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Thread: Motor Questions

  1. #1
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    Motor Questions

    OK, I'm somewhat at a loss here.

    I have a Laguna 12" Jointer/Planer, with supposedly a 3HP motor, which has stopped turning its motor. If I open the back of it, sometimes I can turn the pulleys by hand and get it going, other times it starts slowly and eventually gets up to power. Usually at this point, it just doesn't start.

    Tech support thinks the start capacitor has gone bad. Here's the problem, though. I can't find a start capacitor. It has what I think is a run capacitor on it, which is 40MFD at 450V. There is no other capacitor visible on the motor (I have removed it from the unit, so I get a full view).

    Also, there is no identifying motor plate on the motor, so who knows what HP it really is, and who the manufacturer is, and I think that's likely not a good sign.

    Any suggestions where to go from here? Is it possible that the motor does not have a start capacitor and just has a run capacitor? Would it then be likely that the run capacitor is the cause of this?
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  2. #2
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    Sure, it could only have a run cap. The easiest thing to do is to take your capacitor to a local motor supply house/fix-it place and see if they can match it up for you.
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  3. #3
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    Hi Alan, if there is only one capacitor it is a starting cap. An induction motor does not need a run cap, it is only used to increase efficiency where the start capacitor is necessary to get the motor turning in the correct direction. Your symptoms are classic start cap failure since the motor will run when you help it start.

  4. #4
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    John, Chris:

    When I look at the section on motor capacitors on wikipedia (yeah, yeah, insert comment here), it says that start capacitors are always over 70 MFD and have four major voltage classificationa 125V, 165V, 250V, and 330V. None of these are 450V and 40MFD, which seems to line up with their descriptions of run capacitors.

    Also, could a local motor supply shop determine what the HP rating of the motor is? I'm just not happy about no motor identification plate. I've never seen that.
    - After I ask a stranger if I can pet their dog and they say yes, I like to respond, "I'll keep that in mind" and walk off
    - It's above my pay grade. Mongo only pawn in game of life.

  5. #5
    Quote Originally Posted by John Lanciani View Post
    Hi Alan, if there is only one capacitor it is a starting cap. An induction motor does not need a run cap, it is only used to increase efficiency where the start capacitor is necessary to get the motor turning in the correct direction. Your symptoms are classic start cap failure since the motor will run when you help it start.
    John's right. Single phase induction motors need a starting capacitor. If that's the only one capacitor on the motor, it has to be the starting capacitor.

    And as he points out, if you can get the motor to run when you "start" it by hand, that's almost a sure sign the starting cap is bad. Replace the cap and see what happens. They're not very expensive.

    The capacity of the start capacitor is usually not critical, once you pass a certain minimum microfarad size.

    Mike
    Last edited by Mike Henderson; 09-27-2016 at 7:57 PM.
    Go into the world and do well. But more importantly, go into the world and do good.

  6. #6
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    Wikipedia...always...

    I don't suspect that the Chinese (or the Italians if it happens to be a CEG motor) follow what Wikipedia says they are always supposed to do. I have seen caps with the 450V rating though. Like Mike said, get one as close as you can find and give it a shot.

  7. #7
    PSC motors only have a run capacitor. Furnace fans use these as starting loads are small. The fan in my Delta (AP-400) only uses a 50 uf capacitor. When it puked it's guts, fan wouldn't come up to speed. Substituted a 40uf I had on hand, and it was "off to the races."

  8. #8
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    Quote Originally Posted by Alan Lightstone View Post
    Is it possible that the motor does not have a start capacitor and just has a run capacitor?
    Well, a single phase induction motor has to have some means of starting. It's certainly possible to use a single capacitor for both starting and running. This is called a Permanent Split Capacitor (PSC) motor. There's no centrifugal switch. The main disadvantage of this setup is fairly low starting torque, but a jointer/planer doesn't offer much of a load at start.

    If your cutter head is jammed up a bit though the extra load could be beyond the starting capabilities of a PSC motor so be sure to check this.

    Also, I assume you have already checked this but loose pulleys or belts can show similar symptoms.

