01 - Basic Electric Machines

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    Basic Electric MachinesOutside part of machine doesnot move, is stationary

    Is called stator

    Held stationarythrough vibrationdamping

    attachments

    Central part of machine rotates

    Is called Rotor

    Shaft willrotate withinsome form of bearing.

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    Lorentz force on a conductor:

    Open righthand rule

    Bil F !

    S ource: T. Wildi, El ectrica l Machines, Drives and Power Systems , 5th Edition, Prentice-Hall, 2002

    Basic Electric Machines

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    A pole is a magnetic pole, that is north or south.

    Must always have a north and a matchingsouth pole so poles always are in pairs.

    On next slide is a two pole or one pole-pair DC machine.Commutators are mechanical switchesthat change direction of current.

    Basic Electric Machines

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    South pole North pole

    M agnetic fluxnorth to south

    X

    South pole North pole

    elevation

    plan

    Rotation path

    Conductor withcurrent out of page

    Conductor withcurrent into page

    A pply Lenzs Lawopen right hand rule

    Rotates aboutaxis

    Force

    Force

    Basic DC Machines

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    Southpole X

    Force

    Northpole

    X

    X

    A ngular velocity

    Zero Force

    X

    Basic DC Machines

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    Southpole X

    Northpole

    X

    X

    Zero Force

    X

    +V

    Brush andgap sized toavoid shortcircuit.

    X

    Commutator

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    The power flows in electric machines arereversible.

    To operate machine as motor supply electric

    power to get mechanical power. To operate as generator supply mechanical

    power to generate electrical power. To operate DC machine as generator remove

    DC voltage supply and externally rotate shaftConductor moving through (cutting lines of)magnetic flux induces voltage and/or current.

    Commutator

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    Southpole X Northpole

    XX

    X

    +

    VT

    X

    C C

    Terminalvoltage

    VT

    T ime in positionsNot to scaleC

    C

    ExternallyRotate rotor

    D

    D

    D

    D

    E

    A

    B

    E

    A

    B

    E

    E

    F

    F

    G

    G

    H

    H

    I

    I

    J

    J

    A

    AB

    B

    / 2

    90

    3 / 4

    13 5

    -0 .1

    18 1

    - 0 .1

    17 9

    5 / 4

    22 5

    3 / 2

    2 7 0

    7 / 4

    3 15

    2

    3 6 0

    0

    0

    / 4

    45

    F

    F

    G

    G

    XHH

    XI

    I

    J

    J

    XXX

    DC Generator Voltage Plot

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    Southpole

    VT

    +

    Northpole

    PrimeMover

    shaft

    Prime mover rotates shaft

    Commutator Rotate on shaft, rotor

    Brushes

    Fixed to casing, stator

    Conductor,rotor, rotateswith shaft

    DC Generator Plan View

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    Terminalvoltage

    VT

    T ime in positionsNot to scaleC

    C

    D

    D

    E

    E

    F

    FG

    G

    H

    H

    I

    I

    J

    J

    A

    ABB

    / 2

    90

    3 / 4

    13 5

    -0 .1

    18 1

    - 0 .1

    17 9

    5 / 4

    22 5

    3 / 2

    2 7 0

    7 / 4

    3 15

    2

    3 6 0

    0

    0

    / 4

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    No commutator so when direction of induced voltage (current)changes direction the terminal voltage must also change direction.

    T he terminal voltage induced isan A C waveform

    T he frequency of the inducedvoltage is equal to the rotationspeed in revolutions per second.

    A C Generator Voltage Plot

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    This electric machine with solid rings is called asynchronous machine.

    The induced voltage frequency must be thesame as the speed of rotation.

    The frequency is synchronous with the speed of rotation.

    W hen used as a motor the synchronousmachine can only rotate at the frequency of the A C voltage supply.

    The speed inflexibility results in synchronousmachines being used mostly as generators.

    Sy nchronous Machines

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    Synchronous M achines connectedto electricity system

    The electricity system has a constant frequencyof 5 0 [Hz].So (two pole, single pole-pair) synchronousmotor can only rotate at 5 0 revolutions per second. = 3000 [rpm]If synchronous machine is operated as agenerator, connected to the electricity systemthen the prime mover must rotate the shaft at3000 [rpm].Generated A C voltage must be at 5 0 [Hz].

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    It is more economic to operate and constructsynchronous machines inside-out compared to DCmachines.

    The magnetic poles are placed on the rotor and so rotatewith the shaft.

    The conductor remains stationary, in the stator.For voltage to be induced in a conductor the conductor must be moving relative to the lines of magnetic flux.

