ELB044 Lecture 18. Introduction to Induction Machines

Dr. Francisco M. Gonzalez-Longatt 1/41ELB044 Electrotechnology

ELB044 Electrotechnology

Lecture 18

Induction Machines

Dr Francisco M. [email protected]

http://www.fglongatt.org

mailto:[email protected]

http://www.fglongatt.org/


Agenda• Lecture Outline

– Lesson Opening

– Objectives

– Rotating Magnetic Field

– Induction Machine

– Questions and Answers

– Lesson closing and summary


Lesson Opening Electrical Machines

Rotating Machines Transformers (static machines)

AC MachinesDC Machines

Operation Mode:

• Generator

• Motor

Separately Excited

Series

Shunt or Parallel

Compounds: Series+Parallel

Synchronous

Machines

Operation Mode:

• Generator

• Motor

Asynchronous

Machines or

Induction

Machines

Squirrel-Cage

Wound Rotor

Cylindrical Rotor

Salient Pole Rotor

Today’s

Lesson

Last Week Lesson

Why???



General objective

Understand the most basic structure ofinduction machine and to generation ofrotating magnetic field produced due to threecoils using AC current



Specific Objectives

1) Identify the main components of inductionmachine.

(2) Recognize the generation of rotating magneticfield produced due to three coils using AC current



An Introduction to

Induction Machines (IM)

How does it work…


How does it work…

Asynchronous Induction Motor. How does it work.avi. Category: Education, Licence: Standard YouTube Licence

Source: https://www.youtube.com/watch?v=N8LUOTQKXlk

https://www.youtube.com/watch?v=N8LUOTQKXlk



Structure of an Induction

Machine


Stator and Rotor• Every induction machine (motor or generator) has

two main parts:

• Rotating part (rotor) and

• Stationary part (stator).

Stator

Rotor

Machine Shaft

Schematic DiagramPhysical Structure


Additional Components• Additional components with specific uses.

Steel

Frame


Stator• A stationary stator:

– Consisting of a steel frame that supports a hollow,

cylindrical core.

– Core, constructed from stacked laminations, having a

number of evenly spaced slots, providing the space for

the stator winding.


Rotor• A Revolving Rotor:

– Composed of punched laminations, stacked to create a

series of rotor slots, providing space for the rotor

winding.

Single Cage Rotor Wounded Rotor


Rotor Winding• One of two types of rotor windings.

– Conventional 3-phase windings made of insulated wire

(wound-rotor) similar to the winding on the stator.

– Aluminum bus bars shorted together at the ends by two

aluminum rings, forming a squirrel-cage shaped

circuit (squirrel-cage).

Welds at all

joints

Shaft

Iron CoreCooper or aluminium bars

Welds holding copper or

aluminium bars to end

ring

Aluminiu

m or

copper end

rings

Rotor Core

Rotor

winding

Slip rings

Ball bearings

Cooling Fan

Ball bearingsSquirrel Cage

Rotor

Wound Rotor



Rotating Magnetic Field


The Rotating Magnetic Field• The basic idea of an electric machine is to

generate two magnetic fields:

– Rotor magnetic field (Br) and

– Stator magnetic field (Bs) and make the

stator field rotating.

rBb

'a

'c

a

'b

c sBThis Section presents the

fundamentals behind the

stator magnetic field a

rotating magnetic field


Single Coil (1/2)• Single coil AA’:

➀

➁➂

➀ ➁ ➂

'( ) sin

AA Mi t I t

'0

AA RMSI I

Time domain Representation

Phasor domain Representation

'( )

AAi t

'( )

AAB t

The dark green plot in the phase diagrams is the resulting relative

magnitude of the magnetic field created by the sine wave source

currents. Within the vector plots, the red ball represents the time

position and the dashed black vector is the resultant vector when the

phases are added togetherSouce: ACRotatingMagneticFieldPrinciple.cdf


Single Coil (2/2)

Magnetic flux

density (BAA’)

is fixed in the

space and

changing

magnitude and

direction

Souce: ACRotatingMagneticFieldPrinciple.cdf

"AC Rotating Magnetic Field Principle"

from the Wolfram Demonstrations

Project http://demonstrations.wolfram.co

m/ACRotatingMagneticFieldPrinciple/


Two Coils (1/2)• Two coils AA’ and BB’:

➀ ➁ ➂

'

'

( ) sin

( ) sin 90

AA M

BB M

i t I t

i t I t

'

'

0

90

AA RMS

BB RMS

I I

I I



'( )

AAi t

( )sB t






➀

➁➂

'( )

BBi t

sB

'AAB

'BBB


Two Coils (2/2)

Two magnetic flux

density (BAA’ and

BBB’) are fixed in

the space and

changing

magnitude and

direction


Total magnetic flux

density in the

stator (BS) is

rotating and

constant

amplitude






Three Coils (1/2)• Three coils AA’, BB’, and CC’:

➀ ➁ ➂

'

'

'

( ) sin

( ) sin 120

( ) sin 120

AA M

BB M

CC M

i t I t

i t I t

i t I t

'

'

'

0

120

120

AA RMS

BB RMS

CC RMS

I I

I I

I I



( )sB t






➀

➁➂

'( )

BBi t

sB

'AAB

'BBB

'CCB

'BBB

'( )

CCi t

'( )

AAi t


Three Coils (2/2)

Three magnetic

flux density (BAA’

BBB’ and BCC’) are

fixed in the space

and changing

magnitude and

direction


Total magnetic flux

density in the

stator (BS) is

rotating and

constant

amplitude






The Rotating Magnetic Field (1/6)• Simple stator made of three pairs of coils around

iron pole pieces.

