How important is value of start capacitor on motor?

  1. MoreWellie Member

    Messages:
    972
    Location:
    Bedfordshire, UK
    I have a failed start capacitor on the motor of my compressor.

    The original was 70uF but I am struggling to find that value in a size that would fit the existing casing

    I have found a 100uF which would fit (according to the website) but cannot find what affect the different value might have on the motor. I guess it will increase the current flowing but it might also affect the phasing and therefore the motors ability to start

    My alternative would be to fit a bigger box on the compressor

    Both are 450vac rated and are therefore ok from that point of view
     
    123hotchef likes this.
  2. MoreWellie Member

    Messages:
    972
    Location:
    Bedfordshire, UK
    Done some more research and it looks like +-10% is about the acceptable range so I have answered my own question

    Shame you cannot like your own posts, lol
     
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  3. 123hotchef Member

    Messages:
    7,293
    Location:
    Kent
    I am sure the 100UF will be fine
     
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  4. zzr1200

    zzr1200 Working at 650 ft on open steel work

    Messages:
    3,453
    Location:
    Brimington, Chesterfield, Derbyshire
    Agree stick a100uf in it, it'll be fine.
     
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  5. carbon

    carbon Member

    Messages:
    1,105
    Location:
    S. Glos. U.K
    How many capacities are fitted? Not sure 70uf would be a start cap but depends on speed and size. If it’s a run cap the fitting a higher capacitance cap can lead to lower torque and overheating of the winding
     
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  6. MoreWellie Member

    Messages:
    972
    Location:
    Bedfordshire, UK
    Just the one

    I have now found an alternative supplier that has confirmed the size as being correct to fit so I can stick to the original value. I know this is the original capacitor as I have owned this compressor from new and haven't had to change it before

    Thanks to all for contributing
     
  7. carbon

    carbon Member

    Messages:
    1,105
    Location:
    S. Glos. U.K
    As it only has one cap then 70uf makes more sense
     
  8. colnerov

    colnerov Member

    Messages:
    3,098
    Location:
    Nr Gatwick UK
    Hi, Now that the OP is sorted, can someone confirm my findings. I have an Ingersol Rand compressor bought new in '85, a few years later one of the caps went bang but it continued to work OK. A couple of years ago it was relocated to my workshop and runs off a heavy extension lead. Periodically it blows the 13A fuse in the plug so I thought it might be an idea to replace the caps. The caps are covered in paint and the bits that aren't have the markings worn away by the clamp. However on the data plate on the motor it says -

    2.2kw
    2820 rpm
    240V
    11.5A
    Cap 180 - 225/40

    Does that sound right? Is the value of the second cap a range of 225 to 240 or is the 40 referring to something else?

    Thanks in advance, hopefully.

    Colin
     
  9. carbon

    carbon Member

    Messages:
    1,105
    Location:
    S. Glos. U.K
    From that I’d take it that the start cap is approx 200uf. (180-225) and the run cap is 40uf
     
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  10. colnerov

    colnerov Member

    Messages:
    3,098
    Location:
    Nr Gatwick UK
    Hi, Ahh, the other way round from my thinking. That makes more sense. Thanks. I will look for them then.

    Colin
     
  11. graffian

    graffian Seer unto the end of his beard

    Messages:
    2,659
    Location:
    gatwick
    I have not experimented with single phase start caps, I just put as close to original as I could.

    But when making a phase converter I had to start from scratch with values. I decided on 200uf for start caps
    when I was buying caps. I found with 200uf of start caps the motor started, accelerated up to full speed, in less
    than a few seconds and the motor made a sort of growl.
    I had two 100uf caps in parallel, so I tried 100uf. With 100uf the motor started sweetly but took about 5 seconds or
    more to accelerate up to speed.

    So I would guess the value isn't at all critical.
     
  12. Screwdriver

    Screwdriver Member

    Messages:
    4,869
    UK London
    I think the precise value for a start cap isn't terribly important so long as it's in the right sort of ballpark. The capacitance value does a couple of things. It shifts the phase of the incoming current a bit like you'd want a starting handle or a crank pedal to be in the "perfect" position for getting going. It also provides a reservoir of current which can be dumped into the motor windings, bit like a battery.

    The phasing is important, you can get going with the pedal starting at the top but it takes a fair bit more effort. Once you get going it doesn't matter, similarly, the start cap is disconnected once the motor is up to speed.

    If the capacitance value is too far off, it can almost stall (like trying to press down on a pedal at the top of its stroke) and the additional load can blow the thing apart. Quite literally causing the capacitor to explode. They are physically large so they can survive higher voltages and the heat generated by releasing all the stored energy they contain. If the value is too far from the recommended design parameter, it's more likely to affect reliability and longevity than function. The motor would probably start anyway if you connected it to a wet sock wrapped around a lemon... :dontknow:
     
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  13. Hopefuldave Intergalactic pot-mender

    Messages:
    854
    Location:
    The Shed of Danger, surrey, England
    similar, my phase converter has 300 uf of start capacitors, starts up almost as quickly as I can get my finger off the start button and shows a brief but huge startup current - it's at 415v with a 5.5 (or switching in the second motor, 8.5) KW motor. It starts OK with 200 uf but a little slower, 100uf seems a bit too slow and grumbly and actually shows a higher huge startup current! A bit like a duff/low-value start cap on single-phase.

    What seems to be more critical is the values for balance capacitors, a bit like the run caps on a single-phase motor they're establishing the voltage and phase of the third leg [run winding on 1-P), still haven't got them exactly right (generated phase is about 380 vs 415 for the others) and there's a difference in values and voltages depending on load...

    Dave H. (the other one)
     
  14. Seadog

    Seadog Forum Supporter

    Messages:
    4,194
    Location:
    NE London - UK
    I'd describe its action slightly differently, Steve.

    When a capacitor is connected, there is an instantaneous pulse as current flows into it, i.e. it's acting like a short circuit. This pulses the start winding, and the pulse has to be of sufficient magnitude to overcome the inertia of the rotor. It's only got 10ms to do this in the case of 50Hz. The 90° phase shift which leads the applied voltage dictates the direction of rotation. Hence, reversing the polarity of the start or, for that matter, the run windings determines the direction of rotation.
     
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  15. Screwdriver

    Screwdriver Member

    Messages:
    4,869
    UK London
    Well, I blame Benjamin Franklin for guessing the "wrong" charge on the electron. Current rushes in when electrons flood out. It's a disaster. We should have taken the hit and swapped it around so that electricity flows properly. :mad:
     
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  16. graffian

    graffian Seer unto the end of his beard

    Messages:
    2,659
    Location:
    gatwick

    That is low, is that with something running or while idling?
    I balanced mine to 426v, 426v, that was with the lathe taking a cut. Idling the voltages are 426v and something like 455v.
    Running my compressor makes the idler motor rock. I can't remember if I have ever measured the voltages, it must be dragging the
    voltage down and I do intended on adding caps to the compressor motor[5kw] but it's been OK for 15 years.
     
  17. Turbo Member

    Messages:
    2,994
    Location:
    Fermanagh, Northern Ireland
    In answer to the original post, it depends if it's just a start capacitor or a start/run capacitor.

    The size is not so critical on just a start capacitor, once it starts the motor it is disconnected from the circuit & has no further effect.

    With a start/run capacitor the size is critical as it is in the circuit all the time & the size is crucial to how well the motor runs. You may get away with a 5% increase in uf, any more than this the motor will run hotter & you will reduce the life of it.
     
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