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Thread: Unisaw motor hums but won't start

  1. #1

    Unisaw motor hums but won't start

    have a 3hp Unisaw that suddenly stopped working. When I push the switch to start, I hear the motor humming, but the motor doesn't start rotating. I can easily rotate blade by hand and see the belts rotating the motor pulley. Any ideas on what the problem is and what I would need to do in order to get saw working again?

  2. #2
    Join Date
    Jan 2004
    Location
    Lewiston, Idaho
    Posts
    23,619
    Rick,

    2 things can generally cause this:

    1. The starting capacitor goes bad.

    2. A lot of motors have a centrifugal switch. When the motor isn't turning, the starting capacitor is put in the circuit. Once power is applied and the motor gets up to speed, the starting capacitor is removed from the circuit.
    What I have seen happen is sawdust gets into the centrifugal switch and prevents the contacts from closing properly. Thus the starting capacitor is never properly put in the circuit.

    I have fixed this 2nd problem by removing the end cover of the motor and blowing it out with my air compressor air.
    Ken

  3. #3
    Thanks, I'll give 2. a try.

  4. #4
    Join Date
    Jun 2007
    Location
    Wichita, Kansas
    Posts
    1,795
    Rick, while you're looking at the switch, look at it from the other direction, also. Not only could the switch fail open preventing the start cap from being connected into the circuit, but it could also fail closed preventing the start cap from being removed from the circuit at the proper time.

    If the second possibility is the case, and I have had that happen, you can kiss the start cap goodbye. They are almost always intermittent duty devices and being in the circuit continuously will destroy them in a very short time.

    If you find that cleaning the switch doesn't cure the problem, you should be able to get a replacement capacitor at any local motor repair shop for a few bucks. Take the cap with you. It needs to be matched, not only for electrical size (mF and volts) but for physical size also.
    Tom Veatch
    Wichita, KS
    USA

  5. #5
    I'd check all wire connections first.
    Plug, inside the switch, and at the motor.


  6. #6
    Join Date
    Feb 2008
    Location
    Northwestern Connecticut
    Posts
    5,863
    Unisaw motor hums but won't start.

    Does it hum a pleasant tune, like "Singing in the rain" ? If so leave it alone and get another saw. I prefer a good melody to the roar of a TS any day.

  7. #7

    More help- where is centrifical switch and capacitor?

    I took one end cover off motor ( opposite pulley) and found just fan. There is another cover on the same end that is held by two nuts that come out from parallel to the motor. I didn't want to tackle taking these nuts off without checking to see if this is correct.

    (note- I have 80's model unisaw model 34-802f)

  8. #8

    problem solved for now

    Took cover off box on Unisaw were electrical wires come in and blew air all around. This solved problem and saw started mormally. Thanks for all input.

  9. #9
    Join Date
    Jan 2006
    Location
    mid minnesota
    Posts
    64
    Rick You should check all of those connections, one must be a little loose. Steve

  10. #10

    Thanks-

    I'll check connections.

  11. #11

    Unisaw start capacitor -wiring order

    My saw was not starting-just humming. Found problem was start capacitor wire was broken. I took the capactior out to clean sawdust caked in the housing and now am not sure if it matters which of the two wires goes with the capacitor ends. Does it matter? ( one of the wires is gold).

  12. #12
    Join Date
    Nov 2006
    Location
    Los Angeles, Calif.
    Posts
    1,977
    Maybe it's humming because it doesn't know the words.

    Sorry, couldn't resist!

    Quote Originally Posted by Rick Cansler View Post
    have a 3hp Unisaw that suddenly stopped working. When I push the switch to start, I hear the motor humming, but the motor doesn't start rotating. I can easily rotate blade by hand and see the belts rotating the motor pulley. Any ideas on what the problem is and what I would need to do in order to get saw working again?

  13. #13
    Join Date
    Dec 2006
    Location
    Toronto Ontario
    Posts
    6,468
    Quote Originally Posted by Rick Cansler View Post
    My saw was not starting-just humming. Found problem was start capacitor wire was broken. I took the capactior out to clean sawdust caked in the housing and now am not sure if it matters which of the two wires goes with the capacitor ends. Does it matter? ( one of the wires is gold).
    Hi Rick, if there are only two wires, it doesn't matter which capacitor terminal you connect them to.

    The capacitor is non polarised.............Rod.

  14. #14
    Join Date
    Nov 2004
    Location
    Belden, Mississippi
    Posts
    2,175
    Just a note of caution. Don't forget that some caps will "store" a charge. You can get a rather sudden surprise from 'em. Used to be a standard prank to play on the newbie in the days of hard wired electronics.
    Bill
    On the other hand, I still have five fingers.

  15. #15
    Join Date
    May 2006
    Location
    Central Florida
    Posts
    250
    Depending on the type of capacitor the polarity can be important. I know electrolytic capacitor if reverse insert can be damaged, I am not sure what type of cap is used in this application.

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