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Thread: Start capacitor questions

  1. #1

    Start capacitor questions

    Couple questions for the hive mind.

    First a little background. I recently purchased an older Powermatic model 60-A 8", 2HP jointer. The motor is a Baldor L3515T-BD wired 220v on a dedicated 20a 220v GFI circuit. GFI is built into the circuit breaker in the panel.

    When I bought the jointer I noticed it had a shudder/vibration for about a second when it started so I assumed the started capacitor was on its last legs.

    Today, I was milling my first large project on it and after 20+/- minutes of operation with multiple stops and starts, it started tripping the GFI circuit breaker when ever I tried to turn it on. Moved it to a non-gif circuit and it started once with a great deal of effort. Would not start again but made a humming sound like it was trying for a second or two..

    So I pulled the start capacitor and it reads 640 uf or 690 uf, depending on how I have the leads arranged. Spec on the capacitor is 645-774uf

    Are there other things I should check before replacing the start capacitor? Just looking for confirmation that the start capacitor is the likely culprit since the capacitance reading are so close to spec and when I physically examined the capacitor, it had no indications of being fault.

    The Baldor replacement, manufactured by BMI, is $75. Amazon has another BMI start capacitor with the EXACT same specs on the label as the Baldor branded capacitor, with the exception of the part number, for $15. Is there any reason not to use the less expensive, non-Baldor branded capacitor?

    Thanks,
    Carl.

  2. #2
    Get the start capacitor for my Amazon. You can always go higher in UF but never lower.

  3. #3
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    If you have an electric motor shop nearby that's a good option as well. Remove the belt since you are there anyway. Try spinning the motor when you try starting it. If it spins up then it's start circuit related. However make sure you hear the centrifugal starting contacts click when the motor spins down when you turn it off. You may not hear them as it spins up but when it's coasting down you should. If not that might be the issue as well. There are people who can better advise you than I here but that's the gist of it.

  4. #4
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    Without getting to technical, and sending this thread down the rabbit hole, You should have two ratings stamped on the capacitor. One is the rated voltage, the other is capacitor value.
    There is probably no difference between the Baldor cap for $75.00, and the generic version at $15.00, other than one has Baldor's name stamped on it. As long as the capacitors are physically the same dimensions, so it fits under the cover, just go with the cheaper version.

    As a "general rule" with capacitor you can always go to a higher rated voltage capacitor, but unless you can do the math, don't change the capacitance values. If Baldor has a spec, use it. They're much better at the math.

    One other area that can be causing you issues is any accumulation of dust inside the housing. It happens. Clean it out with nice dry air and you'll be good.
    "The first thing you need to know, will likely be the last thing you learn." (Unknown)

  5. #5
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    It sounds like the capacitor is fine, check the centrifugal starting switch, sounds like it’s open……Rod.

  6. #6
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    Suspect the start switch is open and sawdust will not let it close. Time to open up the endbell and vacuum the start switch at the power end. You might buy a ebay spare capacitor for a lot less then $15. I would be surprised if any of those three is not made in China.
    Bill D
    Last edited by Bill Dufour; 12-26-2021 at 11:16 AM.

  7. #7
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    I agree with the others, check the starting switch. If I found that the switch looked like it has something like sawdust in it that was preventing it from working I would go one step further and replace the bearings if the motor is around 20 years old or more.

  8. #8
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    Especially what Alex said. If it doesn't spin silkie smooth and quiet while your there replace the bearings. They are inexpensive and if you have the end bell off already why not?

  9. #9
    Thanks everybody for the input. I tested the resistance across the centrifugal start switch and it read 1.8 ohms. I assume this is a reasonable reading. Time to order the less expensive capacitor from Amazon.

    Another question popped up though. This motor has a grease zerk on the pull end. Never seen that on a small motor like this before. Should I be greasing it like any other grease zerk?

    Thanks,
    Carl.

  10. #10
    You should grease it SPARINGLY! Excess grease winds up in the motor.

  11. #11
    I believe the capacitor in question is a RUN capacitor. If your motor has a centrifugal start switch then it has a start winding and therefore does not need a start capacitor. Humming noises are typically indicative of run capacitor issues. 1.8 ohms is okay.
    Wood working is like a vicious cycle. The more tools you buy the more you find to buy.

  12. #12
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    Carl said " I tested the resistance across the centrifugal start switch and it read 1.8 ohms. I assume this is a reasonable reading. "

    When I am measuring something with a very low resistance, I will touch the ohm meter leads together to determine what the lowest reading is. (It is almost never actually zero ohms but may be higher - - like a few tenths of an ohm up to more than an ohm). So, when you take a reading of your switch, you should subtract the shorted lead resistance from your measurement.

    But I'm thinking that 1.8 ohms, if that is the actual resistance, is actually a little high. If the inrush or start up current of the motor is, say, 20 amps, then there would be a 36 volt drop across the centrifugal switch.

    If it was me, I'd clean the contacts and take some super fine sandpaper (like 1000 grit or finer) and clean the contacts. If "no joy", I would then replace the capacitor. Although the capacitance value is very good, it is possible that the ESR (effective series resistance) of the cap may be high. Also, it is possible that there could be some leakage from either side of the cap to its metal case which could cause the GFCI to trip. You "could" measure this leakage with your ohm meter on a high ohm scale.



  13. #13
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    Quote Originally Posted by Bill Winter View Post
    I believe the capacitor in question is a RUN capacitor. If your motor has a centrifugal start switch then it has a start winding and therefore does not need a start capacitor. Humming noises are typically indicative of run capacitor issues. 1.8 ohms is okay.
    The motor would have a start capacitor and may have a run capacitor.

    The motor, regardless of whether it’s capacitor start or capacitor start and run will have a starting winding……Rod

  14. #14
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    If it was me, I'd clean the contacts and take some super fine sandpaper (like 1000 grit or finer) and clean the contacts. If "no joy", I would then replace the capacitor. Although the capacitance value is very good, it is possible that the ESR (effective series resistance) of the cap may be high. Also, it is possible that there could be some leakage from either side of the cap to its metal case which could cause the GFCI to trip. You "could" measure this leakage with your ohm meter on a high ohm scale.


    [/QUOTE]
    Before I use any abrasive on the contacts I would use. a piece of brown paper bag drawn a few times between the closed contacts. I find that is often abrasive enough to clean out the gunk.
    Bill D

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