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Chapter 3: INDUCTION MOTOR

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Home air conditioners. Kitchen fans. Washing machines. Industrial machines. Compressors ... A rotating flux F is produced in the air gap. ... – PowerPoint PPT presentation

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Title: Chapter 3: INDUCTION MOTOR


1
Chapter 3 INDUCTION MOTOR
BEE2133 ELECTRICAL MACHINE POWER SYSTEM
2
Learning Outcomes
  • At the end of the lecture, student should be able
    to
  • Understand the principle and the nature of single
    phase and 3 phase induction machines.
  • Perform an analysis on induction machines which
    is the most rugged and the most widely used
    machine
  • in industry.

3
CHAPTER OUTLINE
  • 3.1 Introduction
  • 3.2 Overview of single phase IM
  • 3.3 Overview of Three-Phase IM
  • 3.4 Construction
  • 3.5 Principle of Operation
  • 3.6 Equivalent Circuit
  • Armature reaction
  • Power Flow, Losses and Efficiency
  • Torque-Speed Characteristics
  • 3.7 Speed Control

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3.1 INTRODUCTION
  • A induction machine can be used as either a
    induction generator or a induction motor.
  • IM transform electrical energy into mechanical
    energy
  • IM is a type of asynchronous AC motor where power
    is supplied to the rotating device by means of
    electromagnetic induction

4
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3.1 INTRODUCTION
  • popularly used in the industry and are used
    worldwide in many residential, commercial,
    industrial, and utility applications.
  • Main features cheap and low maintenance
  • (absence of brushes)
  • Main disadvantages speed control
  • is not easy

5
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3.2 OVERVIEW OF SINGLE PHASE IM
  • Construction similar to 3? induction motor
  • A single-phase motor is a rotating machine that
    has both main and auxiliary windings and a
    squirrel-cage rotor.
  • Supplying of both main and auxiliary windings
    enables the single-phase machine to be driven as
    a two-phase machine.

6
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3.2 OVERVIEW OF SINGLE PHASE IM
  • Home air conditioners
  • Kitchen fans
  • Washing machines
  • Industrial machines
  • Compressors
  • Refrigerators

8
3.2 OVERVIEW OF SINGLE PHASE IM
  • Types of 1? induction Motor
  • Split Phase Motor
  • Capacitor Start Motors
  • Capacitor Start, Capacitor Run
  • Shaded Pole Induction Motor
  • Universal Motor (ac series motors)

9
3.3 OVERVIEW OF 3 PHASE IM
  • Simple and rugged construction
  • Low cost and minimum maintenance
  • High reliability and sufficiently
  • high efficiency
  • The speed is frequency dependent
  • ? not easily to control the speed
  • ? thanks to power e

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3.3 OVERVIEW OF 3 PHASE IM
  • can be part of a pump or fan, or connected to
    some other form of mechanical equipment such as a
    winder, conveyor, or mixer.

10
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3.4 CONSTRUCTION
  • Basic parts of an AC motor rotor, stator,
    enclosure
  • The stator and the rotor are electrical circuits
    that perform as electromagnets.

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3.4 CONSTRUCTION (stator)
  • The stator - stationary part of the motor.
  • Stator laminations are stacked together forming a
    hollow cylinder.
  • Coils of insulated wire are inserted into slots
    of the stator core.
  • Each grouping of coils, together with the steel
    core it surrounds, form an electromagnet.

13
3.4 CONSTRUCTION (rotor)
  • The rotor is the rotating part of the motor
  • It can be found in two types
  • Squirrel cage (most common)
  • Wound rotor

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3.4 CONSTRUCTION (rotor)
  • Squirrel cage type
  • Rotor winding is composed of copper bars embedded
    in the rotor slots and shorted at both end by end
    rings
  • Simple, low cost, robust, low maintenance

14
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3.4 CONSTRUCTION (rotor)
  • Wound rotor type
  • Rotor winding is wound by wires. The winding
    terminals can be connected to external circuits
    through slip rings and brushes.
  • (similar with DC motor, with the coils connected
    together that make contact with brushes)
  • Easy to control speed, more expensive.

