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8/3/2019 16514 A Synchronous Motor
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AC MACHINESAC MACHINES
The machines which use alternating voltagefor their operation.
3 PHASE INDUCTION MOTORThe 3 phase induction motors are used in
industry for bulk power conversion from
electrical to mechanical .
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Advantages over DC MotorAdvantages over DC Motor
y Low Cost and more reliable.
y They can operate in dusty environments.
y
Speed in RPM is of order of 12000.y High Efficiency.
y 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:StatorIt is a stationary frame consisting of a
steel frame that supports a hollow, cylindrical coreof stacked laminations. The stator is a stationary
winding which can be a star connected or deltaconnected.
y It Consists of a number of stampings with slots tocarry three phase windings. It is wound for a
definite number of poles. The windings aregeometrically spaced 120 degrees apart.
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ROTORROTOR
y Rotor is a rotating winding.
y This winding is not connected to any external
supply.
y The current flows in rotor winding is due toinduction thats why it named as Inductionmotor.
y
It has two types : 1. Squirrel Cage Rotor2. Wound Rotor.
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Two types of rotors are used in Inductionmotors
y Squirrel-cage rotor
y Wound rotor
Squirrel-cage windings, which produce asquirrel-cage induction motor (most common)
Conventional 3-phase windings made ofinsulated wire, which produce a wound-rotor
induction motor (special characteristics)
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STATOR CONSTRUCTION:STATOR CONSTRUCTION:
The stator and the rotorare electrical circuits thatperform as
electromagnets. Thestator is the stationaryelectrical part of themotor. The stator core ofmotor is made up ofseveral hundred thinlaminations.
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STATOR WINDINGS;STATOR WINDINGS;
Stator laminations are
stacked together forming ahollow cylinder. Coils ofinsulated wire are insertedinto slots of the stator core.
Each grouping of coils,together with the steel coreit surrounds, form anelectromagnet.Electromagnetism is the
principle behind motoroperation. The statorwindings are connecteddirectly to the 3 PhaseSupply
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ROTOR CONSTRUCTION:ROTOR CONSTRUCTION:
y
The rotor is the rotating partof the electromagnetic circuit.
The most common type ofrotor is the "squirrel cage"
rotor.y The rotor consists of a stack
of steel laminations with evenly
spaced conductor bars aroundthe circumference.
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A wound rotor or Slip ring rotorhas a 3-
phase winding, similar to the statorwinding.
y The rotor winding terminals are connectedto three slip rings which turn with the rotor.
The slip rings/brushes allow external
resistors to be connected in series with thewinding.
y Low Efficiency.
y
External Resistance is connected to develophigh Torque.
y E.g. Cranes, Elevators etc.
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Squirrel cage rotorconsists of copper bars,
slightly longer than the rotor,which are pushed into theslots.
y
The ends are welded tocopper end rings, so that allthe bars are short circuited.
y External Resistance cannot be
connected.y No Slip Rings and Brushes.
y Fans, Water Pumps, Blowersetc.
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Graphical RepresentationGraphical Representation
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Operating principleOperating principle ue to three phase stator current a rotating magnetic field is
produced.The Rotating magnetic field rotates at a synchronous speed
(Ns = 120 f / p)
Firstly the Rotor winding is stationary and rotating magnetic
field cuts the stationary rotor conductors and induces an emfin rotor winding.
A voltage E = BLv is induced in each conductor (FaradaysLaw)
The induced voltage produces currents which circulate in aloop around the conductors (through the bars).
Since the current-carrying conductors lie in a magnetic field,they experience a mechanical force (Lorentz force).
The force always acts in a direction to drag the conductoralong with the magnetic field.
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Number of Poles Synchronous Speed
The rotating speed of the revolvingflux 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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Induction Motors: Slip
The difference between the synchronousspeed and rotor speed can be expressed as apercentage of synchronous speed, known asthe slip.
s = Fractional slip, Ns = synchronous speed
(rpm), N = rotor speed (rpm) At no-load, the slip is nearly zero (
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Frequency induced in the rotorFrequency induced in the rotor
The frequency induced in the rotor depends
on the slip:
fR= sf
fR = frequency of voltage in the rotor
f = frequency of the supply and stator field
s = slip
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Uses of Synchronous MotorsUses of Synchronous Motors
y Synchronous motors find applications in
all industrial applications where constantspeed is necessary.
y Low power applications include
positioning machines, where high
precision is required, and robot actuators.
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Advantages of SynchronousAdvantages of Synchronous
motor over Three phase motormotor over Three phase motor
y Speed is independent of the load,
provided an adequate field current isapplied.
y Accurate control in speed and position.
y They run either at the synchronous speed
or they do not run at all.
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