Electric Saw Motor

Discussion in 'General Electronics Chat' started by comcity, Jul 11, 2008.

  1. comcity

    Thread Starter Member

    Jul 11, 2008
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    I'm a neebie and Not a Professional electrician so please excuse my ignorance.

    I bought an electric brick saw off Ebay because the ad said "Tested and works Great!". Well, it doesn't work great and now I'm trying to fix it to save the $150 I paid for it. Sending it back would cost $75 alone.

    The saw motor starts up fine but runs for only about 10 seconds and then shuts off. It has a "self-protection" mechanism in it so if its under excessive load, it shuts off to protect itself. However, this seems to be happening all the time. I've opened up the box and the only thing that is there is a Motor Start capaciator of 150MFD/125VAC,a Run capaciator that is 50uF/350VAC, and a circuit breaker that detects the excessive load.

    So I took the motor off the saw and completly free of all load, brought it into my cool house and it ran great. I put it back on the saw and it ran great twice. Then after about 5 minutes and after it started "presumably" heating up, the problem returned. The motor starts up runs for about 10 seconds and stops. I can tell that the centrifugal relay switch is Not opening up when this occurs. In fact, its obvious the motor does not go to full speed (only 50-75% speed) when this is going to happen so I can tell in about 1 second whether the motor is going to stop 9 seconds later

    This morning, it was cooler and I went outside and started the saw motor up and once again this time, it ran fine and came up to 100% speed. I left it run for 10 minutes no problem. Then I turned it off and tried to turn it back on and the problem returned. It ran for 8-12 seconds and stopped.

    I believe the problem is either the run capacitor or the start capacitor. Can someone please confirm if I'm on the right track and which one of these capacitor would be the issue if they indeed are the issue?

    I found suitable replacements I believe on Grainger. Perhaps I should just buy both for $20 and just see if this fixs the problem. I've already invested a lot of money into something that doesn't seem to be working already.

    Thanks in Advance for your assistance.
     
  2. Externet

    AAC Fanatic!

    Nov 29, 2005
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    If there is a non-operating centrifugal switch ; it would be better to investigate why it gets stuck.
    The shut-down circuit breaker may be doing its job properly.
     
  3. gerty

    AAC Fanatic!

    Aug 30, 2007
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    when running this outside are you using a long small gauge extension cord?
    If you are, the cords resistance drops the voltage to the saw causing the problems.
     
  4. comcity

    Thread Starter Member

    Jul 11, 2008
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    Its not operating because the motor is not "running fast enough". When the motor comes up to full speed, I hear it click on and I hear it click off when it slows back down after stopping. I looked at the switch and I don't see anything really there that can fail....just springs around a contact point. The contact points are a bit dirty...should I clean them with some emery cloth to make sure they have good electrical contact?

    The motor has a belt that operates another pulley that drives the saw axle suspended with a couple of bearings on each side. The other thing that I was originally suspecting was that these bearings were rusty. When I first took things apart...I could barely turn the axle. However, with some oil, they seemed to free up. It would take a gear/pulley puller to get that off and then probably a bearings puller to get further into that.

    What does the centrifugal switch do? Is it suppose to switch current through the run capacitor?

    Thanks for your help.
     
  5. comcity

    Thread Starter Member

    Jul 11, 2008
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    No, unfortunately I plugged directly into the wall to ensure this was not the case and ruled that out already. The wall plug is GFI.

    If the centrifugal switch is what actually is suppose to trip the run capacitor...then I guess I would go back to suspecting there is too much resistance within the run axle area and bearings again.

    If the run or start capacitor is what is suppose to get the motor up to full speed and is not dependent on the centrifugal switch then I would think its the capacitors.
     
  6. gerty

    AAC Fanatic!

    Aug 30, 2007
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    The centrifugal switch causes it to go from "start" windings to "run" windings in the motor. If a motor continues to run on start windings it will heat up and quit.Start windings typically provide more torque for starting under load but are not desinged for long run times.
     
  7. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Will it run ok with belt removed? It might be that there is still a drag on the motor not letting it come to full speed.
     
  8. comcity

    Thread Starter Member

    Jul 11, 2008
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    Ok...but doesn't it need the "run capacitor" to work properly to charge the motor up to its full run speed once the switch is engaged?

    So the problem is either the run capacitor is bad
    or, there is too much drag on the motor keeping it from coming up to the speed necessary to engage the switch.

    It seem to be "heat" activated though. When everything is cool, its running fine. After heating up, it stops. That would support that its more likely the run capacitor to me since the drag on the motor should not be as effected by heat and if anything, once the oil heated up in the bearings...would support it getting better with running not worse.

    Can a capacitor be affected by heat such that it works fine when cool but starts to fail once it heats up but then starts working again once it cools?
     
