INDUCTANCE

                                 

                                  INDUCTANCE

The study of inductance presents a very challenging but rewarding segment of electricity. It is challenging in
the sense that, at first, it will seem that new concepts are being introduced. The reader will realize as this unit
progresses that these "new concepts" are merely extensions and enlargements of fundamental principles that have been acquired previously in the study of magnetism. The study of inductance is rewarding in the sense that a thorough understanding of it will enable the reader to acquire a working knowledge of electrical circuits more rapidly and with more surety of purpose than would otherwise be possible.inductance is the characteristic of an electrical circuit that makes itself evident by opposing the starting,stopping, or changing of current flow. The above statement is of such importance to the study of inductance
that it bears repeating in a simplified form.
Inductance is the characteristic of an electrical conductor, which opposes a CHANGE in
current flow.
   One does not have to look far to find a physical analogy of inductance. Anyone who has ever had to push a heavy load (wheelbarrow, car, etc.) is aware that it takes more work to start the load moving than it does to keep it moving. This is because the load possesses the property of inertia. Inertia is the characteristic of
mass that opposes a CHANGE in velocity. Therefore, inertia can hinder us in some ways and help us in
others. Inductance exhibits the same effect on current in an electric circuit as inertia does on velocity of a
mechanical object. The effects of inductance are sometimes desirable and sometimes undesirable.
Michael Faraday started to experiment with electricity around 1805 while working as an apprentice
bookbinder. It was in 1831 that Faraday performed experiments on magnetically coupled coils. A voltage
was induced in one of the coils due to a magnetic field created by current flow in the other coil. From this
experiment came the induction coil, the theory of which eventually made possible many of our modern
conveniences such as the automobile, doorbell, auto radio, etc. In performing this experiment Faraday also
invented the first transformer, but since alternating current had not yet been discovered the transformer had
few practical applications. Two months later, based on these experiments, Faraday constructed the first direct
current generator. At the same time Faraday was doing his work in England, Joseph Henry was working
independently along the same lines in New York. The discovery of the property of self-induction of a coil was
actually made by Henry a little in advance of Faraday and it is in honor of Joseph Henry that the unit of
inductance is called the HENRY.

                               Unit of Inductance

The unit for measuring inductance (L) is the HENRY (h). An inductor has an inductance of 1 henry if an emf
of 1 volt is induced in the inductor when the current through the inductor is changing at the rate of 1 ampere
per second. The relation between the induced voltage, inductance, and the rate of change of current with
respect to time is stated mathematically as:
Vinduced = L I
tΔΔWhere Induced is the induced emf in volts, L is the inductance in henrys, and ΔI is the change in current in
amperes occurring in Δt seconds. (Delta, symbol Δ, means "a change in.......")
The henry is a large unit of inductance and is used with relatively large inductors. The unit employed with
small inductors is the millihenry, mh. For still smaller inductors the unit of inductance is the microhenry, μh.
Self-Inductance
Even a perfectly straight length of conductor has some inductance. As previously explained, current in a
conductor always produces a magnetic field surrounding, or linking with, the conductor. When the current
changes, the magnetic field changes, and an emf is induced in the conductor. This emf is called a SELFINDUCED
EMF because it is induced in the conductor carrying the current. The direction of the induced emf
has a definite relation to the direction in which the field that induces the emf varies. When the current in a
circuit is increasing, the flux linking with the circuit is increasing. This flux cuts across the conductor and
induces an emf in the conductor in such a direction as to oppose the increase in current and flux. This emf is
sometimes referred to as counter-electro-motive-force (Cemf) or back emf. Likewise, when the current is
decreasing, an emf is induced in the opposite direction and opposes the decrease in current. These effects
are summarized by Lenz's Law, which states that the induced emf in any circuit is always in a direction to
oppose the effect that produced it.