16514 A Synchronous Motor

Embed Size (px)

Citation preview

  • 8/3/2019 16514 A Synchronous Motor

    1/20

    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 .

  • 8/3/2019 16514 A Synchronous Motor

    2/20

    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.

  • 8/3/2019 16514 A Synchronous Motor

    3/20

    INDUCTION MOTOR:INDUCTION MOTOR:

  • 8/3/2019 16514 A Synchronous Motor

    4/20

    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.

  • 8/3/2019 16514 A Synchronous Motor

    5/20

    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.

  • 8/3/2019 16514 A Synchronous Motor

    6/20

    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)

  • 8/3/2019 16514 A Synchronous Motor

    7/20

    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.

  • 8/3/2019 16514 A Synchronous Motor

    8/20

    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

  • 8/3/2019 16514 A Synchronous Motor

    9/20

    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.

  • 8/3/2019 16514 A Synchronous Motor

    10/20

    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.

  • 8/3/2019 16514 A Synchronous Motor

    11/20

    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.

  • 8/3/2019 16514 A Synchronous Motor

    12/20

  • 8/3/2019 16514 A Synchronous Motor

    13/20

    Graphical RepresentationGraphical Representation

  • 8/3/2019 16514 A Synchronous Motor

    14/20

    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.

  • 8/3/2019 16514 A Synchronous Motor

    15/20

    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

  • 8/3/2019 16514 A Synchronous Motor

    16/20

    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 (

  • 8/3/2019 16514 A Synchronous Motor

    17/20

    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

  • 8/3/2019 16514 A Synchronous Motor

    18/20

    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.

  • 8/3/2019 16514 A Synchronous Motor

    19/20

    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.

  • 8/3/2019 16514 A Synchronous Motor

    20/20