Basic AC Reactive Components
According to Lenzs Law (refer to Module 1, Basic Electrical Theory), the induced voltage
always opposes the change in current. Referring to Figure 1, with the current at its maximum
negative value (point a), the induced EMF is at a zero value and falling. Thus, when the current
rises in a positive direction (point a to point c), the induced EMF is of opposite polarity to the
applied voltage and opposes the rise in current. Notice that as the current passes through its zero
value (point b) the induced voltage reaches its maximum negative value. With the current now
at its maximum positive value (point c), the induced EMF is at a zero value and rising. As the
current is falling toward its zero value at 180° (point c to point d), the induced EMF is of the
same polarity as the current and tends to keep the current from falling. When the current reaches
a zero value, the induced EMF is at its maximum positive value. Later, when the current is
increasing from zero to its maximum negative value at 360° (point d to point e), the induced
voltage is of the opposite polarity as the current and tends to keep the current from increasing
in 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 shown
in Figure 1. The applied voltage must be equal and opposite to the self-induced EMF at all
times; 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 by
90°. Diagrams of this type are referred to as phasor diagrams.
A 0.4 H coil with negligible resistance is connected to a 115V, 60 Hz power
source (see Figure 2). Find the inductive reactance of the coil and the current
through the circuit. Draw a phasor diagram showing the phase relationship
between current and applied voltage.
Figure 2 Coil Circuit and Phasor Diagram