    The A C voltage is induced as the lines of magnetic fluxare moving as the magnetic poles are rotated on therotor.

    Sy nchronous MachinesConstructions

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    Synchronous Generator Plan View

    Northpole

    VT

    +

    South

    pole

    PrimeMover

    shaft

    Prime mover rotates shaft

    M agnetic poles

    Rotate on shaft, rotor

    Conductor,fixed to stator. M agnetic core toprovide flux path.Is on stator

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    Field winding

    In the majority of situations it is moreeconomic to induce the magnetic poles

    electro-magnetically. True for both DC machines andsynchronous machines.

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    ElectromagnetismM agnetic field produced by a solenoid: n i

    l

    Ni Q

    Q

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    F ield winding

    In the majority of situations it is moreeconomic to induce the magnetic poleselectron-magnetically.

    The current used to induce the magneticpoles (flux) is called the field current.

    The winding (coil or solenoid) is called the

    field winding.Controlling the field current also allowscontrol of the magnetic flux density.

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    A rmature winding

    The conductor carrying the current thatpasses through the lines of magnetic fluxis called the armature winding.

    Thus the current is called the armaturecurrent.

    The magnitude of the armature currentcontrols the force (Lenzs law) and so thetorque exerted on the shaft.

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    Sy nchronous Generator Plan View

    Northpole

    VT

    +

    South

    pole

    PrimeMover

    Prime mover rotates shaft

    Conductor,fixed to stator.

    M agnetic core to provideflux path. Is on stator

    +

    Vf

    A rmature winding A rmature current

    M agnetic core for field winding,

    rotates with shaft

    Field winding

    Field current

    I f

    I a

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    Sy nchronous Motor Plan View

    VT

    +

    PrimeMover

    Prime mover rotates shaft

    M agnetic core to provideflux path. Is on stator

    +

    Vf

    A rmature winding A rmature current

    M agnetic core for field winding,

    rotates with shaft

    Field winding

    Field current

    I f

    I a

    VT

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    Sy nchronous Motor If remove DC voltage supply to field winding then no fieldcurrent flows.If no field current then no force exerted on field winding(rotor) so shaft will stop rotating.

    There is an A C current flowing through the armaturewinding.

    This armature current will induce a magnetic flux. The current is A C and so the magnetic flux induced will

    be varying (moving). The flux will pass around the magnetic core including the

    field core.So the field winding will be in a moving magnetic field.T he conductor will be cutting lines of magnetic flux.

    A voltage will be induced across the field winding

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    Sy nchronous Machine Plan ViewM agnetic core to provideflux path. Is on stator

    A rmature winding A rmature current

    M agnetic core for field winding

    Field winding

    Field current

    I f

    I a

    VT

    V voltmeter

    Zero field

    current so rotor (shaft) will notrotate

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    Trans f ormer model The armature winding is stationary on the stator. An A C voltage applied across the armature

    winding. The rotor is stationary. An A C voltage is induced across the rotor (field)winding.

    Both windings are stationary so this machine is atransformer.

    The armature winding on the stator is theprimary winding of the transformer.

    The rotor winding is the secondary winding of the transformer.

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    Trans f ormer Machine ModelM agnetic core toprovide flux path.Is on stator

    A rmature winding

    M agnetic corefor field winding

    Rotor windingVT

    Vvoltmeter

    Zero field

    current so rotor (shaft) will notrotate

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    Induction Motor M agnetic core toprovide flux path.Is on stator

    A rmature winding

    M agnetic corefor field windingRotor windingVT

    A A mmeter short-circuit

    Rotor currentso rotor (shaft)will rotate

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    Induction Motor

    W ant high rotor current for high torque.Recall transformer turns ratio

    Keep large turns ration N 1 > N 2Small voltage in rotor winding but large current

    The force Lenzs law and so the torque depends onthe current

    rat ioturnsa N

    N

    i

    i

    V

    V

    2

    1

    1

    2

    2

    1

    !!!!

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    Induction Motor (Machine)

    The field (rotor) winding has been short circuitedso no need for slip rings could short circuit therotor winding on the rotor.

    No electrical connection between the rotor andthe outside.Brushes are a mechanically weak point due tofriction.

    So induction motors that do not need brushesare mechanically simpler and more reliable thanDC or synchronous machines.

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    Induction Motor M agnetic core toprovide flux path.Is on stator

    A rmature winding

    M agnetic corefor field windingRotor windingVT

    Rotor currentso rotor (shaft)will rotate

    Shortcircuit onrotor