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Thread: Unisaw motor help

  1. #1

    Unisaw motor help

    Hi all!


    Last fall, I acquired a Rockwell Unisaw 34-466 (mid-late 70's). At the time I had no way of testing it out, but took it with the idea that being a project could be fun.


    Flash forward to now, and I am so close to having the machine up and running after a _long_ list of replacement parts and repairs. The problem that needs addressing this time is with the motor (Rockwell 83-651: 1ph, 3hp, wired for 230v) / electrical.


    The saw has new cords (magnetic switch to electrical box, motor to electrical box, electrical box to outlet). Initially, the motor turned on and off with only a slight lag and hum. I assumed the start capacitor might needed to be replaced and because the motor is still outside the cabinet, now seemed like a good time to do it. Well, the old capacitor specs were printed on a paper label that over time bled and faded and are completely illegible. An internet search revealed someone else with the same motor set up who provided the specs on their capacitor so I purchased a similarly spec'd model. (220-250 VAC, 270-324 mfd)


    I installed the new capacitor today and now nothing. Wrong capacitor? If so, what specs should I look for? I'd love to have this saw up and running as soon as possible. Thanks for any advice!

  2. #2
    Join Date
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    You might try over at OWWM dot org (Old Woodworking Machines) Everything Electrical forum. Lots of experience there.

  3. #3
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    Dust inside the starting switch internal to the motor is a common problem.
    Bill D.

  4. #4
    Update: I ended up checking the connection to the new capacitor and it might not have been on so well. Made sure the connection was good and tried again.

    I now get the hum again, but nothing moves. I can give the pulley a little push and it does spin, but would take 1˝ to 2 seconds to make a revolution. Or it would if the breaker didn't trip before one full turn.

    Thoughts, anyone?

  5. #5
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    There's some small chance that the new capacitor is faulty as well, but as Bill D. suggested, the centrifugal switch inside the motor may need a good cleaning.
    Chuck Taylor

  6. #6
    Try this. Wrap a thin rope or twine around the shaft. Turn the power on and then pull the rope to give the shaft a good spin. See if the motor spins up to speed.

    If so, something is wrong with the starting circuit: Either the start capacitor is bad, the centrifugal switch is not closing, or your starting coil is bad.

    If it doesn't spin up to speed, you have some other problem, for which I have no suggestions right now.

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

  7. #7
    "Wrap a thin rope or twine around the shaft. Turn the power on and then pull the rope to give the shaft a good spin. See if the motor spins up to speed."

    Tried this and the motor spun right up! So....

    If it's the start capacitor, is the new one I put on even the right one? (See OP for specs)

    If the centrifugal switch isn't closing, how do I remedy that? Same for the starting coil?

    I'll add that after shutting the motor down, starting it back up again a minute or so later tripped the breaker. Don't know if that helps diagnose anything, but thought I'd add it here just in case. (The wiring in the building should be good. It was redone about 6 months ago and the electrician was able to see what kind of power requirements this motor needed.)

    Admittedly, dealing with this kind of stuff is not my forte so I really appreciate any help (particularly when it's descriptive)!

  8. #8
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    I would take it to a motor shop. They will be able to fix anything short of fried windings, efficiently and correctly. I would have them change the bearings, too, and then you likely won't have to worry about the motor for 25 years or more.

    John

  9. #9
    Quote Originally Posted by phillip kerrigan View Post
    "Wrap a thin rope or twine around the shaft. Turn the power on and then pull the rope to give the shaft a good spin. See if the motor spins up to speed."

    Tried this and the motor spun right up! So....

    If it's the start capacitor, is the new one I put on even the right one? (See OP for specs)

    If the centrifugal switch isn't closing, how do I remedy that? Same for the starting coil?

    I'll add that after shutting the motor down, starting it back up again a minute or so later tripped the breaker. Don't know if that helps diagnose anything, but thought I'd add it here just in case. (The wiring in the building should be good. It was redone about 6 months ago and the electrician was able to see what kind of power requirements this motor needed.)

    Admittedly, dealing with this kind of stuff is not my forte so I really appreciate any help (particularly when it's descriptive)!
    Let's assume the starting cap you put in is good and you installed it correctly. The next most likely problem is dirt in your centrifugal switch keeping the switch from closing. You have to take the end off the motor (the end opposite the driving shaft) to get to the centrifugal switch. By hand, open the switch and make sure there's no swarf between the contacts. Take some fine sandpaper and lightly burnish the contacts. Put back together and try the motor again.

