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inductive reactance

Discussion in 'Electronic Basics' started by conrad, May 3, 2007.

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  1. conrad

    conrad Guest

    With inductive reactance, the phase shift that
    is induced where voltage leads current by
    90 degrees or pi/2 radians, is the
    voltage of the self-induced emf vs'
    the current from our applied voltage?
  2. Phil Allison

    Phil Allison Guest


    ** You have mixed up two separate situations.

    1. Where an AC voltage is APPLIED to an inductor causing current to flow
    with phase lag.

    2. Where an AC current is made to flow in an inductor from a source that
    has a series impedance of its own.

    In case 1, the reference AC voltage & phase is simply that of the source.

    In case 2, the reference AC voltage & phase is that appearing across the

    ........ Phil
  3. conrad

    conrad Guest

    And when they say voltage leads
    current by 90 degrees, is this the
    voltage from case 2 and the current
    in case 1? or no?
  4. Phil Allison

    Phil Allison Guest

    "Phil Allison"

    ** In either case, a 90 degree shift exists between the AC voltage *across*
    and the current flowing * through* an ( ideal) inductor or capacitor.

    The confusion exists is only in your mind.

    ........ Phil
  5. It may be easier to think of just the current lagging the voltage for
    inductance, as this is what happens when you apply a voltage to an inductor
    (which is usually the case). For a capacitor, it may be easier to
    conceptualize voltage lagging the current. Leading and lagging are just
    different ways of saying the same thing, but I find it easier to picture a
    second quantity lagging (happening later in time than) the first quantity,
    rather than the concept of leading, which implies the second thing is
    happening before the first thing. I hope that helps, although in rereading
    it might be even more confusing.

  6. Rich Grise

    Rich Grise Guest

    An inductance opposes a change in current, and a capacitor opposes a
    change in voltage.

    When you first apply a voltage to an inductor, the current begins to
    rise, as the voltage decreases due to series resistance in the supply,
    or by design in your circuit. (the series resistance drops more voltage,
    leaving less for the inductor.)

    When you feed that circuit with AC, the result turns out to be a 90 degree
    phase lag for the current. i.e., when you change the applied voltage, the
    current doesn't change right away, like it does with a resistor.

    A capacitor is just the opposite - try to apply a voltage to it, and for
    a very short time it looks like a short circuit - the current first goes
    as high as it can, and the voltage can follow it as it looks less like
    a short circuit - ergo, the voltage lags the current in/on a cap.

    Hope This Helps!
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