Iron Pole

Pieces

Phase connections

A’

A

B

B’

C

C’

Iron

Stator

Ring

Phase

Coils

Current enters coil

A and leaves coils

A’

Magnetic flux set

up in coils with

North Pole at the

bottom and South

Pole at the top


The Rotating Magnetic Field (2/6)• Changing which coils are energised alters

direction of magnetic flux

AA’ energised CC’ energised BB’ energised

N

S

C

B’

C’

A

B

A’

C

B’

C’

A

B

A’

C

B’

C’

A

B

A’


The Rotating Magnetic Field (3/6)• Energizing two sets of coils together in sequence

•

• Compass settles half way between poles


The Rotating Magnetic Field (4/6)• Sequence produces one complete rotation of the

magnetic field

➀ CC’ & B’B

➁ AA’ & B’B

➂ AA’ & C’C

➃ BB’ & C’C

➄ BB’ & A’A

➅ CC’ & A’A

➆ CC’ & B’B


The Rotating Magnetic Field (5/6)• Three-Phase supply provides the correct sequence

for stator coils➀ CC’ & B’B

➁ AA’ & B’B

➂ AA’ & C’C

➃ BB’ & C’C

➄ BB’ & A’A

➅ CC’ & A’A

➆ CC’ & B’B0AI

0AI

0CI

0CI

0BI

0BI


The Rotating Magnetic Field (6/6)• Three-Phase supply provides the correct sequence

for stator coils➀ CC’ & B’B

➁ AA’ & B’B

➂ AA’ & C’C

➃ BB’ & C’C

➄ BB’ & A’A

➅ CC’ & A’A

➆ CC’ & B’B0

AI

0AI

0CI

0CI

0BI

0BI

0AI

0AI


Animation

http://www.ece.umn.edu/users/riaz/animations/spacevecmovie.html

http://www.ece.umn.edu/users/riaz/animations/spacevecmovie.html


Rotational Speed (1/4)

• Relationship between electrical frequency

and speed of field rotation

• The stator rotating magnetic (BS) field can be

represented as a north pole and a south pole.

a’

b

c’

a

b’

c

ω

N

ω

SSB

ω

These magnetic poles

complete one

mechanical rotation

around the stator

surface for each

electrical cycle of

current.

N S


Rotational Speed (2/4)• The mechanical speed of rotation of the magnetic

field equals to the electrical frequency (Two Pole

Machine)a’

b

c’

a

b’

c

ω

N

ω

SSB

ω[ ] [ ] mf Hz f rps

[ ] [ ]sec sec

e m

rad rad

Two

Poles

Machine N S


Rotational Speed (3/4)• What if 3 additional windings will be added? The

new sequence will be: a-c’-b-a’-c-b’-a-c’-b-a’-c-b’ and, when

3-phase current is applied to the stator, two north poles

and two south poles will be produced.

In this winding, a pole moves only halfway around the

stator. 2a

'

1b

1c

'

1a

1b

'

2c1a

'

2b

2c

'

2a

2b

'

1ca’

b

c’

a

b’

c

One south Pole

One North PoleTwo south Pole

Two North Pole


Rotational Speed (4/4)• The relationship between the electrical angle e

(current’s phase change) and the mechanical angle m(at which the magnetic field rotates) in this situation is:

• Therefore, for a four-pole stator:2a

'

1b

1c

'

1a

1b

'

2c1a

'

2b

2c

'

2a

2b

'

1c

mB B

B B

S

S N

N

m

m

m

2e m

[ ] [ ] mf Hz f rps

[ ] [ ]sec sec

e m

rad rad

Four

Poles

Machine


Rotational Speed: General Case• Relationship between electrical frequency

and speed of field rotation:

- For an AC machine with Npoles in its stator:

- Relating the electrical frequency to the machine

speed in rpm:

2

poles

e m

N

2

poles

m

Nf f

2

poles

e m

N

120

polesNf n


Rotational Speed: General Case • A rotating magnetic field with constant

magnitude is produced, rotating with a speed.

Npoles 50 Hz 60 Hz

2 3000 3600

4 1500 1800

6 1000 1200

8 750 900

10 600 720

12 500 600

120e

sync

poles

fn rpm

N



Example

Demonstrative example of rotational magnetic field


Example:• What is the frequency (f) of a four-pole alternator

operating at a speed of n = 1500 rpm?

• Applying the mathematical definition for the

frequency on AC machine:

where Npoles = 4 and mec = 1500 rpm, then

4 1500

120f 120

polesN

f n 50f Hz

120

poles

e

Nf n

50f HzAnswer



Closure and Summary


Induction Machines• Basic structure

• How does it work…

Rotor

Stator

Faraday's law of induction

Stator created a

rotating magnetic field

ELB044 Lecture 18. Introduction to Induction Machines

Education

Transcript of ELB044 Lecture 18. Introduction to Induction Machines