Electric Motor help

Discussion in 'General Electronics Chat' started by repairman, Nov 2, 2009.

  1. repairman

    Thread Starter New Member

    Oct 31, 2009
    4
    0
    I have a 20 year old Sears compresor that sometimes would not start, just ssort of hum and pop the motor breaker. It was complicated by a loose pump pully that would jam. That's fixed now. Now, when power is
    applied, the motor runs slow for several seconds, then the motor breaker pops, even with the belt removed. Applying power turns the motor slowly but the shaft is not binding. I disconneced what I thought was the start capicator, on top of the motor, 704 uf, 110v and tried running the motor. It still started to turn slowly with the cap removed, which makes me wonder if this is a run capicitor. Also, I removed the cap to test it with an ohmmeter, and it acts normally, no shorts or opens, resistance builds.
    The motor is 110v, 17amp, 3450rpm, a case like a 1/3 or 1/2 horse size.
    Do you think replacing the cap will solve the problem or is it something more serious?
     
  2. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    It certainly could be the cap, depending on how it's connected in your circuit. If one end goes to a positive source and the other a negative, it could no doubt be shorting your circuit. Check for signs that the cap is bad, this may include a bulging top (not on the sides) and sometimes leakage.

    Take your multimeter and put it on the resistance setting on both sides of the cap, when the breaker pops you might see a short, which will verify whether the cap is bad or not.

    Austin
     
  3. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    The cap need not be shorted to have failed, it could have "lost" its capicantance ( sp ). I think you should try a new cap.
     
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    If it is a Start cap (rather than Run), there may also be a starter switch or relay unit which switches the cap into circuit. It could be a centrifugal switch on the motor.
     
  5. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    at that value, it will be a start cap. do a continuity check to make sure your centrifugal switch is engaged and that your start winding isn't open.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Take the belt off and when you try to start the motor, VERY CAREFULLY help the pulley spin in whichever direction it is trying to go If it is just humming, direction won't matter. If it is a bad start switch, start capacitor or burned start winding, the motor should take off and come up to full speed. Otherwise, I would say it's toast.
     
  7. repairman

    Thread Starter New Member

    Oct 31, 2009
    4
    0
    Just to clear some things up:
    When the cap is tested with an ohmmeter, the resistance slowly rises to a high nimber. Reversing the leeds does the same. Connecting the ohmmeter across the cap leads with it disconnected is an open circuit,

    With the belt off the motor, it turns freely.
    When power is applied the motor turns slowly by itself until the breaker pops. This is the case WITH or WITHOUT the cap connected. Spinning the already spinning shaft does nothing to change the speed.
     
  8. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    The behavior of the ohmmeter is consistent with the cap being ok, but is not definifitive.
     
  9. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    I agree. What is the resistance of your run and start windings?
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    My Dad had a Craftsman air compressor that he bought in the 70's. It had a capacitor start motor.

    I'll bet that the switch contacts on the centrifugal switch are burned after making/breaking contact many times.

    You'll need to open up the motor, polish the switch contacts using a very fine jeweler's file or 600 grit wet/dry sandpaper (removing as little material as possible), make certain that the mechanical portions of the switch are moving freely, and reassemble it.

    Don't use emery cloth or regular sandpaper on the contacts; they'll get embedded in the contact material and they will very quickly get burned again. Don't try to make the contacts perfect, just draw sandpaper or a file through them several times to remove the worst of it. You can take the material off, but you can't easily put it back on.
     
  11. Duane P Wetick

    Active Member

    Apr 23, 2009
    408
    19
    The first thing that I would do is replace that capacitor. Shining up the centrifigal swich contacts will help (if there is one). Check the windings with an ohmmeter and check for shorts to ground too. The reason your CB trips is that the motor is trying to get up to rated speed and can't. The time delay built into the CB will hold (for a while) to let the motor get up to speed.

    Cheers, DPW
     
    Last edited: Nov 8, 2009
  12. repairman

    Thread Starter New Member

    Oct 31, 2009
    4
    0
    Will pull the motor and open it to check the switch. Will probably be a week till it gets done.
     
  13. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    just check the resistance of your start winding through the switch. You'll know if the switch is open or corrupt.
     
  14. repairman

    Thread Starter New Member

    Oct 31, 2009
    4
    0
    The best idea seemed to be to check the switch. The motor is almost impossible to remove without dis-assembling the entire unit, so I decided to check it installed. As I began to release tension on the 4 thru bolts that hold the motor end pieces in place, I heard a "click" just like a centrfugal switch. I tightened the bolts and the motor runs fine so far.
    Only time will tell how long
    Thanks to all for the suggestions
     
  15. rvh002@gmail.com

    Active Member

    May 15, 2009
    118
    2
    It is most certainly the centrifugal switch that does not work
     
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