Text: Inductors: Regardless of it's size or use, practically every type of electrical equipment uses inductors/or transformers. Many types of equipment have a transformer in the power supply, although this isn't the only place where transformers are used. Whenever one circuit supplies voltage and current to another, if this combination isn't correct for best operation, a transformer permits the voltage and the current values to be changed to the correct values for proper circuit operation. That is, when the voltage is too low, the transformer increases the voltage and reduces the current, or where a large current is needed, the transformer increases the current and reduces the voltage. For this action, the transformer makes use of several important factors. One of these is that a voltage can be induced into a wire or coil whenever the wire or coil either cuts or is cut by a magnetic field. In a transformer, however, the coil remains stationary while the magnetic field "moves" (expands and contracts). The second important factor used in transformers is that an alternating current produces a moving magnetic field. Self-Induction: Electron flow through a wire produces a magnetic field. Let's consider a direct current passing through a wire, as long as there is no complete circuit, there is no current flow and therefore no magnetic field around the wire, but when the circuit is complete, the current flow through the wire causes a magnetic field to build up around the wire. If there is another wire close to the first, a current flow will be caused in that wire as well. The current and voltage in the second wire is called induced voltage and current. The electron flow in the second wire will be of the opposite polarity of the first, because the magnetic lines of force induced in the second wire are flowing in the opposite direction. A similar opposite current is generated in the first wire. Since the voltages in each strand oppose each other, the effective voltage in each of them is the difference between the original supply voltage and the induced voltage. Now, by recom- bining the two as one wire, the voltage induced in the wire by the expanding field opposes the applied voltage so that the resulting voltage is lower. There is also less current. This induced voltage is called a Counter Electromotive Force (cemf). Inducing a voltage into a conductor is called Self-Induction and like all forms of induction, it occurs only while the magnetic field is changing. Although this action takes place very rapidly, this self- induction does prevent the current from rising instantaneously to its full value. Self-induction in a coil of wire is much greater than in a straight piece of wire of the same length, because the magnetic field around each turn cuts not only the turn that sets it up, but also the turns close to it. As this overlapping of magnetic lines occurs between all of the turns, the induced CEMF will be much greater than in a single pair of wires. The actual amount of self-induction in a coil depends on the ampere-turns and the reluctance of the magnetic circuit. That is, the stronger the magnetic field, the greater the self-induction and the greater the CEMF. The effects of self-induction are as follows: 1. Whenever a circuit current increases, the induced counter emf opposes the change of current and prevents it from rising instantly to it's steady value. 2. Whenever a circuit current decreases, the induced emf is in a direction to oppose the change and tends to maintain the current. Notice particularly that the induction occurs only while the current is changing, and the counter emf is always in a direction to oppose the change of current level. With a uniform or steady current, there is no induction, and therefore, no counter emf. Inductance: When there is a variation of current in a circuit, the magnetic flux also varies, expanding as the current increases and contracting as the current decreases. In moving, the magnetic lines of force cut any conductor which is within range and induce a voltage that is always in such a direction as to oppose the current change. The ability to produce a voltage by electromagnetic induction when the current changes is a property possessed by a circuit because of its physical arrangement and is known as Inductance. A conductor has inductance whether or not there is current in it. The inductance of a circuit depends on the number of magnetic lines that cut the conductor for each ampere change in current. Anything that increase the number of flux lines cutting the conductor for each ampere change in current increases the inductance. A conductor wound into a coil has a greater inductance than a straight wire. This is because the flux developed by one turn of wire in the coil also cuts practically every other turn as it expands or contracts. As a result of this additive action, the inductance of a coil is proportional to the number of turns squared. For the same reason, an iron core placed within the coil increases the inductance because the lower reluctance of the magnetic circuit permits a greater number of flux lines, and therefore, more lines cut the conductor as the current changes. Since a coil of wire possesses the property of inductance it is called an Inductor(L). Induction is the result of electromagnetic action in a circuit containing inductance, and as a general definition: Self-inductance is the ability of a circuit to produce a voltage within itself by induction when the current in it changes. Do not confuse self-induction with self-inductance. Self-induction is the ACT or process of inducing a voltage in the wire or coil, While self-inductance is the ABILITY to induce the voltage. The unit of measurement for inductance is the Henry(H), named for the American Physicist Joseph Henry (1797 - 1878), who in 1831, discovered the voltage caused by self-induction. The Definition: The Henry is the amount of inductance which induces a counter emf of one volt when the current is changing at the rate of one ampere per second.

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