Chapter 3: INDUCTION MOTOR

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Chapter 3INDUCTION MOTOR
BEE2133 ELECTRICAL MACHINE POWER SYSTEM
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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.

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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

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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

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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.

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3.2 OVERVIEW OF SINGLE PHASE IM

• Home air conditioners
• Kitchen fans
• Washing machines
• Industrial machines
• Compressors
• Refrigerators

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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)

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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.

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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.

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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

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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.

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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
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3.5 PRINCIPLE OF OPERATION
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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.

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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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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)
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Example 4 (Cont)
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Example 4 (Cont 1st Method)
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Example 4 (Cont 2nd Method)
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Power Flow Diagram
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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
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Efficiency
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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

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Example 5 (Cont from Ex 4)
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Torque-Equation

• Torque, can be derived from power equation in
  term of mechanical power or electrical power.

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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
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Torque-Equation
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Example 6 (Cont from Ex 4)

• Calculate
• Mechanical torque
• Output torque
• Starting torque
• Maximum torque and maximum slip

solution
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Example 6 (Cont from Ex 4)
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Example 6 (Cont from Ex 4)
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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

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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

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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

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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