The
Properties that make a Conductor an Inductor
are found in the following:
If you take a Magnet and move it relative to
a Conductor, a current is made to flow in that conductor. The magnitude
of that Current is dependent on many things, including the Magnetic Flux
density cutting the wire; the relative Speed of the Magnet; the number
of turns the conductor makes, etc. It can be said that the Moving
Flux lines "cutting" the conductor's path induces an Electro Motive Force,
an "E.M.F." --------
Conversely, if an electrical Current is passed
through a Conductor, a Magnetic Field (Flux Lines) forms around the Conductor.
With a sufficient number of turns of wire one has an Electro Magnet.
If one were to place two Conductors side by side; one passing a steady
Electric Current; the other wire is unaffected. Although there are Magnetic
Flux lines "cutting" the second Conductor, No E.M.F. is generated since
there is No Relative Motion.
However, if the Electric Current is made to vary in Magnitude and/or
Polarity, then there would be an Induction
of an E.M.F into the second Conductor, i.e., this variation/change in Magnetic
Flux has the same Effect as Relative Motion between the Magnetic source
and the Conductor.
Thus far, we have covered the properties sufficient
to create a Generator, a Motor and a Transformer.
The Generator and the Motor work by virtue of Polarity Reversal of strong
Magnetic Fields; and will not be covered here.
Step Function
Counter
E.M.F. Retards Current Flow
When
a Voltage (step function) is applied to an Inductor, a current is made
to flow through its conductors creating an expanding Magnetic Field (Flux).
This expanding Field induces a current (Counter
E.M.F.) possessing an opposite polarity from that of the applied voltage.
The amount of this counter E.M.F. is directly related to the derivative
of the applied current, i.e., the faster the rise/fall the greater the
current. The effect of this is to impede the rate of expansion of
the Magnetic field.
Next, the Magnetic
field stops expanding and becomes steady-state. At this point the Inductor
is now only a Resistive load to the applied Voltage Source.
Finally, the
applied voltage is removed--made to equal Zero (Vapplied
= 0 volts); the steady-state Magnetic Field now starts to collapse, which
induces an E.M.F into the inductor thus causing a Current to flow (of the
same polarity as that of the original applied voltage source). As in the
applied case there is a Counter E.M.F. generated by the Current
generated from the collapsing Magnetic Field, again impeding the collapse
of said Magnetic Field.
The
transfer of energy to the magnetic field of an inductor represents work
performed by the source of the voltage. Power is required for doing the
work, and since power is equal to Current multiplied by Voltage, there
must be a voltage drop in the circuit while energy is being stored in the
field. This voltage drop, exclusive of any voltage caused by the resistance
(IR drop) in the circuit, is the result of an opposing voltage induced
in the circuit while the field is building up to its final value.
Once the field becomes constant, the induced voltage or back-voltage
disappears, because no further energy is being stored. The induced voltage
opposes the voltage of the source and tends to prevent the current from
rising rapidly when the circuit is closed. The amplitude of the induced
voltage is proportional to the rate at which the current changes (and consequently
the rate at which the magnetic field changes) and to a constant associated
with the circuit itself: the inductance (or self-inductance) of the circuit.
The effect that an Inductance has on Impeding current
flow (due to the derivative of the applied current & the Inductance--measured
in
Henrys) is analogous to the Effect of Resistance on impeding
current flow in a D.C. Circuit. However, this is a Reactive Impedance (Z);
in this case Inductive Reactance (XL) measured in Ohms (Z =
XL).
From the above, one can imagine if the Rate at which the applied
voltage were fast enough, even a short piece of wire could exhibit relatively
high impedances. --And they Do!
