EEE1012
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

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
×
·
·
Current coming out
Stator
Rotor winding
×
Rotor
Stator Field
Rotor Field

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)
DC
Type Winding Type Location Current Type
Armature Rotor DC
Field Stator DC
Synchronous Armature Stator AC
Field Rotor DC
Induction Primary Stator
Secondary Rotor
AC
AC

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
R f
R a
I a
L f
I f
V f
Separately
Excited
L a
V a

Configurations
R f
L f
I f
R a
V f
I a
L a
V a
Shunt
Connected
R a
L f
L a
V f
Series
Connected
V a

Configurations
R a
I a
L a
Series
Winding
V a
Shunt
Winding
Short-Shunt
Compound
Shunt
Winding
R a
I a
L a
Series
Winding
V a
Long-Shunt
Compound

Steady State Equations
• Referring to the following DC machine model, we can deduce some formulas for motor and generator at constant speed
R f
R x
I f
L f
R
S
L
S
I s
R a
I a
L a
E b
, ω m
V
L or V s

Steady State Equations
• Generator
E b
 k a
 m
T
V
L


P
 m
E b
I a

I
S



I f
E b
 m
I
I a
R a a

 k a

I
I
S
R
S a

Steady State Equations
• Motor
E b
 k a
 m
T
V
L


P
 m
E b
I s

I f



I a
E b
 m
I a
I a
R a

 k a

I
I s
R
S a

Machine Constant
• The armature constant of k a k a
 pN
2

M p = number of magnetic poles
N = number of conductors per coil
M = number of parallel paths in armature winding

Conversions n

60
2

 m n = round per minute, r/min
ω m
= radian per second, rad/s
1 horse power = 746 watts

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
Single
Phase
Coils at stator
·
×
×
N
·
S
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 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