1	Chapter 13 Inductance and Inductors
2	Inductors Common form of an inductor is a coil of wire Used in radio tuning circuits In fluorescent lights Part of ballast circuit
3	Inductors On power systems Part of the protection circuitry used to control short-circuit currents during faults
4	Electromagnetic Induction Voltage is induced When a magnet moves through a coil of wire When a conductor moves through a magnetic field
5	Electromagnetic Induction Change in current in one coil can induce a voltage in a second coil Change in current in a coil can induce a voltage in that coil
6	Electromagnetic Induction Faraday’s Law Voltage is induced in a circuit whenever the flux linking the circuit is changing Magnitude of voltage is proportional to rate of change of the flux linkages with respect to time
7	Electromagnetic Induction Lenz’s Law Polarity of the induced voltage opposes the cause producing it
8	Induced Voltage and Induction If a constant current is applied No voltage is induced If current is increased Inductor will develop a voltage with a polarity to oppose increase
9	Induced Voltage and Induction If current is decreased Voltage is formed with a polarity that opposes decrease
10	Iron-Core Inductors Have flux almost entirely confined to their cores Flux lines pass through the windings Flux linkage as product Flux times number of turns
11	Iron-Core Inductors By Faraday’s law Induced voltage is equal to rate of change of NF
12	Air-Core Inductors All flux lines do not pass through all of the windings Flux is directly proportional to current Induced voltage directly proportional to rate of change of current
13	Self-Inductance Voltage induced in a coil is proportional to rate of change of the current Proportionality constant is L Self-inductance of the coil-units are Henrys (H)
14	Self-Inductance Inductance of a coil is one Henry If the voltage created by its changing current is one volt When its current changes at rate of one amp per second
15	Inductance Formulas Inductance of a coil is given by l is the length of coil in meters A is cross-sectional area in square meters N is number of turns µ is permeability of core
16	Inductance Formulas If air gap is used, formula for inductance is Where µ_o is permeability of air A_g is area of air gap l_g is length of gap
17	Computing Induced Voltage When using equation If current is increasing, voltage is positive If current is decreasing, voltage is negative Di/Dt is slope for currents described with straight lines
18	Inductances in Series For inductors in series Total inductance is sum of individual inductors (similar to resistors in series)
19	Inductances in Parallel Inductors in parallel add as resistors do in parallel
20	Core Types Type of core depends on intended use and frequency range For audio or power supply applications Inductors with iron cores are generally used
21	Core Types Iron-core inductors Large inductance values but have large power losses at high frequencies For high-frequency applications Ferrite-core inductors are used
22	Variable Inductors Used in tuning circuits Inductance may be varied by changing the coil spacing Inductance may be changed by moving a core in or out
23	Circuit Symbols
24	Stray Capacitance Turns of inductors are separated by insulation May cause stray or parasitic capacitance At low frequencies, it can be ignored At high frequencies, it must be taken into account Some coils are wound in multiple sections to reduce stray capacitance
25	Stray Inductance Current-carrying components have some stray inductance Due to magnetic effects of current Leads of resistors, capacitors, etc. have inductance These leads are often cut short to reduce stray inductance
26	Inductance and Steady State DC Voltage across an inductance with constant dc current is zero Since it has current but no voltage, it looks like a short circuit at steady state For non-ideal inductors Resistance of windings must be considered
27	Energy Stored by an Inductance When energy flows into an inductor Energy is stored in its magnetic field When the field collapses Energy returns to the circuit
28	Energy Stored by an Inductance No power is dissipated, so there is no power loss Energy stored is given by
29	Troubleshooting Hints Use ohmmeter Open coil will have infinite resistance Coil can develop shorts between its windings causing excessive current Checking with an ohmmeter may indicate lower resistance