Basic AC Reactive Components INDUCTANCEAccording to Lenz’s Law (refer to Module 1, Basic Electrical Theory), the induced voltagealways opposes the change in current. Referring to Figure 1, with the current at its maximumnegative value (point a), the induced EMF is at a zero value and falling. Thus, when the currentrises in a positive direction (point a to point c), the induced EMF is of opposite polarity to theapplied voltage and opposes the rise in current. Notice that as the current passes through its zerovalue (point b) the induced voltage reaches its maximum negative value. With the current nowat its maximum positive value (point c), the induced EMF is at a zero value and rising. As thecurrent is falling toward its zero value at 180° (point c to point d), the induced EMF is of thesame polarity as the current and tends to keep the current from falling. When the current reachesa zero value, the induced EMF is at its maximum positive value. Later, when the current isincreasing from zero to its maximum negative value at 360° (point d to point e), the inducedvoltage is of the opposite polarity as the current and tends to keep the current from increasingin the negative direction. Thus, the induced EMF can be seen to lag the current by 90°.The value of the self-induced EMF varies as a sine wave and lags the current by 90°, as shownin Figure 1. The applied voltage must be equal and opposite to the self-induced EMF at alltimes; therefore, the current lags the applied voltage by 90° in a purely inductive circuit.If the applied voltage (E) is represented by a vector rotating in a counterclockwise direction(Figure 1b), then the current can be expressed as a vector that is lagging the applied voltage by90°. Diagrams of this type are referred to as phasor diagrams.Example: A 0.4 H coil with negligible resistance is connected to a 115V, 60 Hz powersource (see Figure 2). Find the inductive reactance of the coil and the currentthrough the circuit. Draw a phasor diagram showing the phase relationshipbetween current and applied voltage.Figure 2 Coil Circuit and Phasor DiagramRev. 0 Page 3 ES-08

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