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    REVIEW OFELECTROMAGNESTISM

    NOORLINA MOHD ZAINUDDIN

    EEP 3243: ELECTRICAL MACHINES

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    Learning Outcomes At the end of the chapter, students should be able to:

    Understand the fundamental lawsfundamental laws in the dynamicdynamicmagnetic systemsmagnetic systems and theirrelation to the electricalrelation to the electricalmachinesmachines.

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    Introduction to Electrical Machines

    An electric machine is a device which converts electricalconverts electricalpowerpower (voltages and currents) into mechanical powermechanical power(torque and rotational speed).

    A motormotor describes a machine which converts electricalelectrical

    power to mechanical powerpower to mechanical power; a generator (or alternator)generator (or alternator)converts mechanical power to electrical powermechanical power to electrical power.

    Many electric machines are capable of performing both asperforming both as

    motors and generatorsmotors and generators;

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    Introduction to ElectricalMachines

    The capability of a machine performing as one or the otheris often through the action of a magnetic field, to performsuch conversions.

    To understand how an electrical machines works, the key isto understand how the electromagnet works.

    The principles of magnetism play an important role in the

    operation of an electrical machines.

    http://electronics.howstuffworks.com/electromagnet.htmhttp://electronics.howstuffworks.com/electromagnet.htm
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    Review of

    ElectromagnetismThe basic idea behind an electromagnet is extremely

    simple: a magnetic field around the conductor can bea magnetic field around the conductor can beproduced when current flows through a conductorproduced when current flows through a conductor

    In other word, the magnetic field only exists when electricmagnetic field only exists when electriccurrent is flowingcurrent is flowing

    By using this simple principle, you can create all sorts ofthings, including motorsmotors, solenoids, read/write headsforhard diskshard disksand tape drivestap

    e drives, speakers

    speakers, and so on

    http://electronics.howstuffworks.com/motor.htmhttp://electronics.howstuffworks.com/motor.htmhttp://electronics.howstuffworks.com/hard-disk.htmhttp://electronics.howstuffworks.com/hard-disk.htmhttp://electronics.howstuffworks.com/hard-disk.htmhttp://electronics.howstuffworks.com/cassette.htmhttp://electronics.howstuffworks.com/cassette.htmhttp://electronics.howstuffworks.com/cassette.htmhttp://electronics.howstuffworks.com/speaker.htmhttp://electronics.howstuffworks.com/speaker.htmhttp://electronics.howstuffworks.com/speaker.htmhttp://electronics.howstuffworks.com/cassette.htmhttp://electronics.howstuffworks.com/hard-disk.htmhttp://electronics.howstuffworks.com/motor.htm
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    Magnetic Field

    idH =

    .

    Unlike electric fields (which start on +q and end on q),magnetic field encircle their current source.

    Field is perpendicular tothe wire and that thefield's direction dependson which direction thecurrent is flowing in thewire

    A circular magnetic fielddevelops around the wirefollows right-hand rulesright-hand rules

    The field weakens as you move away from the wire Amperes circuital law - the integration path lengthis longer

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    Example of

    ElectromagneticAn electromagnet can be madecan be made by winding the conductorwinding the conductorinto a coil and applying a DC voltageinto a coil and applying a DC voltage.

    The lines of flux, formed by current flow through theconductor, combine to produce a larger and strongermagnetic field.

    The center of the coil is known as the core. In this simpleelectromagnet the core is airair.

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    Adding an Iron Core

    IronIron is ais a better conductor of fluxbetter conductor of flux thanthan airair. The air core of anelectromagnet can be replaced by a piece of soft iron.

    When a piece of iron is placed in the center of the coil morelines of flux can flow and the magnetic field isthe magnetic field isstrengthened.strengthened.

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    Strength of Magnetic Field (Cont)

    Because the magnetic field around a wire is circular andmagnetic field around a wire is circular andperpendicular to the wireperpendicular to the wire, an easy way to amplifyamplify thewire's magnetic field is to coilcoil the wire

    The strength of the magnetic field in the DCelectromagnet can be increased by increasing theby increasing thenumber of turns in the coil.number of turns in the coil. The greater the number ofThe greater the number ofturns the stronger the magnetic field will be.turns the stronger the magnetic field will be.

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    Faradays Law :Faradays Law : If a magnetic flux, , in a coil is changing in time (nchanging in time (nturns),turns), hence a voltage, Vab is induced

    Lenzs Law :Lenzs Law :if the loop is closed, a connected to b, the current would

    flow in the direction to produce the flux inside the coil opposing theoriginal flux change.(in other words, Lenzs Law will determine thepolarity of the induced voltage)

    tNV

    =

    Vt

    =

    Faradays Law and Lenzs Law

    ab

    V = induced voltageN = no of turns in coil

    = change of flux in coilt = time interval

    If no turns :

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    Faradays Law

    The effect of magnetic field:

    Induced Voltage from a Time ChangingMagnetic Field

    Production of Induced Force on a Wire

    Induced Voltage on a Conductor Moving ina Magnetic Field

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    Voltage Induced from a time changing

    magnetic field

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    Voltage Induced in a conductor

    moving in a magnetic fieldoFaradays Law for moving conductors :Faradays Law for moving conductors : For coils in which wire (conductor)

    is moving thru the magnetic flux, an alternate approach is to separate thevoltage induced by time-varying flux from the voltage induced in a movingconductor.

    oThis situation is indicates the presence of an electromagnetic field in a wire(conductor). This voltage described by Faradays Law is called as the fluxcutting orElectromotive forceElectromotive force, oremfemf.

    o

    oThe value of the induced voltage is given by

    E = Blvwhere

    EE= induced voltage (V)BB = flux density (T)ll= active length of the conductor in the magnetic field (m)vv= relative speedspeed of the conductor (m/s)

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    Induced Force

    The electrical circuit consists of battery, resistor, two stationary rails,and movable bar that can roll or slide along the rails with electricalcontact.