15
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3.4 CONSTRUCTION (enclosure)
  • The enclosure consists of a frame (or yoke) and
    two end brackets (or bearing housings). The
    stator is mounted inside the frame. The rotor
    fits inside the stator with a slight air gap
    separating it from the stator (NO direct physical
    connection)

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3.4 CONSTRUCTION (enclosure)
  • The enclosure protects the electrical and
    operating parts of the motor from harmful effects
    of the environment in which the motor operates.
  • Bearings, mounted on the shaft, support the rotor
    and allow it to turn. A fan, also mounted on the
    shaft, is used on the motor shown below for
    cooling.

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Nameplate
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3.5 PRINCIPLE OF OPERATION
19
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3.5 PRINCIPLE OF OPERATION
20
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Rotating Magnetic Field
  • When a 3 phase stator winding is connected to a 3
    phase voltage supply, 3 phase current will flow
    in the windings, which also will induced 3 phase
    flux in the stator.
  • These flux will rotate at a speed called a
    Synchronous Speed, ns. The flux is called as
    Rotating magnetic Field
  • Synchronous speed speed of rotating flux
  • Where p is the number of poles, and
  • f the frequency of supply

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Slip and Rotor Speed
  • Slip s
  • The rotor speed of an Induction machine is
    different from the speed of Rotating magnetic
    field. The difference of the speed is called
    slip.
  • Where ns synchronous speed (rpm)
  • nr mechanical speed of rotor (rpm)
  • under normal operating conditions, s 0.01
    0.05, which is very small and the actual speed is
    very close to synchronous speed.
  • Note that s is not negligible

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Slip and Rotor Speed
  • Rotor Speed
  • When the rotor move at rotor speed, nr (rps), the
    stator flux will circulate the rotor conductor at
    a speed of (ns-nr) per second. Hence, the
    frequency of the rotor is written as
  • Where s slip
  • f supply frequency

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Principle of Operation
  • Torque producing mechanism
  • When a 3 phase stator winding is connected to a 3
    phase voltage supply, 3 phase current will flow
    in the windings, hence the stator is energized.
  • A rotating flux F is produced in the air gap. The
    flux F induces a voltage Ea in the rotor winding
    (like a transformer).
  • The induced voltage produces rotor current, if
    rotor circuit is closed.
  • The rotor current interacts with the flux F,
    producing torque. The rotor rotates in the
    direction of the rotating flux.

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Direction of Rotor Rotates
  • Q How to change the direction of
  • rotation?
  • A Change the phase sequence of the
  • power supply.

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Equivalent Circuit of Induction Machines
  • Conventional equivalent circuit
  • Note
  • Never use three-phase equivalent circuit. Always
    use per- phase equivalent circuit.
  • The equivalent circuit always bases on the Y
    connection regardless of the actual connection of
    the motor.
  • Induction machine equivalent circuit is very
    similar to the single-phase equivalent circuit of
    transformer. It is composed of stator circuit and
    rotor circuit

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Equivalent Circuit of Induction Machines
  • Step1 Rotor winding is open
  • (The rotor will not rotate)
  • Note
  • the frequency of E2 is the same as that of E1
    since the rotor is at standstill. At standstill
    s1.

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Equivalent Circuit of Induction Machines
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Equivalent Circuit of Induction Machines
  • Step2 Rotor winding is shorted
  • (Under normal operating conditions, the rotor
    winding is shorted. The slip is s)
  • Note
  • the frequency of E2 is frsf because rotor is
    rotating.

30
Equivalent Circuit of Induction Machines
  • Step3 Eliminate f2
  • Keep the rotor current same

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Equivalent Circuit of Induction Machines
  • Step 4 Referred to the stator side
  • Note
  • X2 and R2 will be given or measured. In
    practice, we do not have to calculate them from
    above equations.
  • Always refer the rotor side parameters to stator
    side.
  • Rc represents core loss, which is the core loss
    of stator side.

