Transcript Chapter 9: Introduction to Electric Machines

EEE1012
Introduction to Electrical &
Electronics Engineering
Chapter 9: Introduction to Electric Machines
by Muhazam Mustapha, October 2010
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
Rotating Machines
• Electromechanical machines are
commonly rotational in nature
• The machines require one 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
Rotor and Stator
Current
going in
Stator
winding
Rotor
winding
×
×
·
Stator Field
·
Rotor Field
Current
coming out
Stator
Rotor
Commutator Action
Commutator reverses
current in coil every half
cycle
There can be more
than 1 pair of
commutators
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
Machine Types (Generator & Motor)
Type
DC
Synchronous
Induction
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
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
Configurations
• DC Machines can
be constructed in a
few configurations
depending on
series or parallel
structure or the
availability of a
second power
source
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
Series
Connected
Va
Configurations
Ra
Ia
Series
Winding
La
Shunt
Winding
Ra
Series
Winding
Ia
Va
La
Shunt
Winding
Short-Shunt
Compound
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
Is
LS
If
Ra
Rx
RS
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
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
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
Conversions
60
n  m
2
n = round per minute, r/min
ωm = radian per second, rad/s
1 horse power = 746 watts
Synchronous Machines
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
Alternator
×
×
N
×
·
Single
Phase
S
·
·
Coils at
stator
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
Induction Machines
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?
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
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
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
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