    If that doesn't work, I'd try another start capacitor (make sure it's the right size) and make sure it's connected properly. If that doesn't work, take it to a motor shop.

    Mike

    [Starting coils don't go bad very often. It's highly likely your problem is either a starting cap (probably 90% of the time) or swarf in the centrifugal switch (more than 9%, but not 10%).]
    Last edited by Mike Henderson; 05-05-2021 at 4:00 PM.
    Go into the world and do well. But more importantly, go into the world and do good.

  10. #10
    Thanks for the well explained advice!

  11. #11
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    I think you’d spend a few hundred bucks at a motor shop, if they’d even work on a little toy like that (to them). Do some research and fix it yourself. Looks like you going down that road.

  12. #12
    OP here...

    Just wanted to hop back in to share results. It took little time to get around to it, but cleaning up the contacts on the centrifugal switch did the trick. They were not just dirty, but slightly corroded so I sanded just enough to increase the amount of surface area available (using a fairly fine grit sand paper and creeping upon on it slowly). After putting it back together, the motor is now running like a charm! (Still trips the breaker every once in a while, but i'll deal with that later....)

    In case someone in the future stumbles across this thread and could also use some help with this problem, I found a video online that does a good job of explaining centrifugal switches as well as showing how to clean them:https://youtu.be/F-jOBxn1w1I. For someone who is not great with motors, I found it very helpful.

    Thanks again to all who offered thoughtful advice and responses!

  13. #13
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    Quote Originally Posted by phillip kerrigan View Post
    OP here...

    Just wanted to hop back in to share results. It took little time to get around to it, but cleaning up the contacts on the centrifugal switch did the trick. They were not just dirty, but slightly corroded so I sanded just enough to increase the amount of surface area available (using a fairly fine grit sand paper and creeping upon on it slowly). After putting it back together, the motor is now running like a charm! (Still trips the breaker every once in a while, but i'll deal with that later....)

    In case someone in the future stumbles across this thread and could also use some help with this problem, I found a video online that does a good job of explaining centrifugal switches as well as showing how to clean them:https://youtu.be/F-jOBxn1w1I. For someone who is not great with motors, I found it very helpful.

    Thanks again to all who offered thoughtful advice and responses!
    That's a terrific video/tutorial/tip on the centrifugal switch!

    I bought a 5 hp unisaw which I originally had a long cord connected to it. It would start and run with no problem. As soon as I shortened my cord I started tripping my breaker. I wanted to keep the shorter cord so I ended up putting some Varistors in the circuit to create a higher starting resistance which quickly becomes low resistance due to the way a Varistor works. This method is ok but determining the right values and such is a bit of a guessing game at first and you have to use the correct components to make this work. There are other devices available to reduce the inrush current at startup and can get rather expensive.

    So my easy tip is to use a longer cord. I'm not recommending a smaller gage cord. Do make sure your cord is capable of carrying the sustained current during operation. But a longer cord adds additional impedance to the circuit which can reduce the the inrush some. This method isn't a sure thing but may be enough to keep the inrush current under the trip curve of your breaker at startup.

  14. #14
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    There is another problem that can occur with the centrifugal start switch that hasn't been covered yet. This is a common problem when motors haven't been used in a long time.

    While you have the end cap of the motor off and you have cleaned the centrifugal start switch contacts, you should also service the flyweight mechanism on the motor shaft that operates the centrifugal switch. The weights are spring loaded to hold them against the motor shaft, but swing out as the motor speed increases from stop. When they swing, they move a hard plastic collar along the shaft. It is this collar that operates the centrifugal start switch. Usually what happens is that the shaft gets dirty or slightly rusted in the area where this collar must slide, and the collar can no longer slide easily. A simple cleaning using fine emery cloth of this part of the shaft, plus a very light film of thin oil on this area after cleaning, will fix the problem. Be very careful not to use much oil, since you don't want it flying off when the motor runs.

    Now, make certain that the centrifugal switch contacts are clean and going tightly together, then re-assemble the motor and give it a test run.

    Charley

  15. #15
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    This is exactly why I really like three phase motors, so much simpler.

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