  9. comcity

    Thread Starter Member

    Jul 11, 2008
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    Yeh, it runs fine without the belt... Runs like crazy and the switch almost instantly engages...at least "inside" of my house with the A/C on. I guess I need to test it outside in the heat.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    If the bearings have rusted, the races & balls will be pitted, and bearing failure will occur shortly. :(

    I'm afraid the only real "fix" for that is to replace the bearings ASAP. If you don't, they will soon seize entirely (causing damage) or fail completely (causing damage).

    As things are right now, you've been cycling the saw motor through many high-temp warmups and cool downs, which will shorten it's life.

    Contacts: I suggest that you do NOT use emery; pieces of the emery will imbed in the points, causing them to fail earlier. You can use some new fine-grit wet/dry sandpaper folded double, and pull it between the contacts. Dress them just enough to remove the high points. Excessive burnishing is counterproductive; you can't add more material to the contacts. Wash your hands w/soap well before handling the sandpaper, as finger oils will contaminate the points, making them burn. You could use some 91% isopropyl alcohol (available from any pharmacy) on a small brush to ensure that the contacts are clean. Allow it to dry completely before attempting to start the motor; isopropyl alcohol burns with a near-invisible flame.
     
  11. gerty

    AAC Fanatic!

    Aug 30, 2007
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  12. comcity

    Thread Starter Member

    Jul 11, 2008
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    Well I am able to hand spin the axle so I can understand why the motor can't. I don't think they are that rusty but the outside is sealed so I can't see the balls or the racers. I tried pulling the pulley off which is keyed but was unable to do so. I would need some kind of tool. And then after that, the bearings are pressed in and around the axle and not sure how I could get those out either.

    I did hit the axle with a hammer to see if I could hammer it out but it wouldn't budge.

    The only reason I suspect the axle is because it seemed like the hammering at one point has freed up the axle when I was working on it...also it seemed like one time only, I hammered the axle and it caused the motor to work for one shot. However, like I said...now I'm able to hand spin the axle so its really hard for me to think this is still a problem.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Try dressing the contacts of the centrifugal starter switch using wet/dry sandpaper or an ignition file (wet/dry sandpaper preferred). They may be causing an intermittent high-resistance connection to the starter capacitor, which will keep the motor turning slower than it should.

    Double-check the wiring connections from the motor to the cap. These are frequently male/female spade connectors; the male portion can get corroded, the corrosion makes a high-resistance connection which heats up, causing the female connectors to loosen up. Then you get burned connectors. If you're careful, you may be able to use a pair of pliers to slightly squeeze the female connectors while they're disconnected from the males to make the connection more solid. (this is dicey; if you squeeze them too much you will weaken the female connector to the point that it must be replaced.) Any corrosion on the mating surfaces will cause a problem. You might be able to use an "ignition file" (a small, thin, flat file for ignition points and spark plugs available at most auto supply stores) or jeweler's file or small wire brush to remove the corrosion.
     
  14. comcity

    Thread Starter Member

    Jul 11, 2008
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    Well I just did the test outside in the heat and disconnected the belt and the saw and the motor won't spin up past the point of basic startup running even with NO load at all. And after 12 seconds it shuts off.

    I tried cleaning the points but it didn't change anything.

    So what could this be?
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    Starting capacitor.

    You tried the cheap stuff first.

    When things get warm, their dimension change. The starting cap may have internally fractured connections between it's plates. This can happen after many heating/cooling cycles; sort of like flexing the tab to a pop can back and fourth a bunch of times. Metal fateague. The metal crystallizes, and cracks apart.
     
  16. comcity

    Thread Starter Member

    Jul 11, 2008
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    Really? Not the Run capacitor?

    I would think the starting capacitor is ok because its starting up to 40% or so speed. But perhaps its not taking it up "high" enough is what you are saying.

    I have a VOM, there is no way to test it is there?
     
  17. comcity

    Thread Starter Member

    Jul 11, 2008
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    I put a VOM on the Start capacitor without disconnecting it and its continuous. They look like electrolytic capacitors but I don't see a + sign anywhere.
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    Sorry, I'm working on a project, and not looking at the board continuously.

    You're reading continuity throught the motor windings.

    Start/run caps are non-polarized electrolytic capacitors. Basically, they're sort of like two polarized electrolytics back-to-back (not really, but that's the easiest way to explain them in one sentence).

    You need to try another cap with the same voltage and uF rating.

    Unless you have a cap tester that can check for ESR and leakage at high voltage, you won't be able to test it.
     
  19. theamber

    Active Member

    Jun 13, 2008
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    Yes heat affects capacitance but in your case is negligible.
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    Wow, I replied and it dissappeared. ???

    Anyway, sorry - I'm not looking at the board constantly.

    Those are non-polarized caps. They're sort of like two polarized electrolytics back to back - not quite, but that's the easiest way to explain it in one sentence.

    Unless you have a capacitance checker that has HV leakage test capability, and a way to test it over temperature, you won't really be able to test the cap.

    Try substituting a new start capacitor. That's the one giving you trouble.
     
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