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phasors and power calcs

Discussion in 'Electronic Basics' started by [email protected], May 25, 2005.

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  1. Guest

    Hello,

    I am studying an introductory electronics course and have been given the
    following problem:

    A small factory has the following loads operating from the 240Vrms 50Hz
    supply
    a) Forty 50 watts incandescent lamps with unity power factor
    b) Thirty-five 40 watts fluorescent lamps with a lagging power factor of 0.9
    c) One 2400 watts air conditioning system with a lagging power factor of
    0.65.

    I am asked to draw the complete phasor diagram and determine the total
    peak current drawn and overall power factor of the factory.

    I'm a little stuck :( I have some phasor diagrams put together which I
    am a little unsure of, but I don't really know how to determine peak
    current from this, can anyone shed some light or point me in the
    direction of some material to explain this sort of problem? I've been
    digging through a number of text books and am not really progressing.

    Thank you

    Jan
     
  2. Guest

    Jan,

    I can give you a little help along your way, but since you've already
    formulated an answer then my explanation can help you validate your
    work.

    Expressing sinusoidal signals (such as AC RMS) with the same frequency
    (in this case 50Hz) as phasors is useful for consolidating by addition
    the signals into a signal formula. The key to phasors is that this can
    be done when the frequency is the same, but the amplitudes and phases
    differ. For example,

    N
    Sum of A(k) cos(W(0) * t + Phi(k)) = A cos(W * t + Phi)
    k=1

    Note that on the left hand of the equation, I have written in ASCII
    text as follows:

    "W(0)" is short hand for writing W subscript 0 and represents the
    radian measure of angle,
    "Phi(k)" is short for Phi subscript k,
    "t" is used as the classic symbol representing time.

    This equation states that the sum of N cosine signals of differing
    amplitudes and phase shifts, but with the same frequency, can always be
    reduced to a single cosine signal of the same frequency. [Excerpted
    from text "DSP First, A multimedia approach", by McClellan, et al. pp
    31]. We know that this is only one half of the fun. Euler provided us
    with complete fun by showing us that we could express cosine signals as
    the real part of e^(W*t + Phi) * j. His brilliance helps us in one way
    because it saves us from having to write annoying trigonometric
    identities all over the place, and reduces the math to addition in the
    exponents. Thus,

    e^(Omega * j) = cos(Omega) + j * sin (Omega)

    Real{ e^(Omega * j) } = cos(Omega)
    Imaginary{ e^(Omega * j) } = sin(Omega)

    Notes:

    Omega is the English literal translation of the Greek symbol O,
    j represents the imaginary symbol representing square root of -1.

    Hence,

    N
    Sum of A(k) cos(W(0) * t + Phi(k))
    k=1
    = Sum of A(k) Re{ e^(W(0) * t + Phi(k)) * j }
    = Sum of A(k) Re{ e^(W(0) * t) * j + e^(Phi(k)) * j }
    = Re { e^(W(0) * t) * j } * Sum of A(k) * Re { e^(Phi(k)) * j }

    So much for phasors. From your problem statement we have,

    W(0) = 50 Hz
    V = 240 Vrms

    We know that, "Peak Voltage: Peak voltage tells you how far the voltage
    swings, either positive or negative, from the point of reference... The
    RMS voltage of a pure‡ sine wave is approximately .707*peak voltage."
    [http://www.bcae1.com/voltages.htm]


    V = I*R [Ohms law]

    Thus,

    I = 240 Vrms/ 1 Ohm.
    = 240rms cos(2*pi*50*t + Phi)
    = 240/0.707 Peak Current * cos(2*pi*50*t + Phi)

    a) Forty 50 watts incandescent lamps with unity power factor

    Power = Watts = V * I

    40 * 50 W = 240/0.707 Peak Voltage * cos(2*pi*50*t + 0) * I,
    I = (40 * 50 W) / (240 / 0.707) Peak Current * cos(2*pi*50*t + 0)
    = (40 * 50 W) / (240 / 0.707) * Re { e^(2*pi*50*t)*j }

    b) Thirty-five 40 watts fluorescent lamps with a lagging power factor
    of 0.9

    35 * 40 W = 240/0.707 Peak Voltage * cos(2*pi*50*t + 2*pi*0.9) * I
    I = (35 * 40 W) / (240 / 0.707) Peak Current * cos(2*pi*50*t +
    2*pi*0.9)
    = (35 * 40 W) / (240 / 0.707) Peak Current * Re { e^(2*pi*50*t +
    2*pi*0.9)*j }
    = (35 * 40 W) / (240 / 0.707) Peak Current * Re { e^(2*pi*50*t)*j *
    e^(2*pi*0.9)*j }

    c) One 2400 watts air conditioning system with a lagging power factor
    of
    0.65.

    1 * 2400 W = 240/0.707 Peak Voltage * cos(2*pi*50*t + 2*pi*0.65) * I
    I = (1 * 2400 W) / (240 / 0.707) Peak Current * cos(2*pi*50*t +
    2*pi*0.65)
    = ...

