3 Capacitor single phase motor?

Discussion in 'General Electronics Chat' started by strantor, Aug 30, 2013.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    What manner of sorcery is this? Has anybody here seen it before? Single phase, 230V 5HP motor, 3 different size/different voltage caps inside. Not a field kluge, shipped from the factory this way, and the caps match the nameplate specs. Smallest and the medium size cap say "start cap" and the large cap doesn't.

    I got it fixed by replacing the obviously unhappy large cap, but I need closure. What the heck is this? My theory, as the drawing implies, is that it's actually a 3 phase motor, maybe robbed from one of WEG's other product lines, and they (WEG) slapped a bunch of capacitors inside in such a way to generate a 3rd phase for single phase operation.

    I cannot find this motor on WEG's website nor anywhere on the web. Type "EM" does not exist apparently. Closest match by physical appearance is type "JP." And it's red, which makes little sense since WEG motors in red mean either farm duty or auger drive, and this is a hydraulic pump. I'm thinking maybe it was a special order.

    I don't find much of anything when googling "3 capacitor motor" or similar.

    also, what's the thing in the bottom of the box with the 2 yellow wires?
     
  2. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    The way you drew that, it looks like the 2 large capacitance parts are in parallel to make a huge start cap. Drawing correct?
     
    strantor likes this.
  3. Ramussons

    Active Member

    May 3, 2013
    557
    92
    The unit with the Yellow wires is a Thermal Cut out.

    Seen them on the body of Sealed Compressors of Air Conditioners.

    Ramesh
     
    strantor likes this.
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,542
    2,369
    It appears to me as a standard 1ph 4 pole split phase motor, with a start cap and run cap.
    The label seems to bear it out?
    Max.
     
    strantor likes this.
  5. tinkerman

    Member

    Jul 22, 2012
    136
    34

    Single phase. Cap start, cap run. The unit with yellow wires is the overload as mentioned earlier in the thread. The black units are the start caps. They should be in parallel and are switched out when the motor reaches about 75% speed. The run cap is in series with the start winding and stays in service. Increases the torque and PF.

    Standard single phase arrangement.
     
    Last edited: Aug 30, 2013
    strantor likes this.
  6. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    Thanks everybody

    I guess at this point it's obvious I don't deal with many single phase motors ;).

    The stuff I work with on a daily basis is variable speed 95% of the time; either 3 phase or DC. From that perspective, this thing baffled the crap out of me. Well, another lesson for the books; I'll know what I'm looking at next time.
     
  7. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    The puzzlement was about why this would puzzle you.:confused:

    I guess you started with the big stuff and never met the fractional horsepower motors that are wired this way. I was assuming you knew about the small stuff.:rolleyes:

    My bad.:p
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,542
    2,369
    The way it is done (split phase) on very small motors such as bathroom fans is to use a copper 'Shading Ring' this is essentially a shorted turn on one part of the stator, that performs a bit of phase shift to the incoming AC field in that part of of the stator, out of phase with the supply.
    You may also have seen it on contactor coils also, for the same reason.
    Max.
     
    strantor likes this.
  9. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    LOL. Yep, I went from playing with shaded pole fan motors as a kid to commissioning 1150HP GE motors on drilling rigs as my first industrial job. I missed out on the stuff in between. You know, starting at this end and working back, this dual cap start/cap run setup seems like a kluge. I couldn't believe it was legit when I saw it. I guess no big deal for you, it's normal. But it just doesn't seem right to me.
     
    #12 likes this.
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,542
    2,369
    The main thing is it has worked for many decades, simple and cheap! ;), Usually the weak link is the centrifugal switch, hence the external start relay.
    Max.
     
    strantor likes this.
  11. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    Those large capacitors are called, "dry electrolyte" and they have a failure mode that is about heat (current). If the motor is mechanically stalled when you try to start it, it will only be a matter of seconds before great clouds of white (magic) smoke come out of the capacitor.

    This suggests that 2 capacitors were used to distribute the heat better during a start surge. It also suggests that you make a habit of noticing the escape port next to the connectors. It's a hole in the top, less than 1/4 inch diameter, and originally plugged with what looks like rubber.

    You probably know that the "run" capacitor is able to survive being connected directly to the power line, and it will just sit there passing current, all day. A quote from my teaching days: An oil filled run capacitor can be connected directly across the power line and it will never get hot. If it does, it is defective."
     
    Last edited: Aug 30, 2013
    strantor likes this.
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,542
    2,369
    Oddly it appears the 42μf which I would expect to be the run cap seems to be the culprit, It may not be a standard AC motor-rated run cap to have failed like it did?
    I would insure the replacement is AC run.
    Max.
     
  13. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    It was the original as far as I could tell. It was WEG brand, and matched the nameplate specs 45μF. I replaced it with this.
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,542
    2,369
    Should Work!
    Max.
     
  15. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    Misprint, Max. It's a 45/440.

    Can't believe the price! Those cost me about $12 at my A/C wholesaler.
     
  16. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    My customer saved a lot more than that, by me driving straight to grainger down the road where I knew I could get it, than spending the time to shop around for a better deal @ $136/hr ;). Now, if I were to carry these things around like you do, it would be a different story. I could buy them @ $12 ea and sell them for a lot more. But something tells me they aren't worth carrying, for me.
     
  17. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    I very much agree. You know (from photos I have posted) that I carry a box of caps on my truck. No surprise to install 4 or 5 per month. At $3 for a 5uf@370, or even $27 for a 45@440, the travel time is worth way more than the capacitor.

    ps, I read the price wrong the first time I looked and thought it was $45 for the 45@440.
     
Loading...