total impedance

Discussion in 'Homework Help' started by djstar, Oct 31, 2010.

  1. djstar

    Thread Starter Active Member

    Jan 26, 2008
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    hi ive been given a question which im quite stuck on. weve only just done complex numbers so im trying to get my head around them . i think ive worked out the total impedance correctly but im unsure how to work out the phase angle or the current. any help would be greatfull. If theres any easier way to work out the impedance then i'd appreciate some help.

    liam
     
    Last edited: Nov 1, 2010
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Reup ur file as a pdf. not as doc.
    I am having trouble opening git
     
  3. djstar

    Thread Starter Active Member

    Jan 26, 2008
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    Hi Thanks for the reply, ive just saved the attachment as a pdf
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Your impedance calculation is correct - could be simplified a little more.

    Z=3.969+1.446i

    The current I=100/Z=100/(3.969+1.446i)

    Convert the result to polar form and the angle component is the phase angle of the current relative to the 100V source (with assumed phase angle 0°). The result will give a lagging (-ve) phase angle for the current.
     
  5. djstar

    Thread Starter Active Member

    Jan 26, 2008
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    hi thanks for the great help. ive worked the current out as (22.24-8.105J) and worked that out as 20.6A and the phase agnle i used (-8.105/22.24) tan-1 = a phase angle of -20.1 degrees. Does that sound about right?
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The impedance

    Z=3.969+1.446i=4.22<20° [that's meant to be polar form]

    So what's the result of ...

    I=100/(4.22<20°)= ....

    It wont be 20.6A. The angle is about right. But your rectangular form for the current is incorrect. Looks like you may have slipped the decimal on the imaginary part.

    I suggested using the polar form for the impedance - did you take that on board? You'll possibly note that, in this situation, the polar form gives a better appreciation of phase relationships. For instance the 20° term in the denominator immediately flags a phase lag of 20° between the voltage and current.
     
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