3ph i.m introduction

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    Instructional Objectives

    Construction

    Types of rotor

    Operating PrincipleEffect of slip in rotor

    Torque equation- Starting & Running

    Torque-Slip Characteristics

    By

    P.Anitha,Lecturer

    EEE Dept

    Three-phase induction motor

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    Type of Electric Motors

    Electrical current reverses direction

    Two parts: stator and rotor Stator: stationary electrical component

    Rotor: rotates the motor shaft

    Speed difficult to control

    Two types

    Synchronous motor

    Induction motor

    AC Motors

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    AC motors

    Alternating current (AC) motors use an

    electrical current, which reverses its direction

    at regular intervals.

    An AC motor has two basic electrical parts: a

    "stator" and a "rotor". The stator is in the

    stationary electrical component. The rotor is

    the rotating electrical component, which inturn rotates the motor shaft.

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    The main advantage of DC motors over AC

    motors is that speed is more difficult to control

    for AC motors. To compensate for this, AC motors can be

    equipped with variable frequency drives but

    the improved speed control comes together

    with a reduced power quality.

    Disadvantages of AC motors

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    Type of AC Motors

    There are two types of AC motors:

    synchronous and induction.

    The main difference between the synchronous

    motor and the induction motor is that the rotor

    of the synchronous motor travels at the same

    speed as the rotating magnetic field.

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    Constant speed fixed by system frequency

    DC for excitation and low starting torque:

    suited for low load applications

    Can improve power factor: suited for high

    electricity use systems

    Synchronous speed (Ns):

    AC MotorsSynchronous motor

    Ns = 120 f / P

    F = supply frequency

    P = number of poles

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    AC MotorsSynchronous motor

    A synchronous motor is an AC motor, whichruns at constant speed fixed by frequency ofthe system.

    It requires direct current (DC) for excitationand has low starting torque, and synchronousmotors are therefore suited for applicationsthat start with a low load, such as aircompressors, frequency changes and motorgenerators.

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    AC MotorsSynchronous motor

    Synchronous motors are able to improve thepower factor of a system, which is why theyare often used in systems that use a lot of

    electricity. This motor rotates at a synchronous speed,

    which is given by the following equation Ns = 120 f / P

    Where:

    f = frequency of the supply frequency

    P= number of poles

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    Induction motors are the most common motors used

    for various equipments in industry.

    Their popularity is due to their simple design, they are inexpensive (half or less of the cost of a DC motor)

    High power to weight ratio (about twice that of a DC motor)

    are simple, rugged, low-cost and easy to maintain.

    They run at essentially constant speed from zero-to-full load.

    can be directly connected to an AC power source

    AC MotorsInduction motor

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    AC MotorsInduction motor

    An induction motor has two main electricalcomponents as shown in the figure

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    Construction

    Frame

    - Outer part of the body To support the stator core & windings

    To protect the inner parts

    Serve as a ventilating housing

    Stator core

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    Construction

    The iron core has cylindrical

    shape and is laminated with

    slots.

    The iron core on the figure haspaper liner insulation placed in

    some of the slots.

    In a three-phase motor, the

    three phase windings are placed

    in the slots.

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    Construction

    Stator or primary or field winding

    - Stator winding is 3 ph winding supplied

    from 3 ph source.- 3 ph can be connected in either star or

    delta depending upon the method of

    starting

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    Stator and rotor magnetic circuit

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    AC MotorsInduction motor

    Rotor-Induction motors use two types of rotors:

    A squirrel-cage rotor consists of thick conducting

    bars embedded in parallel slots. These bars are short-

    circuited at both ends by means of short-circuitingrings.

    A wound rotor has a three-phase, double-layer,

    distributed winding. It is wound for as many poles as

    the stator. The three phases are wired internally andthe other ends are connected to slip-rings mounted on

    a shaft with brushes resting on them.

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    Squirrel cage rotor

    Squirrel cage rotor consists of copper bars,slightly longer than the rotor, which are pushedinto the slots.

    The ends are welded to copper end rings, sothat all the bars are short circuited.

    In small motors, the bars and

    end-rings are die-cast in

    aluminum to form an

    integral block.

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    Squirrel cage rotor.

    This rotor has a laminated iron core with slots, and is mountedon a shaft.

    Aluminum bars are molded in the slots and the bars are shortcircuited with two end rings.

    The bars are slanted on a small rotor to reduce audible noise.

    Fins are placed on the ring that shorts the bars. These finswork as a fan and improve cooling.

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    Squirrel cage rotor

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    wound rotor Most motors use the squirrel-cage rotor because of the

    robust and maintenance-free construction.

    However, large, older motors use a wound rotor with

    three phase windings placed in the rotor slots.

    The windings are connected in a three-wire wye.

    The ends of the windings are connected to three slip rings.

    Resistors or power supplies are connected to the slip ringsthrough brushes for reduction of starting current and speed

    control

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    wound rotor

    The external resistors are mainly usedduring start-up under normal runningconditions the windings short-circuited

    externally.

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    AC MotorsInduction motor

    Stator.The stator is made up of a numberof stampings with slots to carry three-phase windings. It is wound for a definite

    number of poles. The windings aregeometrically spaced 120 degrees apart

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    Operating Principle

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    Operation Principle

    The three-phase stator is supplied by balanced threephasevoltage that drives an ac magnetizing current through eachphase winding.

    The magnetizing current in each phase generates a

    pulsating ac flux.

    The flux amplitude varies sinusoidally and the

    direction of the flux is perpendicular to the phase

    winding.

    The total flux in the machine is the sum of the three fluxes.

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    The summation of the three ac fluxes results in a rotating flux,which turns with constant speed and has constant amplitude.

    The rotating flux induces a voltage in the short-circuitedbars of the rotor. This voltage drives current through the bars.

    The induced voltage is proportional with the difference ofmotor and synchronous speed.

    Consequently the motor speed is less than the

    synchronous speed

    The interaction of the rotating flux and the rotor current

    generates a force that drives the motor. The force is proportional with the flux density and the rotor

    bar current

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    The figure shows the three components

    of the magnetic field at a phase angle of

    60. Each phase generates a magnetic field

    vector.

    The vector sum of the component

    vectors a

    , b

    , c

    gives theresulting rotating field vector

    rot,

    The amplitude is 1.5 times the

    individual phase vectoramplitudes,and rot rotates

    with constant speed.

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    Induction motor-Points to be remembered

    The speed of flux cutting is the difference

    between the magnetic field speed and the rotor

    speed.

    The two speeds can be calculated by using the

    radius at the rotor bar location and the

    rotational speed.

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    Induction motor-Points to be remembered

    The voltage and current generation in the rotor bar require a

    speed difference between the rotating field and the rotor.

    Consequently, the rotor speed is always less than the

    magnetic field speed.

    The relative speed difference is the slip, which is calculated

    using

    P

    fN

    N

    NNs

    S

    S

    S