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Inductor Opposition

Discussion in 'Electronic Basics' started by Randy, Jul 18, 2003.

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  1. Randy

    Randy Guest

    A question concerning the characteristic of the Inductor:

    an inductor opposes the current that created it and this opposition is
    toward the source, termed cemf. Can this cemf be great enough to actually
    stop the flow of current?

    For example: an ac source supplying 10 volts @ 1 amp. If the cemf from the
    inductor = 10 volts, would current flow, or would this stop the circuit
    dead until something changed in the circuit?

    I've learned a lot about inductors but, this is one component with
    applications that seem to be endless.

    An Inquiring Mind

    Randy Gross
     
  2. This is not the way to look at it. Neglecting the inductors resistance,
    the voltage across the inductor is *always* the input voltage.

    The formula is V=L.di/dt. That is, the current will be i=V.t/L. for a
    constant applied voltage. The current will therefore build up over time
    in ramp fashion.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  3. If the current were held at a steady zero, there would be no rate of
    change of current, so no cemf, so an inductor cannot hold current
    steady at zero in response to either a DC or AC voltage, except in the
    limiting case of an infinite inductance. For Ac there is the
    interesting case that a perfect inductor (no resistance) will produce
    a cemf equal to the applied voltage (there are only two nodes in the
    circuit, so the applied voltage must be equal to the cemf of the
    inductor) but there is still a current. The strange thing is (if you
    are used to thinking about resistors) is that the current peaks occur
    at the moments when there is zero voltage across the inductor. Of
    course, this same thing happens with a capacitor across an AC source,
    except that the current through the capacitor is 180 degrees out of
    phase with the current through the inductor.
     
  4. Randy

    Randy Guest

    <>...

    : If the current were held at a steady zero, there would be no rate of
    : change of current, so no cemf, so an inductor cannot hold current
    : steady at zero in response to either a DC or AC voltage, except in the
    : limiting case of an infinite inductance. For Ac there is the
    : interesting case that a perfect inductor (no resistance) will produce
    : a cemf equal to the applied voltage (there are only two nodes in the
    : circuit, so the applied voltage must be equal to the cemf of the
    : inductor) but there is still a current. The strange thing is (if you
    : are used to thinking about resistors) is that the current peaks occur
    : at the moments when there is zero voltage across the inductor.

    I think I will save this one until I re-align my brain cells:)

    Of
    : course, this same thing happens with a capacitor across an AC source,
    : except that the current through the capacitor is 180 degrees out of
    : phase with the current through the inductor.
    :
    : --
    : John Popelish

    I found a circuit that I think explains what you are saying. It shows the
    effect of cemf on current flow.
    It states that AC is opposed by a force much greater than its simple dc
    resistance and gives this example:

    A power transformer, with a primary resistance of 6 ohms is plugged into a
    120vac 60hz outlet. The calculated AC should be 20 amps but, the actual
    measurement is 1 amp (this is one serious reduction in current flow). If
    the same transformer had 120vdc applied to the primary, well, let the
    fireworks begin!

    From this I see that the inductor is like a gate valve with a strong spring
    to oppose opening it. The secondary can demand a share of the available
    power but, with a grudge from the primary.

    Thanks, once again
    Randy
     
  5. Fred Abse

    Fred Abse Guest

    Not in an inductor, it doesn't, it LAGS.

    Current leads voltage in a capacitor.
     
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