    When switch is closed:Current will not start immediately as inductance of

    the circuit. (However time constant L/R is verysmall). Hence, current quickly reach V/R.

    Force is exerted on the barForce is exerted on the bardue to interactioninteraction

    between current and magnetic fluxbetween current and magnetic flux to the right andmade the bar move with certain velocitythe bar move with certain velocity. Themechanical power out of the bar.

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    FF = ilB

    Force induced onthe conductor:

    Unit: (N)

    The direction offorce is given bythe right-hand

    rule.

    Induced Force

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    Induced Force (Cont)

    The motion of the barproduces an electromagnetic force.The polarity of the emfis +ve where the current entersthe moving bars. The moving bargenerates a back emfthat opposes the current.

    The instantaneous electrical power into the bar =

    mechanical output power

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    Production of a Magnetic Field

    oThe production of a magnetic field by a current isdetermine by Amperes law:

    o

    = netIdlHH = magnetic field intensitydl = differential element oflength along the path ofintegration

    cl

    NiH = N=number of turns of the coil

    I=current through the coil, in Amplc = length of magnetic circuit inmeters

    Magnetic field intensity:

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    Production of a Magnetic Field

    oThe strength of the magnetic field flux produced in thecore also depends on the material of the core.

    HB =u = magnetic permeability ofmaterial

    r0=

    u0 = permeability of free spaceur = relative permeability ofmaterial

    m/H1047

    0

    =

    Magnetic flux density:

    Unit of B isTesla(Weber/sq.meter)

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    cl

    NiHB

    ==

    BA=

    cl

    NiA=

    Total flux:

    Production of a Magnetic Field

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    Reluctance

    The opposition to magnetic field flux through a given volume ofspace or material. Analogous to electrical resistance. It is similar to:

    resistance in R = in an electric circuit. a Then, Reluctance = mmf unit AT/Wb Also reluctance of any portion of a magnetic circuit propotional to

    the length of the flux path, inversely propotionalto the cross-section of the

    path and inversely propotional to permeability of the magnetic material.

    reluctance, R = = mean length of magneticcircuit in metre.

    0rA

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    Magnetic circuit with air gap

    fringing is neglected, Ag = Ac

    The mmf Ni is now consumed in thecore plus the air-gap

    The multiply the flux are known as thereluctance of core and air gap

    by substituting

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    thus

    Magnetic circuit with air gap (cont.)

    inverted

    0 , Rc Rg

    Multiplies the mmf is known as thePermeance

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    Comparison between magnetic and electrical

    circuitNo Magnetic circuit Electrical circuit

    1 Magneto motive force (MMF)in ampere-turn

    Electromotive force E.m.f involts

    2 Flux in weber. The flux doesnot flow

    Current in ampere, the electriccurrent actually flows

    3 Flux= MMFReluctance

    Current = E.M.FResistance

    4 Flux density in web/sq metre Current density in amp/m2

    5 Reluctance in Amp-turnsReluctance = 1/0/r

    Resistance in ohms R = a

    6 Permeance = 1 / Reluctance Conductance = 1 / Resistance

    7 A switch is not possible to stop

    the flux in a magnetic circuit

    A switch is enough to stop the

    flow of current in an electriccircuit

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    Example 1

    A coil of 200 turns of wire is closely wound on an iron ring of circularcross section of 6 sq.cm. If the mean diameter of the ring is 14cm. Find them.m.f. produced reluctance of the magnetic circuit, magnetic force,reluctance, flux in the ring , and xflux density when a current of 2 ampereflow through the coil. Take relative permeabilty of iron as 1000.

    Example 2

    A steel ring having a uniform cros-sectional area 4 cm2 and mean diameterof 30 cm. A coil of 500 turns wound uniformly around the steel ring.Assuming relative permeability of the steel to be 380, calculate the

    following.

    a)Reluctance of ringb)The current through the wire, the coil to produce a flux of 70 micro web,

    in the steel ring.

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    Example 3

    The magnetic circuit shown in figure a has dimensions Ac= 9 cm2

    , Ag = 9cm2,g = 0.050 cm, c= 30 cm, and N = 500 turns. Assume the value r= 70,

    000 for iron.Finda) the reluctances Rc and Rg. Assume that the magnetic circuit is operating

    with Bc = 1.0 Tb) The flux c) The current i

    Figure a

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    Example 4

    The magnetic structure of a synchronous machine is shown schematically infigure b. Assuming that rotor and stator iron have infinite permeability , findthe air-gap flux and flux density Bg. Given I = 10A, N = 1000 turns, g =

    1cm, and Ag = 2000cm2.

    Figure b

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    Hysteresis Loss

    During a cycle of variation of i (hence H), there is a netenergy flow from the source to the coil-core assembly andreturn to the source

    Energy flowing is greater than energy returned

    This energy loss goes to heat the core

    The loss of power in the core due to the hysteresis effectis called hysteresis loss

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    Eddy Current Loss

    Voltage will be induced in the path of magnetic corebecause of time variation of flux enclosed by the path

    A current, known as an eddy current will flow around thepath

    Because core has resistance, a power loss will be cause by

    the eddy current and appear as heat in the core

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    Eddy Current Loss

    Eddy current can be reduced in 2 ways:

    i) Adding a few percent of silicon to iron to increase the

    resistivity.

    ii) Laminate core with thin laminations and insulated fromeach other.

    Hysteresis loss + eddy current loss = Core lossHysteresis loss + eddy current loss = Core loss

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    THANK YOU FOR

    YOUR ATTENTION!!