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imaginary power

Discussion in 'Electronic Basics' started by Irish LaidE, Apr 25, 2004.

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  1. Irish LaidE

    Irish LaidE Guest

    i've just started to read about imaginary or reactive power my text
    book doesn't seem to cover to much information on this subject.

    any additional information on this subject would be greatly
  2. Bob

    Bob Guest


    There probably is a book, somewhere, with an explanation that will make it
    clear to you. I've never read one that really explained it well, to me.

    Capacitors and inductors (perfect ones, that is) do not dissipate power as
    heat (or light), although they will dissipate some power due to radiation
    (as a function of the driving source frequency or as a function of their
    self oscillations). Ignoring the radiations effects, capacitors and
    inductors are energy storage / energy release devices.

    When a sinusoidal signal is applied to a capacitor or an inductor, the
    resulting current through the device is 90 degrees out-of-phase with respect
    voltage across the device. The voltage "lags" the current in a capacitor (it
    takes time for the charge to separate across its plates (I * t), so the
    resulting voltage (Q/C) builds up as time goes on (it lags). The opposite is
    true with an inductor. It takes time for the current to change in an

    Here are two things to remember:

    1) you can never instantaneously change the voltage across a (perfect)
    capacitor, and you can never instantaneously change the current through a
    (perfect) inductor.

    2) a (perfect) capacitor or inductor will never get hot. In the real world,
    however, this is not the case -- due to parasitic impurities.

    The "so called" real component of power is that component that dissipates
    power. The reactive/imaginary component is that which is being transferred
    back-and-forth between inductors and/or capacitors in the circuit and its
    driving source.

    If you apply a sine wave to a circuit of R's and C's and L's, and look at
    the resulting source's current's phase (with respect to the driving voltage,
    you can always separate the current into two components: the "in phase" part
    of the current and a part which is 90 degrees out of phase (either leading
    or lagging). The heat generated by the circuit will only be due to the
    applied V and its "in phase" component of current. This is the "real"
    component of power. The balance is the "imaginary" component of power (the
    portion that is transferred back-and-forth to/from the source.

    This is probably a pretty good start, for you. Ask more questions after
    you've thought about this.

  3. Fred Stevens

    Fred Stevens Guest

    Reactive or imaginary power is that component of power that is
    transferred between energy storage elements (such as capacitors and
    inductors) with no dissipation. That is, the dissipative component is
    associated with power loss in resistance, whereas the pure reactive
    power component is not dissipated, but moves between (multiple)
    storage elements or is stored in fields in purely reactive devices
    (such as capacitors and inductors).

  4. Real power flows one way, from source to load. Resistors consume
    energy in proportion to the applied voltage squared, so they reach
    their peak energy consumption at the peaks of the voltage waveform,
    but never give any back to the source.

    Imaginary power sloshes back and forth between source and load. It
    makes both trips twice every AC cycle

    Capacitors store peak energy when they reach peak voltage, so a
    capacitor receives energy as the voltage rises from zero to the peaks
    and starts sending energy back at the voltage peaks of the cycle till
    the next zero crossing.

    Inductors across an AC source store peak energy when their current is
    highest. This occurs after the voltage has had the same polarity the
    longest, which is at the voltage zero crossings. The energy flows
    into the inductor from the peaks to the zero crossings and returns ot
    the source from the zero crossings ot the next peak

    If you compare the phase of the current through a capacitor to the
    phase of the applied voltage, you will fond that it leads the voltage
    by 90 degrees. The current through an inductor lags the applied
    voltage by 90 degrees.
  5. Irish LaidE

    Irish LaidE Guest

    ok, so imaginary power all relates back to V leading or lagging I by 90º ?
  6. Ratch

    Ratch Guest

    No, the voltage of a inductor is E = -L*di/dt . The derivative of a
    sinusoidal current is always 90° out of phase with the current in the
    inductance. Since the derivative of the current controls the voltage, that
    explains the 90° phase change between voltage current. It does not relate
    to 'imaginary power'.

    So what is power? Power is the time rate change of energy. The inductor,
    capacitor and the generator are constantly swapping energy back and forth
    according to the sinusoidal input. This happens in a finite time, so power
    is involved. This energy is conserved within the system except for that
    which is dissipated by resistance or radiation losses. The electrical
    community has given this a secondary name of 'imaginary power'. Bad choice.
    The energy stored in a inductor (electromagnetic field) or a capacitor
    (electrostatic field) is just as 'real' as the energy that heats a resistor.
    The rate which energy stores and depletes in a reactance should only be
    called reactive power. Ratch
  7. Imaginary anything (in mathematics) just refers to a 90 degree
    relationship. This right angle just means that two things are
    independent, like height and width. It is a silly word (imaginary) to
    label a second dimension.
  8. Irish LaidE

    Irish LaidE Guest

    thank you all for you help :)
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