9.5 Relationships in Inductive Circuits

This is the fifth week of Module 9, "Relationships of Current, Counter EMF, and Voltage in LR Circuits." These modules are part of the Navy Basic Electricity and Electronics series from the 1970s. The fifth section of this module is titled, "Relationships in Inductive Circuits."

9.1 Rise and Decay of Current and Voltage
9.2 LR Time Constant
9.3 Universal Time Constant Chart
9.4 Inductive Reactance

1. The previous units have set out a number of relationships. For example, the "reactance" of an inductive circuit, the equivalent of its resistance, is found by the equation X_L = 2πfL, where f is the frequency of alternating current and L is the inductance.

So we can substitute X_L for resistance in the earlier equations.

• Since E = IR, then E = I * X_L
• Since I = E/R, then I = E/X_L
• Since R=E/I, then X_L= E/I

2. In normal circuits, power equals current times voltage, P = EI. Or substituting in for E, P = I²R.

In a purely inductive circuit, power is never consumed, but we can speak of "apparent power." It is symbolized by P_a, and it is measured in volt-amperes (va), not watts.

What happens is that the power is supplied by the AC source, stored for part of the cycle in the inductor, and then returned to the source. The amount of volt amps stored in the inductor is called the reactive power (P_x). In a purely inductive circuit, it is the same as the apparent power. We say it is measured in "vars," "volt-amps reactive."