Download - Muhazam

ECA1212
Introduction to Electrical &
Electronics Engineering
Chapter 12: Introduction to Electric Machines
by Muhazam Mustapha, December 2011
Learning Outcome
By the end of this chapter students are
expected to:
• Be able to theoretically explain the various
types of electric motors
• Be able to theoretically explain the various
types of electric generators
• Be able to mathematically solve some
parameters of DC motors
Chapter Content
•
•
•
•
Electric Machines in General
DC Machines
Synchronous Machines
Induction Machines
Electric Machines
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Rotating Machines
• Electromechanical machines are
commonly rotational in nature
• The machines require one part to be static
and the other one to be rotating
– Stator: stationary
– Rotor: rotating
• Both stator and rotor produce magnetic
winding whose field will try to align each
other – this produces mechanical motion
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Rotor and Stator
Current
going in
Stator
winding
Rotor
winding
×
×
·
Stator Field
·
Rotor Field
Current
coming out
Stator
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Rotor
Commutator Action
Commutator reverses
current in coil every half
cycle
There can be more
than 1 pair of
commutators
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Windings
• Two types of magnetic windings:
– Armature: the winding connects to load
– Field: the winding only to produce field
• Either armature or field winding can be
located as rotor or stator
• The location of field and armature
determines the type of the machine
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Machine Types (Generator & Motor)
Type
DC
Synchronous
Induction
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Winding Type
Location
Current Type
Armature
Rotor
DC
Field
Stator
DC
Armature
Stator
AC
Field
Rotor
DC
Primary
Stator
AC
Secondary
Rotor
AC
DC Machines
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DC Machines
• DC Machines are hard to construct, but
easiest to discuss and analyze
• Hence all our mathematical discussion on
machines will be on DC machines
• Other machine type will be covered as
theory
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Configurations
• DC Machines can
be constructed in a
few configurations
depending on
series or parallel
structure or the
availability of a
second power
source
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Ra
Ia
Rf
Lf
If
Vf
Separately
Excited
La
Va
Configurations
Lf
Ra
Rf
Ia
Vf
Ra
La
Va
Vf
La
Lf
If
Shunt
Connected
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Series
Connected
Va
Configurations
Ra
Ia
Series
Winding
La
Shunt
Winding
Series
Winding
Ia
Va
La
Shunt
Winding
Short-Shunt
Compound
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Ra
Long-Shunt
Compound
Va
Steady State Equations
• Referring to the
following DC
machine model,
we can deduce
some formulas for
motor and
generator at
constant speed
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Is
If
RS
Ra
Rx
LS
Ia
VL or Vs
La
Rf
Eb, ωm
Lf
Steady State Equations
• Generator
Eb  k am
T
P
m

Eb I a
m
 k aI a
VL  Eb  I a Ra  I S RS
Ia  IS  I f
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Steady State Equations
• Motor
Eb  k am
T
P
m

Eb I a
m
 k aI a
VL  Eb  I a Ra  I s RS
Is  I f  Ia
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Machine Constant
• The armature
constant of ka
pN
ka 
2M
p = number of magnetic poles
N = number of conductors per coil
M = number of parallel paths in armature winding
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Conversions
60
n  m
2
n = round per minute, r/min
ωm = radian per second, rad/s
1 horse power = 746 watts
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Synchronous Machines
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Alternator
• Just another word for AC generator
• Normally a permanent magnet or a DC
powered electromagnet will be placed at
rotor to generate AC current
• Stator would be wound with solenoid that
carries the generated energy – there can
be more than one windings hence it can
generate more than 1 phase of electricity
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Alternator
×
×
N
×
·
Single
Phase
S
·
·
Coils at
stator
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Three
Phase
Synchronous Motor
• Virtually identical to alternator
• Needs a DC voltage exciter at rotor to start
• Called synchronous because it spins at
the same rate as the AC frequency used to
drive it
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Induction Machines
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Induction Motor
• The stator part is almost identical to
synchronous motor
• AC current (single or multi-phase) will be
fed into stator – produces spinning field
• There is no power or permanent magnet
placed in the rotor
• Rotor and stator are electrically separated
• Then how mechanical force is applied to
the rotor?
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Induction Motor
• Mechanical motion is possible by the
induction process that is identical to the
one in transformer
• The changes in the magnetic flux from
stator will induce current into the rotor
winding and causes magnetic attraction or
repel between stator and rotor poles
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Induction Motor
• The changes of the magnetic field need to
involve the cutting of the rotor coils
(Faraday’s Law)
• This cutting is what called ‘slip’ between
the rate of stator’s field rotation and the
rate of rotor’s spin
• Without the slip induction machine couldn’t
work
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Induction Motor
• The ‘slippings’ also means that the rotation
of rotor is not in-sync with the stator field
rotation rate
• This is the main electrical difference
between synchronous machine and
induction machine
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Induction Generator
• Makes use of the same induction concept
in induction motor – slipping process
• It requires a starting power at rotor to
produce magnetic field for the induction
process to start
• After that, the power generated by the
generator itself will be used to produce the
needed rotor magnetic field
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