DC CircuitsINDUCTANCEThe induced EMF, or counter EMF, is proportional to the time rate of change of the current. Theproportionality constant is called the "inductance" (L). Inductance is a measure of an inductor’sability to induce CEMF. It is measured in henries (H). An inductor has an inductance of onehenry if one amp per second change in current produces one volt of CEMF, as shown inEquation (3-1).CEMF = (3-1)L^{DI}DtwhereCEMF = induced voltage (volts)L = inductance (henries)= time rate of change of current (amp/sec)DIDtThe minus sign shows that the CEMF is opposite in polarity to the applied voltage.Example: A 4-henry inductor is in series with a variable resistor. The resistance is increasedso that the current drops from 6 amps to 2 amps in 2 seconds. What is the CEMFinduced?CEMFL^{DI}Dt4^{}2A6A24(2)CEMF8 voltsInductors in series are combinedFigure 4 Inductors in Serieslike resistors in series. Equivalentinductance (L_{eq}) of two inductorsin series (Figure 4) is given byEquation (3-2).L_{eq} = L_{1} + L_{2} + ... L_{n}(3-2)Rev. 0 Page 3 ES-03

Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business