pure capasitive load phase shift not equal 90 degrees...

Discussion in 'The Projects Forum' started by fancy102, Mar 7, 2008.

  1. fancy102

    Thread Starter Member

    Dec 21, 2007
    26
    0
    hi,

    i am currently carrying out power factor measurement system. If my load is pure resistive, i could get unity power factor.
    However, when my load is pure capasitive, i cant get phase shift of 90 degrees. My phase shift will change to different
    value depends on my capasitive value. For 1micro Farad, i get 80 degree,power factor =0.19. For 6 Micro FArad, i get 33 degree, power factor 0.83

    Is it the capasitive value will change my power factor? But the theory stated that if pure capasitive or pure inductive load, there will be phase shift of 90 degrees.Is it practical and theory have difference?I am confused about this.

    Thanks
     
  2. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
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    Are you sure your capacitor is a pure capacitive load?
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Pure capacitance is hard to come by. It is possible to simulate mathematically a pure capacitor but real world capacitors have leakage currents and lead resistances that prevent achievement of pure capacitance.

    hgmjr
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    this is true only for ideal capacitors
    real capacitors are different. you can represent a real capacitor with an ideal one but with one resistor in series with it and one resistor in parallel with it.
     
  5. fancy102

    Thread Starter Member

    Dec 21, 2007
    26
    0
    thanks for the reply. Yes..my load is pure capasitive load.

    I am not undertand why need to add one resistor in series and also in parallel.What is the purpose of this?
     
  6. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    Let me re-state the point... your capacitor is not a pure capacitive load. There are no pure capacitive capacitors. There are no ideal capacitors.

    As had been posted above, all capacitors have series resistances, inductances and leakage currents. These affect the observed power factors.
     
  7. fancy102

    Thread Starter Member

    Dec 21, 2007
    26
    0
    u mean all capasitors has it own series resistance and leakage current right??

    so tat means it is impossible for me to get phase of 90 degress for the pure capasitive and pure inductive load in practical?
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Yes indeed. Just as there are not "ideal" conductors and no "ideal" capacitors, so too is there no "ideal" 90 degree phase shift. The real world runs plus or minus one-tenth percent on a good day. Mine corner of it often goes plus or minus twenty percent.
     
  9. fancy102

    Thread Starter Member

    Dec 21, 2007
    26
    0
    so u mean plus or minus 20%??

    but my result is different based in different capasitive load as below:
    1 micro Farad - 80 degrees
    2 micro Farad - 64 degrees
    3 micro Farad - 54 degrees
    4 micro Farad - 46 degrees
    5 micro Farad - 40 degrees
    6micro Farad - 33 degrees

    when i put pure resistive load, it give unity power factor.However when check with capasitive load, it give the result as shown above.

    so, my problem is how to check the accuracy of the system?thanks...
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What kind of capacitors are you using, and how long are the connecting wires to them?
    The type of caps you use will have a large impact on how they function in your system.

    Try paper & oil, polycarbonate, polypropylene or polystyrene capacitors like Sprague "orange drops" with short leads & connecting wires.

    Electrolytic capacitors will NOT work very well at all for your experiment. Neither will mica, tantalum, nor ceramic.
    You'll find this page interesting:
    http://members.aol.com/sbench102/caps.html

    The larger the value of the capacitor you use, the greater the parasitics (inductance, resistance and leakage) will come into play, no matter the construction. But if you start off with a cap that has a non-linear response curve, you won't even get close to the 90° phase shift you're looking for.

    If possible, try reducing the frequency you're using, as the parasitic effects increase dramatically as you go higher in frequency.
     
  11. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,789
    Sniff....wahhhh! You mean all those authors, professors, and parents lied to us about the existance of ideal components, Santa Claus, and the Tooth Fairy?

    I'm devastated....and wondering why I am standing here beside myself!
     
  12. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    My goodness, you'll be believing political promises before long, too.
     
  13. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Good heavens no! The Tooth Fairy is real. They did lie about "frictionless surfaces" and "adiabatic containers," though.

    I was personally devastated by the absence of "ideal diodes." It completely shot down my free energy design.
     
  14. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,789
    I've seen no scratch from that miserable cheapskate since I was about 8 1/2. I've lost a few molars to lack of flossing. Maybe he's retired.
     
  15. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    May be the small resistance of wire is coming into play here...and since R is not zero the more the value of Xc the more the shift tends to be 90 deg.
     
  16. fancy102

    Thread Starter Member

    Dec 21, 2007
    26
    0
    my capasitor actually is used as my load. i use ceramic capasitor...so u mean if i reduce the frequensy then my inductance and resistance inside the capasitor itself will reduce?then i will get more accurate result??

    so..basically the power factor of the load will change depends on the capasitance value right??i just wana confirm my circuit is work or not...thanks
     
  17. recca02

    Senior Member

    Apr 2, 2007
    1,211
    0
    Well I'm not sure if resistance depends on frequency..And i also do not know if u'll ever find anything with zero resistance...so unless you are running a virtual simulation, you'll never get 90 deg phase shift.

    Also Power factor is given by..Resistance/Impedance ...
     
  18. fancy102

    Thread Starter Member

    Dec 21, 2007
    26
    0
    i do test with multi sim simulation..and i get different phase shift for different capasitance value...
     
  19. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    I think by default the software uses some value for resistance. Check if that is the case.
     
  20. fancy102

    Thread Starter Member

    Dec 21, 2007
    26
    0
    the load is pure capasitance and there is no any resistence there. and i think there are no default resistance value...so should it be correct the phase is change by the capasitance value?thanks
     
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