content_image/573219.jpg content_image/573218.jpg content_image/573226.jpg content_image/573224.jpg content_image/573225.jpg content_image/573220.jpg content_image/573221.jpg content_image/573227.jpg
4. Core material around which the coil is formed— coils are wound on either magnetic or nonmagnetic materials. Some nonmagnetic materials include air, copper, plastic, and glass. Magnetic materials include nickel, iron, steel, and cobalt, which have a permeability that provides a better path for the magnetic lines of force and permit a stronger magnetic field. 

Since AC is in a constant state of change, the magnetic fields within an inductor are also continuously changing and create an inducted voltage/current. This induced voltage opposes the applied voltage and is known as the counter EMF. This opposition is called inductive reactance, symbolized by XL, and is measured in ohms. This characteristic of the inductor may also create a phase shift between voltage and current of the circuit. The phase shift created by inductive reactance always causes voltage to lead current. That is, the voltage of an inductive circuit reaches its peak values before the current reaches peak values. Additional discussions related to phase shift are presented later in this chapter. 

Inductance is the property of a circuit to oppose any change in current and is measured in henries. Inductive reactance is a measure of how much the countering EMF in the circuit opposes the applied current. The inductive reactance of a component is directly proportional to the inductance of the component and the applied frequency to the circuit. By increasing either the inductance or applied frequency, the inductive reactance likewise increases and presents more opposition to current in the circuit. This relationship is given as XL = 2πfL Where XL = inductive reactance in ohms, L = inductance in henries, f = frequency in cycles per second, and π = 3.1416 

In Figure 9-18 , an AC series circuit is shown in which the inductance is 0.146 henry and the voltage is 110 volts at a frequency of 60 cycles per second. Inductive reactance is determined by the following method. 

<img src='content_image/573219.jpg'>

<img src='content_image/573218.jpg'>

X$_{L}$  = 55Ω 

In AC series circuits, inductive reactance is added like resistances in series in a DC circuit. [Figure 9-19]  The total reactance in the illustrated circuit equals the sum of the individual reactances. 

<img src='content_image/573226.jpg'>

<img src='content_image/573224.jpg'>

<img src='content_image/573225.jpg'>

X$_{LT}$  = 25Ω 

The total reactance of inductors connected in parallel is found the same way as the total resistance in a parallel circuit. [Figure 9-20]  Thus, the total reactance of inductances connected in parallel, as shown, is expressed as: 

<img src='content_image/573220.jpg'>

<img src='content_image/573221.jpg'>

X$_{LT}$ = 5Ω 

<img src='content_image/573227.jpg'>

