16515 A Synchronous Motor

8/2/2019 16515 A Synchronous Motor

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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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16515 A Synchronous Motor