Old bench grinder what is this part?

Discussion in 'The Projects Forum' started by themow, Dec 24, 2015.

  1. themow

    Thread Starter New Member

    Dec 28, 2012
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    I was given a heavy old bench grinder 110v ac motor with shaft for a wheel on each side. The motor plate is missing so I dont know anything about the motor.

    Its plug was missing so I shoved the bare wires into an outlet to test and it powered on fine.
    I put a plug onto it and it powered on but sounded different and then started smoking and made a pop sound. I unplugged it and waited an hour and plugged it back in and its just making a buzzing sound.

    The only thing under the hood is a switch and this part in the pictures below. Is the polarity important on this setup?

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  2. MaxHeadRoom

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    Jul 18, 2013
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    Last edited: Dec 24, 2015
    #12 likes this.
  3. themow

    Thread Starter New Member

    Dec 28, 2012
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    I popped the cover where the wires enter the motor casing. Pics attached. I dont see a CAP
    Wires look toasty
    Any clue what caused this?
    WHen i power the unit on, the motor spins forward and back .5" 20151224_140435.jpg
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
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    Some types of motor don't have a start capacitor - you just power the start winding until it spins up, any longer lets the magic smoke out though!

    The relay is a current relay, the motor in the stalled condition draws enough current to pull the relay in, as it spins up the relay drops out.

    Some manufacturers took to putting a PTC thermistor in series with the start winding - once or twice I've got away with bodging them with a TV degauss posistor.

    If the motor uses a start capacitor - the PTC method might not work.
     
  5. #12

    Expert

    Nov 30, 2010
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    From the looks of those windings, all you need now is a list of scrap metal prices.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It looks to me as well as is it cooked!
    Max.
     
  7. ian field

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    Oct 27, 2012
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    Probably the start relay contacts welded shut and let the magic smoke out.
     
  8. MaxHeadRoom

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    The op says it is an older unit, so if it had not been ran for some considerable time, the bearing may have partially seized or become corroded, and if it is the non-capacitor start motor, they require zero load to start, if any considerable resistance they can burn out.
    Max.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    They usually buzz when that happens - most people would give it a shove to see if it starts going, and/or unplug it as the magic smoke starts billowing.

    They usually have impeller blades on one end of the rotor for what little air flow they shift. Failure to open the start contacts on an otherwise running motor may not release all that much magic smoke all at once - the smoke could even go un noticed.
     
  10. themow

    Thread Starter New Member

    Dec 28, 2012
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    Its alive!!!!
    I bypassed the start relay and wired the hot directly to the run winding. As someone mentioned in a previous post, there were 3 windings in this motor. neutral, hot, and start.
    I wrapped a rope on the shaft and turned it on and gave it a pull and she came right to life!!!
    Ran it for 5 minutes and is working fine.
    I hooked up a 250v 10uf capacitor hoping that would get it going but it didnt budge.
    Its probably a 1/3hp 3450rpm motor.
    Will the proper sized cap work? If not I may just stick to the rope method. THis motor is super smooth.

    What I think initially happened was that the relay was pushed up against the base and forced constant contact.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    You could probably flick it round enough with your fingers - but if you forget; the run winding will burn out.

    As I mentioned in an earlier post - some manufactures just put a PTC thermistor in series with the start winding, once or twice I've got away with a PTC element from a CRT TV degauss unit.
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If by passing the switch, make sure you do not power the start winding, if left connected it could burn out the start winding, if not cooked already, if you fitted a manual switch in place of the current relay then you could do it this way as long as you switch it off as just before it comes up to speed, IOW a couple of seconds at best, as long as the start winding is still OK.
    It would be a 2 pole motor for a bench grinder and fractional HP if no capacitor.
    Max.
     
    Last edited: Dec 24, 2015
  13. themow

    Thread Starter New Member

    Dec 28, 2012
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    I guess I was not clear in my last post.
    The start winding is cooked.
    The start relay works, but since the start winding is cooked, I disconnected it.
    Now power only goes to the 2 main windings

    In order to get up to full speed, spinning by hand is no good. Need to wrap a strap around the shaft and give it a good pull. Then flip the switch.
    After that, she purrs

    Can I add a capactior to fix this problem? If not, what about a ptc?
    I tried a cap from my other bench grinder but it wouldn't budge. 250v 10uf
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If the start winding is cooked, then a cap is not going to be of any use, as you would normally put the cap in series with the start winding, but normally there is already some phase shift in this start winding to begin with so that would not be a success, for the price of bench grinders now, it would not be worth getting it rewound.
    Max.
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
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    There is/was a rewind firm near me.

    The quote they gave me for an angle grinder was more than the price of a much better quality tool than the one I wanted to repair.
     
  16. themow

    Thread Starter New Member

    Dec 28, 2012
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    OK I didn't know motors with caps had start windings. Thank you guys for all your help. I'm going to use the rope start method for a while becasue newer Chinese grinders are no where as smooth. Thank you all
     
  17. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    For quick testing, I've briefly shorted where the capacitor goes in a capacitive start motor to start it.
     
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    In my experience usually angle grinders are portable and have Universal motors.
    Max.
     
  19. ian field

    Distinguished Member

    Oct 27, 2012
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    The firm in question would probably want more to rewind a bench grinder than the price of a better one too.......................
     
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