What is the cause burnt running capacitor with an 0.75 kw single phase motor.

Discussion in 'General Electronics Chat' started by shahid chaudhary, Oct 4, 2015.

  1. shahid chaudhary

    Thread Starter New Member

    Oct 3, 2015
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    Good morning dear all senior persons .

    please some one explain me what is the cause burnt running capacitor. i have 0.75 kw single phase motor on start and run capacitor . but Running capacitor burnt second time . please help me to solve this issue
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Not using an AC motor rated run capacitor?
    The other is if the motor is Chinese in origin, and then replacing with exact part No.
    Max.
     
  3. #12

    Expert

    Nov 30, 2010
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    A run capacitor can be connected directly to the power line and sit there all day with no bad results, therefore it is either the wrong capacitor or the motor is causing voltages not expected. "Wrong" includes bad quality parts and parts not able to survive the voltage on their label.
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
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    Not in my experience - I once used a motor run capacitor as a current limiter for charging a motorcycle battery (the type of circuit is forbidden here), the capacitor did a "party popper" across the room.

    There was a haze of smoke and people running about like headless chickens.............
     
  5. MaxHeadRoom

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    Sounds like a start capacitor to me!:eek:
    Max.
     
  6. #12

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    The fact that you lack experience does not mean run capacitors can't survive the voltage on their label.
    I have connected (3) run capacitors in parallel across a 240 VAC line while I taught a class. Then I showed the students that the capacitors had been connected for nearly an hour and they weren't even warm.

    Max has a good idea. Perhaps you had a, "dry electrolytic" start capacitor that exploded.
    That is also a possibility for the Thread Starter. He might be mistaking a dry electrolytic start cap for an oil filled run cap.

    The white smoke is an indicator of a dry electrolytic start capacitor. When a run cap explodes, it spews oil.
     
    Last edited: Oct 4, 2015
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  7. ian field

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    The label said a fair bit more than the mains voltage - something like 380 or 440V.
     
  8. MaxHeadRoom

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    If this was a start capacitor it could have well said 380+ but they are only rated for around 5 seconds operation in a motor start situation.
    I have experienced the same as #12 AFA run capacitors are concerned, I have a stock I use for a device I market, they are not used in a motor start situation, but initially I did a bench test by applying AC for considerable time and found no visual effects or heating.
    The Caps are oil/paper and made in N.A. by CDE.
    Chinese motor capacitors are notorious for early failure.
    Max.
     
  9. #12

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    I see there is another aspect to address. The start caps do not eject their smoke because their voltage rating was exceeded, they fail because the current overheats them.

    You can use dry electrolytic start caps for other purposes, but your calculations are based on a time limit and the current in the circuit. The oil filled run caps are pretty much idiot proof. You can plug those directly into a wall outlet and come back tomorrow without any damage.
     
    Last edited: Oct 4, 2015
  10. ian field

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    I've never seen an "electrolytic" start or run capacitor on any AC motor that uses that type of arrangement.
     
  11. MaxHeadRoom

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    The typical start capacitor is quite a bit larger (μf wise) than the run capacitor, in order to keep the physical size within reason, the bi-polar electrolytic is typically used, but on the constraints that it is used for a very short period, (starting), if left in circuit on AC excessive heating occures, (how do I know this!:eek:).
    High ESR value.
    Max.
     
  12. #12

    Expert

    Nov 30, 2010
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    I guess that explains why your past experience was not sufficient for you to already know how to handle this circuit problem.
     
  13. dannyf

    Well-Known Member

    Sep 13, 2015
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    1. over-voltage, like power-on, or lightening strikes;
    2. in-rush current, like turning on the motor, etc.
    3. ESR.

    They usually last a long-time, 10K or 30K hour ratings are common. So if yours die out that fast, there may be something else, like corrosion, animal nesting, etc.
     
  14. ian field

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    Well the capacitor I used wasn't electrolytic, I checked that.

    NP electrolytics with that sort of voltage rating must be pretty rare - and probably pretty big!
     
  15. ian field

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    I guess you just couldn't resist a chance to mock.
     
  16. MaxHeadRoom

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    How did you check, the outward (bi-polar) appearance and label is not always obvious.
    Max.
     
  17. MaxHeadRoom

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    You should see the oil/paper version!.:(
    Max.
     
  18. ian field

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    It was too long ago to remember exact details of what was printed on it, but the foils and paper expelled were oily and not caustic.
     
  19. MaxHeadRoom

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    Well needless to say, if the capacitor was in fact a AC motor run type, it was either defective or of very poor manufacture.
    It is certainly not the normal behavior for one of that type, and should be able to withstand direct connection to an AC supply.
    Max.
     
  20. ian field

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    Nevertheless - I tend to avoid motor capacitors for that purpose and prefer PFC capacitors from old strip lights.
     
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