Put the leads on the opposite way and the reading goes down. I am sure this is not the correct way to test but it does seem to work. I do t remember the exact values or relationship of the readings if any.


    Sent from my iPad using Tapatalk Pro

  13. #27
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    Quote Originally Posted by forrestmount View Post
    I have checked large capacitors with the ohm reading before. You put the leads on on way and as the capacitor charges the reading goes up. Put the leads on the opposite way and the reading goes down. I am sure this is not the correct way to test but it does seem to work. I do t remember the exact values or relationship of the readings if any.


    Sent from my iPad using Tapatalk Pro
    Mine read 180 but kept going up and down one way and the other way it just read 1 as in the first digit not a actually + or minus as in polarity

  14. #28
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    Quote Originally Posted by forrestmount View Post
    I have checked large capacitors with the ohm reading before. You put the leads on on way and as the capacitor charges the reading goes up. Put the leads on the opposite way and the reading goes down. I am sure this is not the correct way to test but it does seem to work. I do t remember the exact values or relationship of the readings if any.
    All this really does is indicate if the capacitor is either shorted or not, but it's not the correct way to get a reading of the Capacitors actual capacitance. It may not be shorted and able to accept charge but it's actual capacitance may be outside spec which usually means it needs to be replaced.

    If the cap is not shorted, a resistance measurement also only works if your meter has a high enough resistance reading capability relative to the capacitors resistance. If the meter has a resistance limit of 20MΩ but the resistance of the cap is more than this the meter will not show charge or discharge and show it is outside its reading range.

    I tested two capacitors this way with one of my meters that can go to 100MΩ. One 100µF cap read ~18MΩ and showed the charge and discharge characteristic but the other (rated at 2µF) showed it was outside the resistance range but when I tested it with another meter that has a capacitance measuring range it showed it was 2.1µF

    Bottom line is don't rely on resistance measurement for capacitor as this could damage a motor.

  15. #29
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    Got the compressor running to pump up a tyre and it returned to doing what it was doing before I'm at a loss as to what is wrong with it, ready to just throw it out in the rubbish and buy another second hand one

  16. #30
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    Think i may have found the problem and it was me all along, for a very long time i've had a leak in the hose where i knicked the hose with the grinder and instead of releasing the pressure everyday i use the compressor i have been just allowing the air to slowly leak out of the leaking hose i think there might be a memory still in the pressure cut off switch remembering its almost at cut off when i am just firing up the compressor

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