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Motor start capacitor mfd rating..........?

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  • Motor start capacitor mfd rating..........?

    In switching to a different motor in a saw (not under load to start) there's a clearance issue with the top mounted capacitor cover. The current capacitor is 650-750 mfd rated, 1-13/16" diameter by 4-3/8" length.

    Looking in the Grainger catalog there's a smaller capacitor 1-7/16" diameter by 3-3/8" length with 460-552 mfd rating. I have another motor to rob this size capacitor cover off and that would solve my clearance issues.

    My question is, would the 100 mfd less capacitance of the physically smaller capacitor have any negative effects. The motor in question here is Baldor tefc, general purpose motor.

  • #2
    The lower rated cap may not start the motor. Try it and see.

    JL................

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    • #3
      Thanks Joe. That's a possibility. To do that I've got to drive Grainger, buy the cap, install and try. Half a day for that.

      Since this is a general purpose motor I wonder if they use that original capacitance so it could start under load. My situation would be no load except the belt turning the saw's spindle, possibly the lower capacitance would do it. I'm looking for some theories along that line of reasoning.

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      • #4
        It should start without load. Try disconnecting the capacitor and start the motor by just giving a push on the shaft or try the cover donor motor's capacitor.
        Helder Ferreira
        Setubal, Portugal

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        • #5
          Originally posted by Noitoen View Post
          It should start without load. Try disconnecting the capacitor and start the motor by just giving a push on the shaft or try the cover donor motor's capacitor.
          Thanks, yes it will start just giving the shaft a spin on the bench with no capacitor. The donor motor's capacitor is badly corroded and the connection terminals are broken off missing so it's not useable.

          Joe's idea for testing is good, the problem is I would have to install the motor on the saw to verify. Installing this motor is difficult working upside down and backwards. It's a fairly heavy motor.

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          • #6
            There is nothing magic about the mount and cover for the capacitor. Why not just relocate it? In this case it doesn't even need to be in a metal housing, just provide insulation for the terminals and zip tie to, say the motor base.

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            • #7
              Originally posted by jlevie View Post
              There is nothing magic about the mount and cover for the capacitor. Why not just relocate it? In this case it doesn't even need to be in a metal housing, just provide insulation for the terminals and zip tie to, say the motor base.
              Thanks, yes, that's another possibility that occurred to me. The cleanest solution of the smaller capacitor housing is still the one I'd prefer.

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              • #8
                You could always mount the capacitor in a different location other than on top of the motor....worse case scenario that is.
                Giving the motor a push to get it started may work but I would advise against it especially since it's mounted on a saw.
                If there is a local motor shop near you might give them a call and run the motors specs by them and see what they say about the change in cap value and how it might impact the motor on start up.
                You might also check with Newark Electronics or Allied to see if they have a cap with the value and dimensions your looking for.

                JL.....................

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                • #9
                  If there is an HVAC shop near you, they may have some old blowers and fans that you can have for free (or a box of donuts and coffee). There was such a shop next to where I worked and I scrounged a lot from their dumpster, including sometimes new or barely used stuff. Usually the blowers fail because the lubrication of the shaft bearings has failed or become clogged with lint and hair. It is usually cheaper (or more profitable) for them to replace the entire blower.

                  Otherwise, you can find a suitable capacitor on eBay for about $13 including free delivery from CA, from Temco:
                  http://www.ebay.com/itm/Start-Capaci...-/261087516868

                  I assume yours is a 120V system. And I think it should be OK to use a smaller size if the motor is starting with light load. Large capacitors such as this may be more common for pumps that need high starting torque.
                  http://pauleschoen.com/pix/PM08_P76_P54.png
                  Paul , P S Technology, Inc. and MrTibbs
                  USA Maryland 21030

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                  • #10
                    Spinning the motor by hand does not substitute the smaller capacitor. I will only show if it's possible to start or not.
                    Helder Ferreira
                    Setubal, Portugal

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                    • #11
                      With no load, any capacitor at all should start it.

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                      • #12
                        Okay, thanks for all the replies. I talked with a knowledgeable electric motor guy who has helped me with more complicated servo motor issues.

                        His opinion is to try the smaller capacitor, if it starts the motor reasonably quickly with the inertia of the belt and no load spindle then go with it. He feels over time the capacitor might need more frequent replacement than normally. His opinion was not based on theory, but on 40 years of motor experience.

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                        • #13
                          You are actually at 200 uF smaller, since the one is really a 700 uF cap and the other a 500 uF. Might work, might complain at being 30% or so smaller.

                          The saw is not "no -load" to start.... Most saws have a speed-up pulley ratio, (saw faster than motor) and the saw itself is then a reasonably heavy inertial load. Might work, might not, you need to try it.

                          If it starts inside of 2 sec, it should last decently. If it takes much longer than that, life will definitely be reduced, in my estimation. The saw should not spin up slowly.

                          The caps are pretty severely overloaded, and have a short life anyway, in minutes. But since they are usually in-circuit for a second or so at most, even if their expected life were only 10 minutes total, that would be at least 600 starts. Usually they would be rated for more than that, depending on the intended usage.

                          For a 'consumer" saw, used on weekends for incidental cutting, 600 starts might be OK as a design target (seems a bit short, but it's just an example...). For a working contractor, 5000 starts would be a better target. Even that is only 25 starts a day for a year. Many are really rated at 3 or 4 starts per hour.
                          CNC machines only go through the motions

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                          • #14
                            You don't say if your clearance problem is in the diameter or the length of the capacitor. Both are reduced in the substitute that you propose. In either case, you might consider using two smaller capacitors in parallel to get the full capacitance value that you need. Capacitors in parallel will have a total value of the sum of the individual values.

                            A quick check of the Grainger web site shows that they stock many values so finding a pair of capacitors at about 350 uF should not be difficult.
                            Paul A.
                            SE Texas

                            And if you look REAL close at an analog signal,
                            You will find that it has discrete steps.

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