# Inductor Opposition

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

1. ### RandyGuest

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. ### Kevin AylwardGuest

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. ### John PopelishGuest

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. ### RandyGuest

<>...

: 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 AbseGuest

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

Current leads voltage in a capacitor.