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AC MACHINESAC MACHINES
The machines which use alternatingvoltage for their operation.
3 PHASE INDUCTION MOTOR The 3 phase induction motors areused in industry for bulk power
conversion from electrical tomechanical .
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Advantages over DCAdvantages over DC
MotorMotorLow Cost and more reliable.They can operate in dusty
environments.
Speed in RPM is of order of12000.
High Efficiency.
Smaller Size and Weight.
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INDUCTION MOTOR:INDUCTION MOTOR:
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An Induction motor hasAn Induction motor has
basically two parts:basically two parts: Stator It is a stationary frame
consisting of a steel frame that supportsa hollow, cylindrical core of stackedlaminations. The stator is a stationary
winding which can be a star connected ordelta connected.
It Consists of a number of stampings with
slots to carry three phase windings. It iswound for a definite number of poles.The windings are geometrically spaced
120 degrees apart.
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ROTORROTOR
Rotor is a rotating winding.This winding is not connected to any
external supply.
The current flows in rotor winding isdue to induction thats why it namedas Induction motor.
It has two types : 1. Squirrel CageRotor
2. Wound Rotor.
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Two types of rotors are used inInduction motors
Squirrel-cage rotorWound rotor
Squirrel-cage windings, which
produce a squirrel-cage inductionmotor (most common)
Conventional 3-phase windings
made of insulated wire, whichproduce a wound-rotor inductionmotor (special characteristics)
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STATOR CONSTRUCTION:STATOR CONSTRUCTION:
The stator and therotor are electricalcircuits that perform
as electromagnets.The stator is the
stationary electricalpart of the motor.
The stator core ofmotor is made up ofseveral hundred thin
laminations.
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STATOR WINDINGS;STATOR WINDINGS; Stator laminations are
stacked togetherforming a hollowcylinder. Coils ofinsulated wire areinserted into slots of
the stator core. Each grouping of coils,
together with the steelcore it surrounds, form
an electromagnet.Electromagnetism isthe principle behindmotor operation. Thestator windings areconnected directly to
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ROTOR CONSTRUCTION:ROTOR CONSTRUCTION:The rotor is the rotating
part of theelectromagnetic circuit.
The most common type
of rotor is the "squirrelcage" rotor.
The rotor consists of a
stack of steellaminations with evenlyspaced conductor barsaround thecircumference.
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A wound rotor or Slip ring rotorhas a 3-phase winding, similar to
the statorwinding.The rotor winding terminals are
connected to three slip rings which
turn with the rotor. The sliprings/brushes allow externalresistors to be connected in serieswith the winding.
Low Efficiency.
External Resistance is connected todevelop high Torque.
E. . Cranes Elevators etc.
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Squirrel cage rotorconsists of copper bars,
slightly longer than therotor, which are pushedinto the slots.The ends are welded to
copper end rings, sothat all the bars areshort circuited.
External Resistancecannot be connected.No Slip Rings and
Brushes.Fans, Water Pumps,
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Rotating FieldRotating FieldConsider a simple stator with
6poles - windings AN, BN, CN.The windings are mechanicallyspaced at 120 from each
other.
The windings are connectedto a 3-
phase source.AC currents Ia, Ib and Ic will
flow inthe windings, but will bedisplaced in time by 120.Each winding produces its
own MMF,
which creates a flux across thehollow interior o the stator.
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RepresentationRepresentation
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Operating principleOperating principle Due to three phase stator current a rotating
magnetic field is produced.
The Rotating magnetic field rotates at asynchronous speed (Ns = 120 f / p)
Firstly the Rotor winding is stationary and rotatingmagnetic field cuts the stationary rotor conductorsand induces an emf in rotor winding.
A voltage E = BLv is induced in each conductor(Faradays Law)
The induced voltage produces currents whichcirculate in a loop around the conductors (throughthe bars).
Since the current-carrying conductors lie in amagnetic field, they experience a mechanicalforce (Lorentz force).
N b f P l S h
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Number of Poles SynchronousSpeed
The rotating speed of the revolving
flux can be reduced by increasing the
number of poles (in multiples of two).
Ns = 120 f / p
Ns = synchronous speed (rpm)
f = frequency of the supply (Hz)
p = number of poles
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Slip
The difference between thesynchronous speed and rotor speedcan be expressed as a percentage ofsynchronous speed, known as theslip.
s = Fractional slip, Ns = synchronousspeed (rpm), N = rotor speed (rpm) At no-load, the slip is nearly zero
(
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Frequency induced in theFrequency induced in the
rotorrotorThe frequency induced in the rotor
depends on the slip:
fR= sf
fR = frequency of voltage in therotor
f = frequency of the supply and
stator fields = slip
U f S hU f S h
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Uses of SynchronousUses of Synchronous
MotorsMotors
Synchronous motors findapplications in all industrialapplications where constant
speed is necessary.Low power applications include
positioning machines, where high
precision is required, and robotactuators.
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motor over Three phasemotor over Three phase
motormotorSpeed is independent of the load,
provided an adequate fieldcurrent is applied.
Accurate control in speed andposition.
They run either at the
synchronous speed or they donot run at all.
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