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Chapter 3 INDUCTION MOTOR

BEE2133 ELECTRICAL MACHINE POWER SYSTEM

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.

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

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)

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.

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.

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)

Example 4 (Cont)

Example 4 (Cont 1st Method)

Example 4 (Cont 2nd Method)

Power Flow Diagram

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

Efficiency

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

Example 5 (Cont from Ex 4)

Torque-Equation

- Torque, can be derived from power equation in

term of mechanical power or electrical power.

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

Torque-Equation

Example 6 (Cont from Ex 4)

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

solution

Example 6 (Cont from Ex 4)

Example 6 (Cont from Ex 4)

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

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

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

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