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To increase the property of inductance, the conductor can be formed into a loop or
coil. A coil is also called an inductor. Figure 23 shows a conductor formed into a coil.
Current through one loop produces a magnetic field that encircles the loop in the
direction as shown in figure 23(A). As current increases, the magnetic field expands and
cuts all the loops as shown in figure 23(B). The current in each loop affects all other
loops. The field cutting the other loop has the effect of increasing the opposition to a
current change.
Figure 23.  Inductance. Inductors are classified according to core type. The core is the center of the inductor just as the core of an apple is the center of an apple. The inductor is made by forming a coil of wire around a core. The core material is normally one of two basic types: softiron or air. An ironcore inductor and its schematic symbol (which is represented with lines across the top of it to indicate the presence of an iron core) are shown, in figure 24(A). The aircore inductor may be nothing more than a coil of wire, but it is usually a coil formed around a hollow form of some nonmagnetic material such as cardboard. This material serves no purpose other than to hold the shape of the coil. An aircore inductor and its schematic symbol are shown in figure 24(B). Figure 24.  Inductor types and schematic symbols. Factors Affecting Coil Inductance There are several physical factors which affect the inductance of a coil. They include the number of turns in the coil, the diameter of the coil, the coil length, the type of material used in the core, and the number of layers of winding in the coils. Inductance depends entirely upon the physical construction of the circuit, and can only be measured with special laboratory instruments. Of the factors mentioned, consider first how the number of turns affects the inductance of a coil. Figure 25 shows two coils. Coil (A) has two turns and coil (B) has four turns. In coil (A), the flux field set up by one loop cuts one other loop. In coil (B), the flux field set up by one loop cuts three other loops. Doubling the number of turns in the coil will produce a field twice as strong, if the same current is used. A field twice as strong, cutting twice the number of turns, will induce four times the voltage. Therefore, it can be said that the inductance varies as the square of the number of turns. Figure 25.  Inductance factor (turns). The second factor is the coil diameter. In figure 26you can see that the coil in view B has twice the diameter of coil view A. Physically, it requires more wire to construct a coil of large diameter than one of small diameter with an equal number of turns. Therefore, more lines of force exist to induce a counter emf in the coil with the larger diameter. Actually, the inductance of a coil increases directly as the crosssectional area of the core increases. Recall the formula for the area of a circle: A = pr^{2}. Doubling the radius of a coil increases the inductance by a factor of four. Figure 26.  Inductance factor (diameter). The third factor that affects the inductance of a coil is the length of the coil. Figure 27 shows two examples of coil spacings. Coil (A) has three turns, rather widely spaced, making a relatively long coil. A coil of this type has few flux linkages, due to the greater distance between each turn. Therefore, coil (A) has a relatively low inductance. Coil (B) has closely spaced turns, making a relatively short coil. This close spacing increases the flux linkage, increasing the inductance of the coil. Doubling the length of a coil while keeping the same number of turns halves the value of inductance. Figure 2  7.  Inductance factor (coil length). CLOSELY WOUND The fourth physical factor is the type of core material used with the coil. Figure 28 shows two coils: Coil (A) with an air core, and coil (B) with a softiron core. The magnetic core of coil (B) is a better path for magnetic lines of force than is the nonmagnetic core of coil (A). The softiron magnetic core's high permeability has less reluctance to the magnetic flux, resulting in more magnetic lines of force. This increase in the magnetic lines of force increases the number of lines of force cutting each loop of the coil, thus increasing the inductance of the coil. It should now be apparent that the inductance of a coil increases directly as the permeability of the core material increases. Figure 28.  Inductance factor (core material). SOFTIRON CORE Another way of increasing the inductance is to wind the coil in layers. Figure 29 shows three cores with different amounts of layering. The coil in figure 29(A) is a poor inductor compared to the others in the figure because its turns are widely spaced and there is no layering. The flux movement, indicated by the dashed arrows, does not link effectively because there is only one layer of turns. A more inductive coil is shown in figure 29(B). The turns are closely spaced and the wire has been wound in two layers. The two layers link each other with a greater number of flux loops during all flux movements. Note that nearly all the turns, such as X, are next to four other turns (shaded). This causes the flux linkage to be increased. Figure 29.  Coils of various inductances. A coil can be made still more inductive by winding it in three layers, as shown in figure 29(C). The increased number of layers (crosssectional area) improves flux linkage even more. Note that some turns, such as Y, lie directly next to six other turns (shaded). In actual practice, layering can continue on through many more layers. The important fact to remember, however, is that the inductance of the coil increases with each layer added. As you have seen, several factors can affect the inductance of a coil, and all of these factors are variable. Many differently constructed coils can have the same inductance. The important information to remember, however, is that inductance is dependent upon the degree of linkage between the wire conductor(s) and the electromagnetic field. In a straight length of conductor, there is very little flux linkage between one part of the conductor and another. Therefore, its inductance is extremely small. It was shown that conductors become much more inductive when they are wound into coils. This is true because there is maximum flux linkage between the conductor turns, which lie side by side in the coil. Q.7 List five factors that affect the inductance of a coil. 