    Quote Originally Posted by Alan Lightstone View Post
    Also, could a local motor supply shop determine what the HP rating of the motor is? I'm just not happy about no motor identification plate. I've never seen that.
    I see that a lot on imported equipment. The horsepower rating of a motor depends upon the temperature rating of the insulation, the expected lifetime and the thermal dissipation capabilities of the motor housing. It's not something you just measure on a dynamometer. A good motor person could eyeball it or compare DC resistance measurements to other motors and give you an educated guess but I'm not sure what you would do with that information. Can you calculate a required HP knowing material type, width, cut depth, feed rate and duty cycle?
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  9. Sounds like a bad start capacitor.

  10. #10
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    The only reason I'm interested in the HP rating of the motor is to make sure it really is the 3HP motor I paid for.

    I've got the motor out in a box, and am going to bring it in to a local motor place. Laguna sent me a quote for the capacitor, which was quite reasonable (don't get me started about my local AC contractor that just charged me 6 times that for an identical capacitor (and they will lose a customer over it). That being said, since I pulled the motor, I don't mind the extra expense (just as long as it's reasonable) for the motor place to test the motor and replace the capacitor.

    I may drive a Tesla, but Nikola's knowledge of AC motors has not rubbed off on me by just driving one.
    - After I ask a stranger if I can pet their dog and they say yes, I like to respond, "I'll keep that in mind" and walk off
    - It's above my pay grade. Mongo only pawn in game of life.

  11. #11
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    Quote Originally Posted by David L Morse View Post
    Well, a single phase induction motor has to have some means of starting. It's certainly possible to use a single capacitor for both starting and running. This is called a Permanent Split Capacitor (PSC) motor. There's no centrifugal switch. The main disadvantage of this setup is fairly low starting torque, but a jointer/planer doesn't offer much of a load at start.

    If your cutter head is jammed up a bit though the extra load could be beyond the starting capabilities of a PSC motor so be sure to check this.

    Also, I assume you have already checked this but loose pulleys or belts can show similar symptoms.
    Actually, it was when I opened the machine to tighten up the loose pulleys that the problem first presented itself.

    The cutter head isn't jammed. It's the first thing I checked, but good point.

    What does a centrifugal switch look like? Is it inside the motor itself, because I don't see one.
    - After I ask a stranger if I can pet their dog and they say yes, I like to respond, "I'll keep that in mind" and walk off
    - It's above my pay grade. Mongo only pawn in game of life.

  12. #12
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    Also, I have had good luck shopping for and buying start capacitors on ebay. It makes it more of a commodity.
    "the mechanic that would perfect his work must first sharpen his tools.” Confucius

  13. #13
    Quote Originally Posted by Alan Lightstone View Post
    Actually, it was when I opened the machine to tighten up the loose pulleys that the problem first presented itself.

    The cutter head isn't jammed. It's the first thing I checked, but good point.

    What does a centrifugal switch look like? Is it inside the motor itself, because I don't see one.
    Even if you can't see it, you should be able to hear it when the motor slows down when you power it down. Run the motor with no load and then turn it off. As the motor slows down, you'll hear a definite "snap" or "click" which is the centrifugal switch closing. If you don't hear that, you either don't have a centrifugal switch (maybe it's a permanent split capacitor motor), it's very quiet, or it's stuck.

    Mike

    [I don't know why they would put a permanent split capacitor motor on a jointer/planer, but you never know. The size and the voltage rating would lead you towards that alternative. But maybe the cap has been changed earlier.]
    Last edited by Mike Henderson; 09-27-2016 at 10:15 PM.
    Go into the world and do well. But more importantly, go into the world and do good.

  14. #14
    Do a google image search for centrifugal switch, and you'll see several examples. You can usually hear them click in and out, as well.

    Your motor almost certainly has a centrifugal switch and a start capacitor. PSC motors are not as common, and I don't think I've ever heard of one >2HP.

    I suppose it's possible that someone put the wrong value start cap on your motor (but as Mike says, it's not a critical value, so maybe it worked).

    I'd start by replacing the cap with a roughly identical one and see what happens.

  15. #15
    Quote Originally Posted by Mike Henderson View Post
    The capacity of the start capacitor is usually not critical, once you pass a certain minimum microfarad size.
    Mike

    True. However I think you want to find a cap series that can handle a high inrush current, so "just any" cap in that range may not be ideal. There may also be other beneficial electrical characteristics that I'm not aware of.

    Maybe Mike or someone else can recommend a good brand & series to use. (I'd kinda' like to know myself.)

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