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Thread: Help with low motor power on table saw

  1. #1
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    Help with low motor power on table saw

    Had a 7.5 baldor, had a seperate post on this because I thought I figured out what was going on but evidently this is beyond my
    knowledge or experience.

    I replaced caps on 7.5 baldor, no difference, bought new leeson 7.5 hp, both single phase motors,
    got it hooked up, check wiring, cranked it up. No weird sounds or fire so good, but then I went to cut a piece of wood and I could stop this motor with minimal pressure. All I did was put a 1.25 inch piece of kentucky coffee to the blade and I literally stopped the motor.

    So I went to wiring as far as from breaker to table saw, wiring looks good so I did an ohm check came out 3.7 ohms. Specs say it should be around .019 milliohms. But the more I read on ohms the more I dont understand, every different site out there has a different spec or reading you should go by. I took cover off of switch, did not see anything burnt or discolored, not sure what else to look for.

    I am at a loss. Any thoughts appreciated. I did think about the bearings in the table saw, but no rattles or noises, I can turn the blade by hand with very little effort. Blade also runs out true. Table saw is an old delta/rockwell 12/14 I believe model number is 34-350
    "To me, there's nothing freer than a bird, you know, just flying wherever he wants to go. And, I don't know, that's what this country is all about, being free. I think everyone wants to be a free bird." - Ronnie Van Zant

  2. #2
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    Check voltage at the wall. 120, 120, 240. Then check beyond the switch. My guess is you lost one leg.
    I got rid of the 5hp? three phase one on mine and have a 2hp+ single phase on it. It runs a 14" blade fine, in soft wood only so far.
    Bill D

  3. #3
    So both the Baldor and Leeson motors have the same problem? Seems unlikely the issue is the motor, then.

    When you say you measured the ohms, what did you measure?

    Look upstream: are you sure you have 240V at the motor? Put a voltmeter on. Can you measure the voltage while the motor is exhibiting the under-powered symptom (ie - when it is powered on but stalled)? Could be a problem with the supply voltage or with the switch/starter.
    Look downstream: when the blade stops, does the motor also stop? Is it possible you have a slipping belt or sheave? Lost a key on a sheave?

  4. #4
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    Quote Originally Posted by Bill Dufour View Post
    Check voltage at the wall. 120, 120, 240. Then check beyond the switch. My guess is you lost one leg.
    I got rid of the 5hp? three phase one on mine and have a 2hp+ single phase on it. It runs a 14" blade fine, in soft wood only so far.
    Bill D

    at the breaker I have 119.6 on each pole. at the switch I have the same, at the motor I have the same.
    What else could there be but the bearing on the belt pulley......????!!!!
    "To me, there's nothing freer than a bird, you know, just flying wherever he wants to go. And, I don't know, that's what this country is all about, being free. I think everyone wants to be a free bird." - Ronnie Van Zant

  5. #5
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    You have 120 but no idea if they are different phases or not. You need to check for 240.
    Bill d

  6. #6
    Does the saw work in a different outlet on a different circuit?

    Have you confirmed that you've wired the motor for 220 and not 110?

    Have you confirmed that a different 220v appliance works on the outlet?

  7. #7
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    There isn't a key or set screw missing somewhere on the arbor pulley? Does the motor stall?

  8. #8
    I'm not familiar with that particular saw, but you mentioned belt & pulley. Is the belt in decent shape? I had a table saw (only 120v that I got for free) that behaved as you described - I'd turn it on, it would spin up and look like it is working fine. Put a piece of wood to it, and the blade would stop (motor would keep spinning).

    If the motor is still running when the blade stops, this could be a bad belt, or maybe one of the pulleys is spinning and not turning the shaft it is connected to.

  9. #9
    Arbor pulley loose? Motor pulley presumably installed on new motor so it should be OK.

  10. #10
    What does it sound like when the stall occurs? Are you sure the motor is stoping? Do you have a clamp meter to check the amps draw on each leg while the motor is running? Troubleshooting with voltage alone can be uninformative and misleading.
    Last edited by Maurice Mcmurry; 05-31-2024 at 7:21 AM.

  11. #11
    at the breaker I have 119.6 on each pole. at the switch I have the same, at the motor I have the same.
    What else could there be but the bearing on the belt pulley......????!!!!


