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L11 Induction Motors

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    Induction Motors

    Amit Mishra

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    Topics to be DiscussedTopics to be Discussed

    Induction Motors : Applications.

    Construction :

    Stator.

    Rotor : Squirrel cage rotor.

    Wound rotor.

    The emf generated in the rotor.

    Torque generated on the rotor.

    Slip of Induction Motor.

    Frequency of Rotor Currents.

    Speed of Rotation of Rotor-Field.

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    Rotor emf.

    Rotor Impedance.

    Rotor Current.

    Differences from a Transformer.

    Power Flow Diagram.

    Starting Torque.

    Torque-Slip Characteristic Curve. Three Modes of Operation :

    (1) Motor action (0 < s 1).

    (3) Generator action (s

    < 0). Condition for Maximum Torque.

    Maximum Torque.

    Equivalent Circuit.

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    Induction Motors A synchronous motor has limited practical

    applications like control instrumentation, lightdisplays, textile, engineering and allied industries.

    An alternative is the asynchronous motor which we

    usually call induction motor.

    Most motors that we see around are induction

    motors.

    Induction motors

    are more rugged, need less maintenance,

    are less expensive.

    Are efficient but less than synchronous motors.

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    Applications of Induction Motors

    Three-phase induction motors are used for

    high power and industrial applications such

    as

    lifts, cranes, pumps, exhaust fans, lathes etc.,

    Single-phase induction motors find use in

    domestic electric appliances such as

    fans, refrigerators, washing machines, pumps,hair-driers, etc.

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    Construction

    Two Main Parts

    Stator

    Rotor

    Squirrel Cage Rotor

    (Mostly used)

    Phase Wound Rotor

    The statorof an induction motor is similar to

    that of a synchronous machine.

    When three-phase currents flow through the

    stator windings, a magnetic flux is produced that

    rotates at synchronous speed.

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    Induction motor components

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    Stator-Rotor Assembly

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    Stator of a large induction motor.

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    Squirrel cage rotor

    This rotor has a laminated iron core with slots, andis mounted on a shaft.

    Aluminum bars are molded in the slots and the bars

    are short circuited with two end rings.

    The bars and the shorting rings look like a

    squirrel cage.

    Fins are placed on the ring that shorts the bars.

    These fins work as a fan and improve cooling.

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    Squirrel cage rotor

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    Rotor bars (slightly skewed)

    End ring

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    1. It helps in reduction of magnetic hum, thus keeping the

    motor quiet.

    2. It also helps to avoid Cogging, i.e. locking tendency of the

    rotor. The tendency of rotor teeth remaining under the stator

    teeth due to the direct magnetic attraction between the two.

    3. Increased rotor resistance due to comparatively lengthier

    rotor conductor bars.

    4. Increased slip for a given torque.

    Reasons for Having Skewed Rotor

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    Wound rotor

    Most motors use the squirrel-cage rotor because ofthe robust and maintenance-free construction.

    However, large motors use a wound rotor with threephase windings placed in the rotor slots.

    The windings are connected in a three-wire wye.

    The ends of the windings are connected to three sliprings.

    Resistors or power supplies are connected to theslip rings through brushes for reduction of startingcurrent and speed control

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    Slip-ring rotor assembly

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    The emf generated in the rotor.

    Assume that the stator flux rotates anticlockwise.

    With respect to this flux, the rotor conductors move inclockwise direction.

    The emfs in conductorsA andB are induced.

    This is why this type of motor is called an inductionmotor.

    In fact, an induction motor is like a transformer with itssecondary winding short-circuited.

    The only difference is that in an induction motor, thesecondary winding is free to rotate.

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    Torque generated on the rotor

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    Fleming's left hand rule (forelectric

    motors) shows the direction of the thrust ona conductorcarrying a current in a

    magnetic field.

    The left hand is held with the thumb, index

    fingerand middle fingermutually at right

    angles. It can be recalled by rememberingthat "motors drive on the left, in Britain

    anyway."

    The First finger represents the direction of

    the magnetic Field. (north to south)

    The Second finger represents the direction

    of the Current (the direction of the current isthe direction of conventional current; from

    positive to negative).

    The Thumb represents the direction of the

    Thrust or resultant Motion.

    Flemings left hand rule

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    Current-carrying conductorsA and B experience

    force Fas shown (Flemings left hand rule).

    Torque is generated to rotate the rotor in

    anticlockwise direction.

    Torque keeps on generating till there is relative

    motion between the rotating field and the rotorconductors.

    Rotor accelerates, but can never attain the

    synchronous speed.

    Why ?

    Ans. : If it does, the induced voltage and hence

    current would be zero. As a result, the torque would

    become zero.

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    Motor on Load

    On putting load on the shaft, the rotor slows down.

    The relative speed increases.

    The induced emf in the rotor-conductors increases

    and hence the torque exerted on the rotor

    increases.

    The speed adjusts itself to get sufficient torque.

    Obviously, the speed has slightly reduced.

    Note that unlike a synchronous machine, the

    induction motor has field on the stator and armature

    on the rotor.

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    Slip of Induction Motor

    The difference between the synchronous speed Nsand the actual speed Nof the rotor is known as slip

    speed.

    The normalized slip speed, or simply the slip s isexpressed as

    pu or %s

    s s

    N NN

    s N N

    (

    ! !

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    For a given slip s, the rotor speed is given as

    (1 )s N N s!

    When standstill(that is, it is not running), the

    rotor speed Nis zero.

    Hence s = 1.

    The value ofs can never be zero.

    Why ?

    Ans. : Torque generated would be zero.

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    Is it possible to make the slip s have a

    negative value ?

    Ans. : Yes, if the rotor is made to rotate by a

    prime-mover at a speed higher than the

    synchronous speed. The negative slip corresponds to the

    generatoraction.

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    Frequency of Rotor Currents

    When standstill, It is like a transformer.

    fr= f

    When running, fr depends upon the relative

    speed or slip-speed.

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    120r

    s

    f

    N N P !

    120But ; Dividing the two,s

    fN

    P

    !

    ors r r

    s

    N N f f s

    N f f

    ! !

    r f s f @ !

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