An AC motor creating electrical energy

Discussion in 'General Electronics Chat' started by Gdrumm, Jan 29, 2015.

  1. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    A friend asked me to look at an exhaust fan.
    The motor is probably 1/4 horse, and the fan blade is about 9" in diameter.

    He thought it was "single phasing", but when I looked at it, it is a 110v single phase motor, with a frozen rotor shaft.

    I put some automatic transmission fluid on the bearing journals, and hooked it up to my drill motor and gave it a spin to see if I could free it up.

    That didn't work, but as it was hesitantly spinning, I saw one of the two wires (for the plug) touch the metal shroud it was mounted in, and the wire was sparking. It was a white spark.

    I did it once more to be sure I wasn't just hungry os something, and it sparked again.
    What kind of electircal energy was it producting? Just curious.

    Thanks,
    Gary
     
  2. wayneh

    Expert

    Sep 9, 2010
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    You see sparks when you're hungry ? o_O
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The scary kind!:eek:

    Seriously, it could have been some leakage if it was not a full blown low resistance short to ground, which is usually more spectacular.
    Max.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Max, he was spinning it with an attached drill. It was not sparking while plugged in.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    In that case it would be unusual as a induction motor does not usually generate without some form of excitation.
    Could you reproduce it or was it a one shot occasion?.
    Max.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    If the rotor had some residual magnetism I would think it could generate some voltage.
     
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  7. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    I do see sparks sometimes, but then again I hit my head on the concrete a few months back.
    So, I second guessed what I was seeing, and well, it was close to dinner time.

    So I did it again (reproduced it), and sure enough, it did spark.
    A white spark.

    It had a run capacitor hooked up to the motor if that makes any differnece.

    It was just unusual I thought.

    Thanks,
    Gary
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If it was one wire from the motor touched the frame then there would have to be a return path of some kind, unless it was some kind of static generated charge?
    Did it sustain an arc or just a one shot every time?
    Max.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    This thread suddenly reminded me of some advice in a PC servicing article - don't blast the dust out of cooling fans with compressed air (or at least stop it from spinning). Apparently the motor can act as a generator with enough juice to wreck the circuit that generates the motor phases.

    I vaguely remember something about some old British motor cycles having an emergency start position on the ignition switch. In that era they had similar pattern dynamos to the ones on cars of the day - the emergency position made full use of residual magnetisation in the dynamo field poles - if you bump started it fast enough, you could get just enough spark to fire the cylinder, then if you're really lucky the battery should pick up enough to energise the field winding in the normal way.
     
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  10. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Yeah, but the motors that you mentioned are of the permanent magnet type, which can easily function as generators. What's unusual about this case is that he's (apparently) talking about an induction type of motor.
     
  11. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    It was a sustained spark.

    Anyway, it turns out the motor was under warrenty.
    So I called, and they are going to send me a replacement.

    The original motor had the rotor shaft frozen to the bearing of the front cowling.
    After a lot of effort, I freed the shaft from the cowling and bearing, and the shaft was scored pretty bad.
    I cleaned it up by running it in the chuck of my 1/2" drill.
    I ran a fine metal file over it.
    I followed that up with fine emory cloth.

    I put it back together, and it's running (in my bench vise).

    The bearings are starting to heat up now, the front one reads 113 defrees f, and the rear one reads 141 degrees.
    I'm sure it would be running cooler if I had the fan blade attached.

    Is that much heat to be expected in 3-5 minutes of run time?

    It is a 1/5 hp, SIngle Phase, 120v 60 Hz, 2.45 amp motor.

    Thanks,
    Gary
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I take it these are oilite bearings? they usually last for many years, not so long as ball, but should not have failed in a warranty period, smacks of poor assembly?
    They should also run cooler than that, unless it is the partially seized motor heating them up?
    Max.
     
  13. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
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    Okay,
    Thanks,
    I'll keep it in my project box, perhaps I can clean it up a bit more and get it to run cooler.
    What would be an acceptable run temp?
    It's currently 68 degrees f in my garage.

    Thanks again
     
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