    You did not provide *model #'s but i don't think there are any real, single phase cap start/cap run 7.5 HP motors that will run on 120v.
    You need 220V at the saw. (minimum), 240V would be good

    *Check the wiring diagram inside the box where the J cord or whatever you have connects to the motor leads & see if it offers a "high voltage" and "low voltage" hook-up diagram
    If it does have a **"low voltage" (IOW 120 V) then be sure the terminal connections are correct for that voltage. But i do not think that likely - if you pulled 7.5 HP out of the motor on 120V,, it would be at around 60 amps.
    It will be about 1/2 that at 240v.

    **if it is a 3ph motor, "low voltage" in that size means 220 (sometimes 208) - 240v, "high voltage means 440 - 480V

    I probably should not say this, no intention to be snide - is the motor that you are trying to run on 120V single phase, indeed a single phase motor, or is it a 3ph motor?
    (CLEAR) photo of the dataplate?


    Last edited by stephen thomas; 05-31-2024 at 8:00 AM.

  12. #12
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    IF it's a 1 phase motor, at 7.5 hp...should the outlet supplying power be fed with something approaching 8 AWG wire and a 50 amp breaker? If the wiring is not sufficient, would that cause the low power the OP is experiencing?? Just thinking out loud (and trying to learn for my future)

  13. #13
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    It sounds to me like you have a voltage drop problem. If the motor is correctly wired the full load amperage will be somewhere around 40 amps. The resistance of #10 wire which is the minimum size for 40 amps (#8 probably a better choice) is 1 ohm per 1000 ft. If your measurement of 3.7 ohms is correct my calcs say you have a motor stopping problem involving wiring. It can be tricky to measure the voltage at the motor under load but that might reveal the problem. Prior to attempting that check your wiring carefully make sure you have it sized correctly and that all connections are solid and check the breaker they can go bad. I've encountered all of these problems.
    Cheers
    Robert

  14. #14
    I wouldn't worry too much about the ohms at this point. Cheaper DMM's can be very unreliable on the outer edges of their range. So unless your DMM has a setting of 10 Ohms, and you're using that setting, it could just be misreading something a resistance that is so low. Also, the "correct" resistance will depend on a lot of things, so all you can really do is get a ballpark figure to help you diagnose a potential issue. Don't worry about exact figures you read online. In this case 3.7 Ohms doesn't worry me. Between the reliability of your DMM and general variance due to any number of reasons, this is within the ballpark. 37 Ohms would start to concern me, but still not definitively diagnose a problem. 3.7K Ohms would definitely be a problem.

    I would think your two legs have to be out of phase or the motor wouldn't run. Though, that depends on the connection. Is it a 3 wire or 4 wire? If it's just two hots and a ground, it won't run if the two hots are in phase. But if there's a common wire as well, it could. Also, the way most electrical panels are designed, if your using a single 240v breaker (well, two breakers joined together, but not two individual breakers spaced apart), then they pretty much have to be out of phase, unless you've got some serious wiring issues. Either way, a quick measurement should resolve that question.

    My guess is it's something mechanical. The belt, pulley, bearing, something's not transferring the power from the motor to the blade. I would focus on that for now, as you've already replaced the motor and nothing you'd said so far is ringing any alarm bells.

  15. #15
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    Quote Originally Posted by stephen thomas View Post


    You did not provide *model #'s but i don't think there are any real, single phase cap start/cap run 7.5 HP motors that will run on 120v.
    You need 220V at the saw. (minimum), 240V would be good

    *Check the wiring diagram inside the box where the J cord or whatever you have connects to the motor leads & see if it offers a "high voltage" and "low voltage" hook-up diagram
    If it does have a **"low voltage" (IOW 120 V) then be sure the terminal connections are correct for that voltage. But i do not think that likely - if you pulled 7.5 HP out of the motor on 120V,, it would be at around 60 amps.
    It will be about 1/2 that at 240v.

    **if it is a 3ph motor, "low voltage" in that size means 220 (sometimes 208) - 240v, "high voltage means 440 - 480V

    I probably should not say this, no intention to be snide - is the motor that you are trying to run on 120V single phase, indeed a single phase motor, or is it a 3ph motor?
    (CLEAR) photo of the dataplate?



    It is a single phase motor running on 240. with 3 wires. 2 hots and a ground.
    "To me, there's nothing freer than a bird, you know, just flying wherever he wants to go. And, I don't know, that's what this country is all about, being free. I think everyone wants to be a free bird." - Ronnie Van Zant

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