Inductor-Resistor Series Circuit (Disconnection)

Chapter 8.4 Inductor-Resistor Series Circuit (Disconnection)

Fundamental Electrical and Electronic Principles Third Edition Book
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Fundamental Electrical and Electronic Principles Third Edition Book

  • D.C. Transients 259 (a) initialddamp/sso, initialddA/s itELit1110573 33..Ans (b) final current, amptherefore, A IIER11022005.Ans (c) LRsecondhence, ms1..5220682 Since the system takes approximately 5 seconds to reach its new steady state, then the current will reach its fi nal steady value in a time: t56 8234..msms 1Ans 8.4 Inductor-Resistor Series Circuit (Disconnection) Figure 8.16 shows such a circuit, connected to a d.c. supply. Assume that the current has reached its fi nal steady value of I amps. Let the switch now be returned to position ‘ A ’ (at time t 0). The current will now decay to zero in an exponential manner. However, the decaying current will induce a back-emf across the coil. This emf must oppose the change of current. Therefore, the decaying current will fl ow inthe same direction as the original steady current. In other words, the back-emf will try to maintain the original current fl ow. The graph of the decaying current, with respect to time, will therefore be as shown in Fig. 8.17. The time constant of the circuit will, of course, still be L / Rsecond, and the current will decay from a value of I E / R amp. The initial rate of decay will also be E / L amp/s.τ2τ3τ4τ5τ t (s)00.368 Ii (A)I Fig. 8.17 ‘B’‘A’RELi Fig. 8.16