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Thread: Unisaw Problem Starting

  1. #1
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    Unisaw Problem Starting

    I have a Delta Unisaw model 36-816, my Grandpa purchased in New in 1998. Last night I was making really light cuts finishing up some tenons and I shut the saw off to change my setup and the saw wouldn't start again, it just hums. Everything rotates freely, and I replaced the mag switch with a spare I had. That didn't solve the problem. I checked the voltage to the switch and to the motor and that isn't the problem. I am assuming it is the start capacitor. I can turn the saw on and use a board to spin the blade and get it running. There are no schematics of the motor in the original owners manual and I am not sure which of the two capacitors is the start capacitor. Any advice would be appreciated.

  2. #2
    Sounds like the start cap. The start cap will probably be the cap with the largest cap value. Motor caps are pretty cheap so in the worse case, buy two and replace them both.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  3. #3
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    I got a new start capacitor ordered. The start cap had a 10x larger cap value than the other so that seemed obvious enough.

    Next question is can I still run the saw with the capacitor not working? I can use a board to spin the blade and get it moving easy enough. I'm working on a time sensitive project but don't want to hurt the saw.

  4. #4
    Quote Originally Posted by Dylan Wyatt View Post
    I got a new start capacitor ordered. The start cap had a 10x larger cap value than the other so that seemed obvious enough.

    Next question is can I still run the saw with the capacitor not working? I can use a board to spin the blade and get it moving easy enough. I'm working on a time sensitive project but don't want to hurt the saw.
    Yes, once it gets up to speed, it's just like it started normally. Compared to starting with a cap, manual starting is slow so the motor has more time with large current - so I'd start it and leave it running until you're finished. That is, I'd limit how many times I started the motor in a particular time period.

    The additional current will cause heat but once it's running the motor will cool off. But I'd still try to limit how many times I started it in a relatively short period of time.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  5. #5
    If the motor doesn't turn over on its own, you will likely damage it by trying to run it unless you pull start it, which isn't very safe nor practical. You probably should blow out the non-shaft end of the motor to insure that sawdust is not limiting the movement of the centrifugal switch. You may not need to replace the capacitor if the switch is stuck due to sawdust buildup.
    Lee Schierer
    USNA- '71
    Captain USN(Ret)

    My advice, comments and suggestions are free, but it costs money to run the site. If you found something of value here please give a little something back by becoming a contributor! Please Contribute

  6. #6
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    Well no such luck with replacing the start capacitor. Still doing the same exact thing. I hit the start switch and the motor hums and then kicks out the overload in the switch. Any ideas?

  7. #7
    Have you checked the operation of the centrifugal switch? If it doesn't close, the capacitor isn't in the circuit and the motor will just hum.
    Lee Schierer
    USNA- '71
    Captain USN(Ret)

    My advice, comments and suggestions are free, but it costs money to run the site. If you found something of value here please give a little something back by becoming a contributor! Please Contribute

  8. #8
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    Did you look inside the switch box for a schematic? I think that's where mine is. If you can isolate the wires for the centrifugal switch, use an ohm meter to verify the switch is closed when the motor is at rest ( and of course, with the power unplugged).

  9. #9
    Quote Originally Posted by Lee Schierer View Post
    Have you checked the operation of the centrifugal switch? If it doesn't close, the capacitor isn't in the circuit and the motor will just hum.
    I agree with Lee. There's not much to an induction motor. If it runs when you manually spin it up, the problem is in the starting circuit. If it's not the cap, it's the centrifugal switch.

    If that doesn't fix it, then you might have an open starting coil which would be more serious. But that's very unusual.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  10. #10
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    Alright, there is not a schematic in the electrical box. I just pulled the motor out because I can't work on the thing under the table. So my problem is that I know very little knowledge about motors like this. So any help with a schematic of some sort would be good. I already pulled the end off the motor that has all of the guts of the motor tied to it. I blew the inside of the motor out with compressed air and put it back together quickly because it looked way over my head. I would assume the centrifugal switch is inside the motor. Thanks again for the advice so far.

  11. #11
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    You will need to remove the non-shaft end of the motor to inspect the centrifugal start switch. The switch itself will be attached to the inside of the end cover on the motor. On the motor shaft there is a mechanism that operates the centrifugal start switch. There are two weights that are hinged and spring loaded to hold them in against the motor shaft. As the motor spins up to speed these weights swing out and move a plastic collar along the motor shaft about 1/2". This collar presses on the centrifugal start switch to open the circuit containing the capacitor and the start winding of the motor, since once spinning, the start winding function is no longer needed and must be switched off or it will overheat. The centrifugal start switch performs this function. As the motor slows down when powered off, the weights will return to their close to the shaft position, moving the collar back along the motor shaft, and allowing the centrifugal start switch to close and re-connect the start winding and capacitor so it will be ready for the next time that you turn on the motor.

    From my experience, you could have any one of three problems or a combination of them. It's usually best to service all three whenever you are having these problems, but if you have overheated the motor from repeatedly trying to use the motor when it isn't working correctly, and you have let the magic smoke out, none of these will save the motor and make it work properly again. If all of these fail to fix the motor, take it to an electric motor service shop and have them determine the problem.

    1. The size of your replacement capacitor needs to be within about 20% of the value of the original and the working voltage needs to be AC and the same or higher voltage as the original.

    2. The collar part of the centrifugal start mechanism needs to slide on the motor shaft easily. Frequently the motor shaft develops rust in this area if the motor sits unused in a damp area for an extended time. Clean the area of the shaft where the collar needs to slide and apply a very thin coat of light oil, but not enough that it will fly off when the motor runs.

    3. The centrifugal start switch contacts require cleaning and possible adjusting. It should have enough spring tension to close the contacts together when the centrifugal mechanism on the motor shaft is not pressing against it and the contacts should not be heavily pitted. A small file may be needed to re-shape them.


    Charley
    Last edited by Charles Lent; 12-12-2019 at 8:55 AM.

  12. #12
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    Google " centrifugal switch induction motor " and you will find many videos to help repair and troubleshoot this switch.


    Mike

  13. #13
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    Do they make solid state starting switches to replace the mechanical ones? Seems like a simple timer would do the trick. Do modern fractional Hp motors still use mechanical start switches? It may be do to the high amperage that thy have stayed with mechanical switching.
    Bill D

  14. #14
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    Thanks everyone for the information so far. When I get home I will open up the motor and check the centrifugal switch. I did watch a couple youtube videos and it seems easy enough. I will take a couple pictures when I get home if I am still having these issues.

    Charles, the new capacitor is exactly the same size as the old one. I have only started the motor twice since I started having these issues so hopefully I haven't caused any damage.

  15. #15
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    Alright guys, I'm not sure what I did that worked, but the motor is running again. I took the motor apart, cleaned up the contacts on the switch with 400 grit paper. Played with the centrifugal switch to make sure it was all moving freely and put things back together. I hard wired the motor and she fired right up. Then I hooked it to the switch which is the replacement one I had and everything still works so I'm not messing with anything else. Thank you all so much for your help. Saved me about $650 buying a new motor. Now just to get this motor back into the awkward hole under the saw without help.

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