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Thread: What's the deal with electric motors?

  1. #1

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    Question What's the deal with electric motors?

    I have an inexpensive 6" belt sander with (apparently) a non-functioning motor ( 3/4hp). The cheapest replacement motors I can find are in the $80-100 range, while a new sander from Harbor Freight is $60 on sale!

    I can't stand the idea of throwing away the entire rest of the sander, so I'm trying to find a replacement motor I can salvage from something else. Would a motor from a washer or drier work?

    Any other ideas welcome.

  2. #2
    Profkanz's Avatar
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    First check that it's getting power.
    Then check if the switch is OK. Motors usually last a long time. It's often the cord, plug or switch that fail first.

    Almost any motor with a similar HP rating will work. The difficulty comes in mounting the motor.

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    Jon (02-28-2016)

  4. #3

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    I check wiring and switch visually -- looked okay. will try bypassing them ...

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    Beyond the visual check, you really ought to put a meter on each component of the circuit and check for continuity. Sometimes they'll look OK, but one piece or another will be kaput. Just be glad it's not an intermittent failure, as they can be especially challenging to diagnose.

    Ken

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    PJs (03-31-2016)

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    I assume that this is a bench top sander and not a hand held (but same checks apply anyway).
    Check that power point is working.
    Turn sander on - does it hum or is there no sound at all.
    Power to off - remove belt - Power "on" and try to spin the motor or drum by hand (carefully) - if it tries to run or has any torque- renew the capacitor.
    Check the brushes - remove both brushes and inspect - look for burning or excessive wear. Check brushes slide freely in their holders. Renew if worn or clean up the old brushes and bed them in to the commutator. To bed the brushes, (old or new), wrap a narrow strip of fine glass paper around the commutator, tape it in place if necessary. fit the brushes and rotate the armature in the direction of normal rotation by hand until the brush ends take up the curve shape of the commutator. If you use tape you may need to rotate in both directions to get the bulk of the bedding done but try to finish up in the direction of rotation. It is fiddly work.
    While you are there check the state of the commutator. Feel the surface with your finger as you rotate it and there should be no lumps or bumps. It should have a blue/black appearance, (patina), but if it appears burnt it will need dressing or skimming in a lathe. You can usually get away with cleaning it up with very fine glass paper, preferably in the direction of rotation. Clean up any burning between the segments while you are doing this.
    Check the smell of the internals - if it smells badly burnt it may be for the scrap heap.
    If you have a multi meter you can check for continuity (ohms) of the field coils. Each coil should have a similar reading. Similar checks can be done between segments of the commutator.
    If your sander is from Harbor Freight check them for a spare motor. In most countries sellers are obliged to carry spares for anything that they sell. They are not obliged to sell these parts cheaply though.
    Remember POWER OFF at the wall and to be safe, pull the plug when you are doing most of this work.

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    PJs (03-31-2016)

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    thanks for checklist! I'm embarrassed to admit I had to put off working on it til I made room on my bench! Nearly there, and your suggestions will be the first I things I try. (Yes, it is a benchtop unit.)

  10. #7

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    Need advice on motor switch wiring -- was: Followup on sander troubleshooting

    [I'm updating this in the hope someone will see it and be able to offer a suggestion. Thanks!]

    I was able to clear my bench and start working on the sander. Had to disassemble quite a bit to get to components. Have a short video at https://www.flickr.com/gp/[email protected]/A4ib04 showing parts. I'm guessing the black component at the top is a capacitor? I checked continuity of power cord which is good. I tried testing the switch and it appears that it's open across all possible 2 terminal combinations when in both "off" and "on" positions so possibly it is the failed part. Can I temporarily jumper it to check? Can you tell from the picture ("on" is up) What's the deal with electric motors?-switch.gif what connections to make?

    There are no markings on the switch that I can see.
    Last edited by richardcrane; 03-25-2016 at 02:26 PM.

  11. #8
    Frank S's Avatar
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    Richardcrane ; as per your drawing of the switch you need a double pole single throw switch
    it would be wired as per your color guide as brown to black blue to red


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