ELB044 Lecture 18. Introduction to Induction Machines

Dr. Francisco M. Gonzalez-Longatt 1/41ELB044 Electrotechnology
ELB044 Electrotechnology
Lecture 18
Induction Machines
Dr Francisco M. Gonzalez-Longatt
f.gonzalez-longatt@lboro.ac.uk
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 of
induction machine and to generation of
rotating magnetic field produced due to three
coils using AC current

Specific Objectives
1) Identify the main components of induction
machine.
(2) Recognize the generation of rotating magnetic
field 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

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 Diagram
Physical 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 Core
Cooper 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 bearings
Squirrel 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.
rB
b
'
a
'
c
a
'
b
c sB
This Section presents the
fundamentals behind the
stator magnetic field a
rotating magnetic field

Single Coil (1/2)
• Single coil AA’:
➀
➁
➂
➀ ➁ ➂
'( ) sinAA Mi t I t
' 0AA 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

• 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’

• Energizing two sets of coils together in sequence
•
• Compass settles half way between poles

• 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

• Three-Phase supply provides the correct sequence
for stator coils
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’B0AI 
0AI 
0CI 
0CI 
0BI 
0BI 

• Three-Phase supply provides the correct sequence
for stator coils
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
0AI 
0AI 
0CI 
0CI 
0BI 
0BI 
0AI 
0AI 

Animation
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
S
SB
ω
These magnetic poles
complete one
mechanical rotation
around the stator
surface for each
electrical cycle of
current.
N S

• 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
S
SB
ω[ ] [ ] mf Hz f rps
[ ] [ ]
sec sec
e m
rad rad
  
Two
Poles
Machine N S

• 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
'
2c
1a
'
2b
2c
'
2a
2b
'
1ca’
b
c’
a
b’
c
One south Pole
One North Pole
Two south Pole
Two North Pole

• 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
'
2c
1a
'
2b
2c
'
2a
2b
'
1c
mB 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
N
f f
2
  poles
e m
N
120
 polesN
f 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
120 e
sync
poles
f
n 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
120
f 120
polesN
f n 50f Hz
120
poles
e
N
f n
50f HzAnswer

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

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