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Motor resistance

Discussion in 'Commercial' started by Texas-Tech, Jun 3, 2017.

  1. Texas-Tech

    Texas-Tech Official Geezer*

    Messages:
    16,701
    Location:
    duhh
    have a 40hp motor on a cw pump, blew out all three fuses. Am getting 45k ohms to the motor shaft so I pulled the motor out because they have one here that was rewound last year. For the heck of it I checked the resistance on that one and have 165k ohms to the shaft.

    Is this common because I'm not used to seeing any resistance to the motor
     
  2. Catpower

    Catpower The Crowd Pleaser

    Messages:
    21,356
    Location:
    in the Cabana

    Is the motor wet? There should be infinite resistance through the windings
     
  3. Daryl_Dixon

    Daryl_Dixon New Member

    Messages:
    3,357
    We have a local motor company test our big motors on site. They use a variety of tools, one of which is an oscilloscope. I have diagnosed motors as "shorted to ground" only to have them do further testing and say the motor was okay. So, there is more to it with these things...
     
  4. Texas-Tech

    Texas-Tech Official Geezer*

    Messages:
    16,701
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    duhh
    It's polling 40 amps, rated for 46
     
  5. Texas-Tech

    Texas-Tech Official Geezer*

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    duhh
    The temp of the case is 187
     
  6. Texas-Tech

    Texas-Tech Official Geezer*

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    16,701
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    duhh
    Well, well, well, hows about an update
    There was a pretty strong power surge there Fri nite, all three 70 amp fuse blew out and wouldn't ya know I don't have them in my truck and all the parts houses are either closed or they don't carry um so the other tech picked up some 100amp from Home Depot on the way down.
    We had just replaced the motor in Sept so it was pretty new. When I took a reading from the mtr to the shaft and saw 48k Ohms I thought the damn motor was bad, pulled it and put the one that,we had rewound in it's place. Cool, no problems start the chiller and away we go.......................until the motor started to get hot. It was around 190°, even the pump was getting warm. :shocked:

    Looked at my partner, we didn't want to come back for any other problems so we pulled the motor back out and installed the other back in. Facker ran like a new motor
    Turns out the motor that was rewound was missing covers inside the housing that helped direct the air from the impellers to go out the sides of the motor. Looking in the ends of the new motor you couldn't see the windings and the air was drawn into the ends and out the louvers in the sides. The one that overheated was pushing some air out each end. The windings were totally visible
    Lessons learned today:
    1. stock up on a variety of fuses
    2. don't be so quick to condemn the equip
    3. more education(at my age:lachen:) about the electrical side of motor testing
    4. buy a good megger
    5. BUT the most important lesson was DON'T ANSWER THE PHONE ON SATURDAY MORNING WHEN YOUR NOT THE LOD
    end of story
    sob
     
    LemonWizard, jpb2, tuna and 9 others like this.
  7. Catpower

    Catpower The Crowd Pleaser

    Messages:
    21,356
    Location:
    in the Cabana

    I will make you a hell of a deal on a Hi Pot tester (high potential) but they have renamed them to Dielectric Withstand Tester

    They are mo better than a megger, it will tell you how much voltage can be applied before it finds ground
     
  8. Texas-Tech

    Texas-Tech Official Geezer*

    Messages:
    16,701
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    duhh
    How much?
     
  9. Catpower

    Catpower The Crowd Pleaser

    Messages:
    21,356
    Location:
    in the Cabana

    $800 it was about $2000 new only used a couple times, but it is more for in house work it could be fitted into a pelican case or something like that for portablity
     
  10. pintofstout

    pintofstout New Member

    Messages:
    1,462
    Location:
    Olean, NY
    Is anyone able to explain why there is resistance to the shaft? I always thought it was the refrigerant oil because i see it on compressors- but here lately I've tested a couple motors on drives and found it's quite common to find some degree of continuity on dry motors.

    I'm not able to explain that. The windings should be perfectly isolated from the rotor.


    Sent from my iPhone using Tapatalk
     
  11. HMD

    HMD does it matter?

    Messages:
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    yep. stator should have zero continuity to rotor
    except, in a refrigerant environment
     
  12. Texas-Tech

    Texas-Tech Official Geezer*

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    I'm gonna find out why if it kills me. That 40 hp motor was a heavy sob and to swap it twice over not knowing what I was seeing is a pisser.
    Like you I'm looking for nothing less than high megaohms not what I saw
     
  13. mike1020

    mike1020 entitled millennial

    Messages:
    17,181
    Location:
    SoDoSoPa
    I had a motor full of metallic dust that grounded out.

    It read 1 ohm to ground, I blew the dust out and it then read infinite to ground.
     
    Tap likes this.
  14. HMD

    HMD does it matter?

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    windings are covered, in varnish, or whatever goo.
    there most likely is a breakdown in that varnish
     
    Tap likes this.
  15. Texas-Tech

    Texas-Tech Official Geezer*

    Messages:
    16,701
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    each motor gave me a reading, one was 45kohms, the other was 358kohms.
     
  16. mike1020

    mike1020 entitled millennial

    Messages:
    17,181
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    SoDoSoPa
    That makes sense.

    I ended up putting a new motor in.
     
  17. HMD

    HMD does it matter?

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    trane had a rule of thumb............ i think it was 1k ohm per volt applied.
    in a refrigerant atmosphere.
     
  18. Texas-Tech

    Texas-Tech Official Geezer*

    Messages:
    16,701
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    I have heard that too but these are just running cw pumps
     
  19. Daryl_Dixon

    Daryl_Dixon New Member

    Messages:
    3,357
    Spark up a conversation with guy at local industrial motor supply/repair center.

    I believe the OEM's of such motors have specs that take the mystery out of it.
     
  20. pintofstout

    pintofstout New Member

    Messages:
    1,462
    Location:
    Olean, NY
    A Trane guy told that i needed to change the oil because i was megging at less then 100K but obviously that related directly to the oil being conductive. Changing the oil did not ultimately help actually, but conducive oil was the premise. I'll bet whatever holds the windings in place is slightly conducive


    Sent from my iPhone using Tapatalk