Inductors in DC Circuits
By Stephen Bucaro
It was Michael Faraday, in 1831, who first noticed that direct current passing through a wire creates
a magnetic field around the wire. He also noted that moving a magnet near a wire induced an electric
current in the wire. Faraday explained electromagnetic induction using a concept he called lines of force.
Today electromagnetic induction is the principle that is used in all electric motors and in electric
transformers. Without electromagnetic induction we would be living in a primitive world indeed.
Magnetic field lines (B, green) of a wire loop carrying an electric current (I, red), illustrating how
the field lines all pass through the interior of the loop, creating a strong magnetic field there.
If the wire is formed into a circle, and an electric current is passed through the wire, the magnetic
lines of force would go through the center of the circle. The magnetic flux would be densest in the
middle of the circle. Conversely, if a magnet was pushed through the center of the circle, it would
induce an electric current in the circle of wire.
If the wire is formed into a coil, and a direct current is passed through the wire, the combined
lines of force will create a magnetic field with with the polarity shown above. Note that the magnetic
lines of force circle around the outside of the coil to complete a magnetic circuit.
When a certain high current is passed through the coil, the coil's magnetic field will become saturated.
Saturation is the state reached when an increase in applied current does not increase the magnetic flux
density of the coil. However the flux density and the strength of the magnetic field can be increased
by placing an iron core in the coil.
• Reluctance is measure of a material's opposition to magnetic flux.
• Permeability is a measure a material's support for the formation of a magnetic field within itself.
An iron core has at least 1,000 more permeability than air. Iron ferrite material can be up to 10,000
times more permeable than air. Therefore an iron or ferrite core inceases the indictance of a coil.
An undesireable side effect of high permeability is magnetic hysteresis. When a magnetic field is
created in ferromagnetic material, even when the field is removed, the material will stay partially
magnetized. It requires a magnetic field in the opposite direction to demagnetize it. This becomes a
desirable effect when used in a computer hard disk to retain memory.
Another undesireable side effect of using ferromagnetic material is that it is a conducting material,
and a changing magnetic field will induce currents within the material itself. These currents, called
eddy currents don't do any useful work, and they produces a loss in the magnetic material and
generate waste heat in the magnetic material.