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  1. #1
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    Default Motor capacitor 3HP DC

    SHMBO was working in the shed and had the DC running for about two hours. Half way through that time she thought a palm branch had fallen on the shed.

    I came in when the DC was switched off and after a time noticed a typical burning smell of electronics / wiring. Found nothing but noticed the DC gate was still open. Check the DC outside and found the following. I stretched the plastic on the CAP back enough with a heat gun to make out the Cap value.

    It's a 60 uf 350 vac.

    I'm hoping that this is a Run cap and that there is no start cut-out switch in these big DC 3 HP motors. Could anyone who has pulled down these typical 3HP DC motors confirm?








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  3. #2
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    Quote Originally Posted by MandJ View Post
    It's a 60 uf 350 vac.

    I'm hoping that this is a Run cap and that there is no start cut-out switch in these big DC 3 HP motors. Could anyone who has pulled down these typical 3HP DC motors confirm?
    All the DCs I've played with have had centrifugal starting switches that the caps are "run caps".

    When you get a replacement see if you can get one rated for 450V.

  4. #3
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    Thanks, if that's the case then would this indicate that the centrifugal switch did not disconnect, as the cap exploded after an hour of run time with no on/off cycles, on the other had, if this is a run cap and it's not in the centrifugal switch circuit then there must be a hidden start cap somewhere? It's not in the switch box.

  5. #4
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    The most likely cause is a breakdown in the electrolyte inside the cap. As the cap ages small leaks develop and some end up self perpetuating so that when the leak is big enough this generates enough heat to pop the cap. This is a pretty common outcome especially with cheap caps. These cheaper caps typically last around 5 years although I have seen some only as old as a few months die like yours.

  6. #5
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    I'll have to have a look in the old parts bin, I still can't bend down or do much so it may have to wait but I may have something close to this value and rated at 450VAC -at least to test it, my concern is the cost of a new unit and finding out that I have an underlying problem with the motor or centrifugal switch, trying to get that switch out will be a pain as I'm pretty sure I'll have to pull the impeller to get to it. Dam - Last thing I need right now.

  7. #6
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    It doesn't sound like the centrifugal switch is the problem. I'd put the replacement cap on and see if it starts. When it gets to full speed turn it off and when you do so, listen for the sound of the centrif switch kicking back on as it winds down - if it does that then that means it turned off when it started.

  8. #7
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    Thanks again, yes that's what I planed on doing, thinking about it, a cap of that rating likely shouldn't have blown in that way unless it was breaking down internally with age. I'll post the result when I get it installed.

  9. #8
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    Fitted a cap and tested, started up fine, listened for a click as it was spinning down and heard nothing, placed a sounding stick against the motor and did the same test - no click as it spun down?

    Thoughts.

    BTW Current draw is still the same as always with all ports closed, don't know if having the start winding permanently in circuit would make a noticeable difference to current? If it's not disconnecting then it wont be long before I burn out the start winding.

  10. #9
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    1: There are NO other caps in this motor - There is only one 60uf CAP.

    2: With the caps connected the no load (intake blocked) current is 4.9A, same as it has always been from day one.

    3: If the cap is disconnected once the motor is running the no Load current jumps to 9.5A.

    If there is a centrifugal switch then it would be simply disconnecting the start winding, however I can see no current difference compared to what it was from new?

    NOTE: I cannot, under any circumstance, hear a centrifugal switch working.

  11. #10
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    Quote Originally Posted by MandJ View Post
    1: There are NO other caps in this motor - There is only one 60uf CAP.

    2: With the caps connected the no load (intake blocked) current is 4.9A, same as it has always been from day one.

    3: If the cap is disconnected once the motor is running the no Load current jumps to 9.5A.

    If there is a centrifugal switch then it would be simply disconnecting the start winding, however I can see no current difference compared to what it was from new?

    NOTE: I cannot, under any circumstance, hear a centrifugal switch working.
    Then it's a permanent split capacitor motors used in situations like fans and blowers which have low starting torques.

    See towards the bottom of the page of
    SINGLE-PHASE INDUCTION MOTORS (Electric Motor)

  12. #11
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    Bob that's what I initially thought but everything I've read seems to imply that it's only used in low power .5 to 1.5 HP motors, that's what made me think it was not. Well if it keeps on going then it must be, don't think the start winding will last long if it's not.

  13. #12
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    I've seen a 3HP version used on an AC fan.

    What's the current with all blast gate valves open?

  14. #13
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    it's around 9A, interestingly with the caps off for a few seconds, there was no change in no load vacuum or motor sound, just almost double the current. It ran like that for an hour the other day afters the cap blew but I don't know what the current was then, still it's rated to 15A.

  15. #14
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    Quote Originally Posted by MandJ View Post
    it's around 9A, interestingly with the caps off for a few seconds, there was no change in no load vacuum or motor sound, just almost double the current. It ran like that for an hour the other day afters the cap blew but I don't know what the current was then, still it's rated to 15A.
    9A is about right.
    if it ran with a blown cap for an hour it was would have been running hot.
    Those motors usually have a thermal cut out on it (have you looked?) so if it got too hot it would have just stopped.
    Being a DC motor (in an enclosure?) it may have been getting plenty of cooling air running past it?

    The 15A rating is just an arbitrary rating to indicate it should be located on a 15A circuit but mostly that is to allow for higher start start up currents.

  16. #15
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    Having never felt the motor after it was running I don't know how hot it would normally get, when I felt it it was hot but not hot enough to make you remove your hand instantly, the 0.5HP motor on the old Woodfast lathe felt about the same after an hours use, I couldn't find my non contact temperature reader at the time.

    Actually I was a bit worried as I now have plastic bags on the bottom and huge pleated filters on top, I had to raise the roof of the DC cabinet/shed about 1/2 meter to fit then in, so not as much air past the motor mounted at the bottom of the housing, I was lucky the exploding cap didn't burn a hole through the one of the plastic bags, it's a bit blackened in one spot but still airtight.

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