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Why Wont This Electric Motor Start?

Discussion in 'ELECTRICAL ISSUES - POWER YOUR MACHINES & SHOP' started by Xyius, Jan 25, 2016.

  1. Xyius

    Xyius United States Iron Registered Member

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    I am working on an electric motor for my Atlas 10" lathe. One day when I was disassembling the lathe to put a new headstock on there, when I re-assembled it, the motor didn't want to start. It just hums. I can start it by hand and it works fine.

    I took the motor apart and checked the resistance of both the starter and the running windings and they are both less than 3 ohms, so there are no opens. The contacts of the centrifugal switch were pretty gunked up (There is a lot of gunk inside) so I cleaned them up with emery cloth and then sprayed contact cleaner on it to get rid of the grit from the cloth. Still doesn't start on it's own. The centrifugal switch moves freely and is working properly. At this point I don't know what else to do. The motor does not have a starting capacitor so I don't know what else there is to check.

    One bizzare thing I found inside the motor, there was an old screw that seemed to be free-floating, as well as a spring. See attached picture. This is approximately where I found it. The armature looks like the screw has been rubbing on it. I have no clue what this is for and why it is in there. Someone clearly made a repair or something, but it just seems very odd.

    Any thoughts?

    20160120_220240.jpg
     
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  2. Ulma Doctor

    Ulma Doctor Infinitely Curious H-M Supporter-Premium

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    the screw is not supposed to be free floating, of that i can assure you.
    i didn't see the spring you described, maybe i din't look good enough...
    did you do a complete dissassemble or just crack the one end open?
    is the wiring correct?
    sometimes a winding failure is a short, but not to ground- it may be shorting inside itself through poor insulation characteristics.

    for hypothetical example,
    a motor was bogging to start for a long period of time, through poor lubrication, wear, high belt tension, or other factors.
    the starting circuit resistance will eventually increase, as the resistance raises- the wire in the windings and in the circuit heat as a direct result.
    the heat will eventually heat the insulation and can crack and fall away.
    any place where there is a failure in the insulation in the winding you have a place for a short.
    overheated windings are less efficient at conducting electricity than windings that have never been overheated, when all other things are equal.
     
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  3. Xyius

    Xyius United States Iron Registered Member

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    Ha yeah I figured as much about the screw. I just wonder why the heck it was even in there. As for the spring. I lost it. Don't know where it fell, but I wasn't up for trying to look for it very hard. I took both end bells off and removed the armature. Didn't seem like I could tear it down any further aside from removing the windings from the housing. I don't know how the windings are attached to the housing but I didn't attempt to remove them. I believe the wiring is correct as it WAS working before, but now is not. I didn't even disconnect any wires when taking it apart.

    Thank you for your reply, I think I will just get a new motor for this thing as it is only a 1/4 hp motor. Just need to work it into the budget. I would have liked to fix this one though.
     
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  4. kd4gij

    kd4gij United States Active User Active Member

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    There should be a start capacitor some where. On my old craftsman 12" it is remote mounted in a switch box that has the on-off and forward-reverse switch.

    The spring may go in the centrifugal switch. It should have a spring to hold it closed untile the RPM's come up.
     
    Last edited: Jan 25, 2016
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  5. Xyius

    Xyius United States Iron Registered Member

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    No capacitor. Some induction motors don't use a starting capacitor. Typically when the starting load isn't too large you can get away without one. The starter windings are at a 90 degree electrical phase shift from the run windings, allowing starting to be easier from a stand still. The spring isn't for the centrifugal switch as the spring loose inside was a coil spring while the switch spring is a leaf type spring. The centrifugal switch works fine btw.
     
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  6. hermetic

    hermetic United Kingdom Active User Active Member

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    Have you checked across the contacts of the centrifugal switch with a multimeter, if they are not making goog contact when stationary, the motor will not start.
     
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  7. David S

    David S Canada Active User H-M Supporter-Premium

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    If the windings are about 3 ohms each, when you measure the resistance of the two input wires when the motor is stationary it should measure about half that. Once it starts up it will kick out the start winding.

    If you can get it to run by spinning it with your hand, then for what ever reason the start winding is not in the circuit. Open winding (which you say isn't), open connection, or open centrifugal switch.

    David
     
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  8. Wireaddict

    Wireaddict United States Active User Active Member

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    I've attached a sketch showing a single-phase motor wiring. One other place I've seen starting caps [if this motor has one] is in the motor base if it's a casting [although most today are stamped brackets which are welded to the motor housing]. You may have had a wire come off somewhere in the starting circuit. Hope you find it.
     

    Attached Files:

  9. bobshobby

    bobshobby Australia H-M Supporter - Premium Content H-M Supporter-Premium

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    Are you sure the spring and screw don't belong to the centrifugal starting switch, might be why it's not starting. Are you sure there is no starting cap, mounted remotely in a switch box.
    Check that the switch is actually closed and conducting while the motor is stationary. If it will run but won't start the problem is clearly in the starting circuit.
     
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  10. 4gsr

    4gsr Global Moderator Staff Member H-M Supporter-Premium

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    I'm surprised that screw didn't short out the winding with a big flash and boom!

    You can tell that some one has been in that motor before. Look at the butt connectors, that's not normal in a factory motor. One of them may be loose not allowing the starting switch to operate. Another thing to pay attention to when putting the motor back together. Make sure the wires don't get in the way of the starting switch. This will not allow the switch to make contact to start the motor. I've encountered this a time or two over the years. And that screw, does not look like any hardware associated with the motor. I'm sure the motor is a open drip proof motor but does not prevent foreign objects from getting into the motor. Usually it's steel shavings that land inside the motor not screws and bolts.
     
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  11. kd4gij

    kd4gij United States Active User Active Member

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    Looks like the OP hasn't been back on the board since he posted this on Jan 25
     
  12. 4gsr

    4gsr Global Moderator Staff Member H-M Supporter-Premium

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    Snake bit us again!!!
     
  13. markba633csi

    markba633csi United States Active Member Active Member

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    Love it, folks write in wanting to know how to fix something, then suddenly they don't. I'm sure in this case it was just a loose connection. I like to follow the thread to the successful conclusion- This is like turning off the TV before the movie is over LOL
    MS
     

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