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Equivalent Circuit of Induction Machines
  • IEEE recommended equivalent circuit
  • Note
  • Rc is omitted. The core loss is lumped with the
    rotational loss.

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Equivalent Circuit of Induction Machines
  • IEEE recommended equivalent circuit
  • Note can be separated into 2 PARTS
  • Purpose
  • to obtain the developed mechanical

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Analysis of Induction Machines
  • For simplicity, let assume
  • IsI1 , IRI2
  • (sstator, Rrotor)

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Analysis of Induction Machines
Note 1hp 746Watt
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36
EXAMPLE 1
  • A 4 poles, 3? Induction Motor operates from a
    supply which frequency is 50Hz. Calculate
  • The speed at which the magnetic field is rotating
  • The speed of the rotor when slip is 0.04
  • The frequency of the rotor when slip is 3.
  • The frequency of the rotor at standstill

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EXAMPLE 2
  • A 500hp, 3? 6 poles, 50Hz Induction Motor has a
    speed of 950rpm on full load. Calculate the slip.

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EXAMPLE 3
  • If the emf in the rotor of an 8 poles Induction
    Motor has a frequency of 1.5Hz and the supply
    frequency is 50Hz. Calculate the slip and the
    speed of the motor.

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EXAMPLE 4
  • A 440V, 50Hz, 6 poles, Y connected induction
    motor is rated at 135hp. The equivalent circuit
    parameters are
  • Rs0.084? RR0.066?
  • Xs0.2? XR0.165?
  • s 5 Xm6.9?
  • Determine the stator current, magnetism current
    and rotor current.
  • Solution
  • Given V440V, p6, f50Hz, 135hp

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Example 4 (Cont)
41
Example 4 (Cont)
42
Example 4 (Cont 1st Method)
43
Example 4 (Cont 2nd Method)
44
Power Flow Diagram
45
Power Flow Diagram
  • Ratio

Ratio makes the analysis simpler to find the
value of the particular power if we have another
particular power. For example
46
Efficiency
47
Example 5 (Cont from Ex 4)
  • Calculate
  • Stator Copper Loss
  • Air Gap Power
  • Power converted from electrical to mechanical
    power
  • Output power
  • Motor efficiency
  • Solution

48
Example 5 (Cont from Ex 4)
49
Torque-Equation
  • Torque, can be derived from power equation in
    term of mechanical power or electrical power.

50
Torque-Equation
  • Note that, Mechanical torque can written in terms
    of circuit parameters. This is determined by
    using approximation method

Hence, Plot Tm vs s
51
Torque-Equation
52
Example 6 (Cont from Ex 4)
  • Calculate
  • Mechanical torque
  • Output torque
  • Starting torque
  • Maximum torque and maximum slip

solution
53
Example 6 (Cont from Ex 4)
54
Example 6 (Cont from Ex 4)
55
Speed Control
  • There are 3 types of speed control of 3 phase
    induction machines
  • Varying rotor resistance
  • Varying supply voltage
  • Varying supply voltage and supply frequency

56
Varying rotor resistance
  • For wound rotor only
  • Speed is decreasing
  • Constant maximum torque
  • The speed at which max torque occurs changes
  • Disadvantages
  • large speed regulation
  • Power loss in Rext reduce the efficiency

57
Varying supply voltage
  • Maximum torque changes
  • The speed which at max torque occurs is constant
    (at max torque, XRRR/s
  • Relatively simple method uses power electronics
    circuit for voltage controller
  • Suitable for fan type load
  • Disadvantages
  • Large speed regulation since ns

58
Varying supply voltage and supply frequency
  • The best method since supply voltage and supply
    frequency is varied to keep V/f constant
  • Maintain speed regulation
  • uses power electronics circuit for frequency and
    voltage controller
  • Constant maximum torque
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