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help with dead compressor??

Discussion in 'The Pub' at netrider.net.au started by bikeboy, Jan 11, 2011.

  1. Hope to get some advice from the lectrickery-whizzes here. My compressor has shat itself, tripping the safety switch. By the time I figured out what it was, the shed stank of burnt electrics (I was inside the house at the time). The motor was real hot, but had not been running. The housing on top that contains a couple of (what I think are) capacitors was melted. I've noticed over the last couple of days that it would be sluggish to kick-in, but once running was fine. The motor still spins freely, as does the compressor pulley, but these two components still stink:

    parts.



    Am I right in thinking these are capacitors of some type? Is their purpose to provide the motor with a high voltage kick in the pants to get it moving? Are the numbers indicative of anything (and why are the different??). Would Dick Smith carry them?

    Any help welcome. I'm hoping if I replace these I'll be good to go. You don't realise how much you depend on these things until they ain't there :-s


    cheers guys
    ian
     
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  2. Have you tried a defibrillator?
     
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  3. Yes they're capacitors, given the size of them I'd suspect they're for letting an electric motor run direct off AC current. Might be too big for the likes of Dick Smith, probably have more luck with Jaycar.
     
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  4. They're capacitors, and they act "kinda" like a primitive battery: they store charge, then release it at once.
    The blue one is a 300 micro-Farad (unit of capacitance) one, the darker one is 40-something?
    The CD60 and CBB60 are both "Motor Starting" capacitors, so you're dead-on with the kick-in-the-pants theory!

    But I don't think Tricky Dicky would carry them, just a guess...
     
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  5. Middys will have them, off the shelf.
    Middendorp, They are all over the place.
     
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  6. Sorry, do not touch the two wires to your Body, they will give you one hell of a jolt.
     
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  7. thanks very much guys. Sounds like a cheap(?) fix. We have both a jaycar and middys here, so I'll go shopping tomorrow.

    good point on the electrocution point too. I hadn't thought of that :eek:hno:

    cheers boys
    ian
     
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  8. Yeah shoulda mentioned that, sorry!
    I erm, know someone who made a stungun using a large cap - strong enough to pit solid copper...
     
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  9. We used the capacitor out of my Bonnie to shock my mate when he was dead drunk.
    Its about 30mm high and 20mm across, a very little one,
    I put it in the 240 outlet and switched it on, Pulled it out of the switch. then a bit of spit on the back of each hand and touched the wires to his hands, He actually jumped out of the chair he was flaked out in.
    I had sparkys for mates, other wise, I would not have known,

    One that size will probably kill you stone dead.
     
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  10. Just to clarify 240V (single phase motors) require a capacitor to start. Something to do with direction.

    Flea-bay.
     
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  11. cool. That was my next question. Can I test the motor without these connected to see if that is fried? Seems not.

    cheers
    ian
     
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  12. something in my memory has my old man starting motors with his hand. So he'd turn the motor on and then yank the pulley in the right direction, then get his hand out of the farking way really quick.

    I really don't advise doing it.

    there would be a few ways of testing caps, but those are getting biggish and you would want to do it the right way.

    10s of micro-farads caps are cheap enough. They don't start getting dear until you get into 10,000s micro-farads (I've never seen them called milli-farad capacitors).

    You won't have to get the value exactly right. Somewhere in the ballpark should be fine.
     
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  13. No you can't, but if you DID pop the caps (judging by the brown smell you describe...), there's a good chance that it saved the windings.
     
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  14. ibast is correct you only need the capacitors during starting and once the motor is up to speed commonly a centrifugal switch will disconnect the capacitors from the circuit. You can start the motor without the capacitors by giving the shaft a spin prior to applying power so that it is already moving in one direction. You will probably need to disconnect the compressor from the motor to do this.

    However as you can see from drawing below there are two windings, main which does all the work and the start winding which the capacitor connects through. It may be that the main winding is intact but the start winding has failed. This is a common fault if the centrifugal switch gets stuck closed because both the capacitor and start winding are short term rated and expect to be only run for short periods. The main winding is continuous rated.

    200px-Condensatormotor.svg.
     
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  15. Yup, starting capacitors which are there to let what is, effectively, a three phase motor run on single phase. The ones on Chinese motors often go pop and stink the workshop out. Previously I've just replaced 'em with others off otherwise dead motors. When I run out I'll have to look at where to get replacements. Dead Chinese motors from the tip I suspect.

    The other common failure mode of these motors is the centrifugal switch that should trip out after the motor starts and spools up to speed. They often stick. Trouble is, the motor will run fine with a stuck switch but will become white-hot after a few minutes. When the motor on my mill did it, the first I knew was when the plastic cooling fan melted and came out through the vents as scalding liquid :shock:. Still, that was two more OK capacitors to add to my stash :D.

    If your switch is OK, you can usually hear it dropping back into contact as the machine slows down on shutdown.
     
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  16. Yeah, when I was a kid we had a lathe like this. It had an ancient three-phase motor, scavenged by my grandfather from God knows where, with a Heath Robinson arrangement of ancient capacitors that, evidently, weren't up to the job. Switch on, spin the chuck by hand and away you went. Until swarf got into the not very well sealed terminal box and caused a bright blue bang, followed by total darkness, anyway. I thought I'd blown meself up somehow :shock:.
     
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  17. Thanks guys. That's some great info there.

    Pat, this is what happened to my motor too. It seems I was off in my diagnosis (not surprising since I know nothing about this stuff!). I took the capacitors in to have them tested at a motor rewind shop down here, and he said they were fine. My motor has shat itself :-s

    I explained to him it would start, eventually, then run fine. But when it trip the RCD and I went to investigate, the fan at the rear end had melted it's way off, and the tray that holds the capacitors on top of the motor had also melted in place. He said the motor is dead :cry:

    Oh well. Just another delay for the project :?

    thanks a heap guys. Lots of stuff to digest here.

    cheers
    ian
     
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  18. I got a much better quality replacement motor for my mill for (IIRC) <$200 off the shelf. Unless you're very unlucky you will have a standard induction motor which can be swapped over with any other standard induction motor with the same frame size. It should be just a case of unbolting the old motor, bolting on the new one, swapping the pulley and connecting up the wires.
     
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  19. whose self pat? the rewind-guy quoted $400 for a 3 HP replacement :cry:

    That sounds like a better deal to me :wink:
     
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  20. It was an outfit called Metrinch in Perth. The mill motor was (I think) a 2hp rather than 3. Might have been 2.5. I'm pretty sure the price was $195. It was something 95 anyway and I doubt if I'd have felt I got off as lightly as I did if it was nearly $300 :D.

    Sorry, I can't be more specific. I'm currently 600km from both mill and banking records.

    It's still Chinese but is visually much higher quality and has worked hard (by amateur standards) for five years. The previous unit lasted 2 months. In spite of having the same HP rating, I remember it feeling significantly more powerful than the original. The original would audibly slow when taking a big cut. The replacement would not for any cut the machine as a whole could handle.

    Pity the mill's still rubbish :LOL:.
     
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