    Etcetera, etcetera.... Now see if you can do the rest.
    You're Welcome,
    -Beagle
     
  3. Dan Hollands

    Dan Hollands Guest

    Draw each of the three phasors

    Break each down to a real and a reactive component

    Add the real components

    Add the reactive components

    Draw the phasor from the the sum of real and reactive components

    The length of the phasor gives the total current

    The angle of the phasor gives the power factor

    --

    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
    585-872-2606

    www.QuickScoreRace.com
     
  4. Terry Given

    Terry Given Guest

    guess the currents are all pretty sine waves, as is the voltage, and
    that the supply impedance is zero (no matter how much load you draw, the
    voltage never droops. not really true, but close enough :)

    the incandescent lamps:
    =======================
    50W/240V = 0.21Arms per lamp. there are 40 of them,
    so 8.33A = 0.21A*40

    power factor = 1 so there is no reactive current (IOW its a resistive load)


    The flouro's:
    =============
    40W/240V = 0.17Arms per flouro. 35 of them,
    so 5.83A = 35*0.17A

    power factor = 0.9 (lagging), so there *IS* reactive current (which is
    at 90 degrees to real current, so think right-angled triangles).

    pf = cos(phi), phi = angle between real and actual current.

    from SOH CAH TOA trigonometic identities, we know
    cos(phi) = Adjacent/Hypotenuse = 0.9.

    so Hypotenuse = Adjacent/cos(phi) = 5.83A/0.9 = 6.48A

    Pythagoras allows us to calculate the reactive current,
    as 6.48^2 = 5.83^2 + Ireact^2
    Ireact = sqrt(6.48^2 - 5.83^2) = 2.83A

    (you can check the cos(phi) comes out right)


    The A/C:
    =============
    2400W/240V = 10Arms

    power factor = 0.65 (lagging), so there *IS* reactive current.

    so Itotal = 10A/0.65 = 15.38A

    Pythagoras allows us to calculate the reactive current,
    Ireact = sqrt(15.38^2 - 10^2) = 11.69A

    (as you can see, the total current is a lot higher than the real
    current, so low PF loads kinda suck)


    The result:
    ===========

    to add vectors, add all the X and all the Y components separately.

    Real current:
    8.33A + 5.83A + 10A = 24.16Arms real

    reactive current:
    2.83A + 11.69A = 14.52Arms reactive (aka imaginary)


    total current I = sqrt(24.16A + 14.52A) = 28.19Arms

    power factor PF = 24.16A/28.19A = 0.86

    Now, just to be a bastard, the question asks for PEAK current. which is
    of course sqrt(2)*RMS current, so Ipeak = 39.87A


    HTH
    Cheers
    Terry
     
  5. Break each of the individual load current phasors into a real (in
    phase with voltage) and imaginary (90 degrees shifted with respect to
    voltage) components. Add all the real components together as the real
    component of the total load phasor, and add all the imaginary
    components together as the total imaginary component of the total load
    phasor. Then put these two components start to end and draw the
    diagonal sum that represents their resultant. Of course, if you have
    a compass and protractor, you could just add the individual load
    phasors one to the next and get the same resultant. Breaking them
    into components just makes it easer to do the sums, graphically
    without those tools The divisions on the graph paper is the only tool
    needed.
     
  6. Ed-

    Ed- Guest

    next question:
    how can i get the electric meter to turn backwards by using some caps?


    : Hello,
    :
    : I am studying an introductory electronics course and have been given
    the
    : following problem:
    :
    : A small factory has the following loads operating from the 240Vrms
    50Hz
    : supply
    : a) Forty 50 watts incandescent lamps with unity power factor
    : b) Thirty-five 40 watts fluorescent lamps with a lagging power factor
    of 0.9
    : c) One 2400 watts air conditioning system with a lagging power factor
    of
    : 0.65.
    :
    : I am asked to draw the complete phasor diagram and determine the total
    : peak current drawn and overall power factor of the factory.
    :
    : I'm a little stuck :( I have some phasor diagrams put together which I
    : am a little unsure of, but I don't really know how to determine peak
    : current from this, can anyone shed some light or point me in the
    : direction of some material to explain this sort of problem? I've been
    : digging through a number of text books and am not really progressing.
    :
    : Thank you
    :
    : Jan
     

  7. Dream on.
     
  8. Don Kelly

    Don Kelly Guest


    You can't.

    Capacitors will change the power factor but not the real power(*time) which
    the meter measures.
    Changing the pf will result in less current and some decrease in losses. If
    you have demand metering, then the demand may be reduced.
     
  9. David Sauer

    David Sauer Guest

    Drawing is the best method, even if it is only a rough sketch, you can
    easily check that the answers are about the right values when you have
    finished.

    And in an exam situation, even a correct drawing will normally get you
    part marks if you stuff up part of the equations.
     
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