Slide 1 Chapter 7:Inductors, Capacitors, and Transformers ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 2 2Inductors• Inductors are another basic building block of electronic circuits.• Like resistors, they tend to impede current flow; however, the opposition offered by inductors is different for AC and DC circuits.• Inductors may also be called chokes or coils.– The term choke refers to how inductors can be used to suppress undesired frequencies.• Electromagnetic induction is at the heart of inductor theory of operation.L147mH ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 3 3Electromagnetic Induction• To illustrate the principle of electromagnetic induction, think of two coils of wire wound on a common core.• The core has high permeability.– Permeability is a measure of how easily a magnetic field can be set up in a material.– As the current changes continuously in the coil, the magnetic field in the core also changes. – Current introduced in one coil will induce a current in the other coil.– This is possible because of the moving magnetic field. ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________Slide 4 4Factors Affecting Induction• Several factors determine the amount and polarity of an induced voltage:– Magnetic field strength.– Rate of relative motion between flux and conductor.– Angle of relative motion between flux and conductor.– Number of turns of wire in the coil.– Direction of relative motion between flux and conductor.– Polarity of the magnetic field.http://micro.magnet.fsu.edu/electromag/java/faraday/index.html ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 5 5Effects of Induced Current• When a current is induced in a coil by a changing magnetic field, the current creates a second magnetic field. The magnetic field produced by the induced current has a polarity that opposes the changes in the original magnetic field.•This is Lenz’s Law.Fig 7-3http://micro.magnet.fsu.edu/electromag/java/lenzlaw/index.html ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 6 6Effects of Induced Current• As the magnet motion moves toward the coil, the magnetic field induced in the coil opposes the magnet (North to North)• The motion induces a current in the coil• If there was no motion there would be no current.• As the magnet moves away in the lower picture, notice the north/south magnetic field switches. ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________Slide 7 7Inductance and its Factors• Inductors are merely coils of wire.• Current flowing through the turns of an inductor will produce a magnetic field around each turn.• Any change in the current flow will produce a corresponding change in the magnetic field.• Any change in the magnetic fields of a coil will produce a voltage change in each of the turns of the coil and that voltage will be opposing the original change in current.L147mH ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 8 8Inductance and its Factors• An inductor opposes any change in current.• The property of an inductor that opposes current flow is called inductance (L).• The unit of measure for inductance is the henry (H).• If current changes in an inductor at the rate of one ampere per second and causes one volt of self-induced voltage, the inductor has a value of one henry. ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 9 9Inductance and its Factors• Concept: When current changes in an inductor, a voltage is induced in the turns of the inductor that opposes the initial change in current.• Concept: An inductor opposes changes in current.Physical factors that affect inductanceµ↑ Inductance ↑DirectPermeability of the core (µ)Turns ↑ (x3) Inductance ↑ (x9)Direct (N2)Number of turns (N)Area ↑ Inductance ↑DirectCross sectional area of the coilLength ↑ Inductance↓InverseLength of the coilExampleProportionalityFactor ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________Slide 10 10Inductive Reactance• An inductor in a circuit that has a sinusoidal current will self-induce a voltage that is sinusoidal, but opposing the inducing current.• The opposition to a sinusoidal alternating current flow is called inductive reactance and is measured in ohms.• The symbol for reactance is X. Inductive reactance is designated by XL.• Inductive reactance is calculated by: XL= 2πfL, where f is the frequency of the sine wave.– From the formula we see reactance is a function of frequency and Inductance. ___________________________________ ___________________________________ ___________________________________ ___________________________________