Topic 2
Electric Machines
Faculty of Electrical Engineering
Universiti Teknologi Malaysia
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Contents
■  Basic construction and principles
■  DC machines
■  Synchronous machines
■  Induction machines
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1
Introduction
One of energy can be obtained from the other form
with the help of converters. Converters that are
used to continuously translate electrical input to
mechanical output or vice versa are called
electrical machines.
The process of translation is known as
electromechanical energy conversion.
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Electrical
system
e, i
Electric
Machine
Mechanical
system
T, n
Motor
Energy flow
Generator
• An electrical machine is link between an electrical
system and a mechanical system.
• Conversion from mechanical to electrical: generator
• Conversion from electrical to mechanical: motor
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2
Electrical
Machines
DC
machine
AC
machine
Synchronous
machine
Induction
machine
• Machines are called AC machines (generators or
motors) if the electrical system is AC.
• DC machines (generators or motors) if the electrical
system is DC.
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Electrical
system
e, i
Coupling
magnetic
fields
Mechanical
system
T, n
Two electromagnetic phenomena in the electric machines:
• When a conductor moves in a magnetic field, voltage is
induced in the conductor.
• When a current-carrying conductor is placed in a magnetic
field, the conductor experiences a mechanical force.
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3
Electric Machines
Basic Structure
a'
b
c
• The structure of an electric
machine has two major
components, stator and rotor,
separated by the air gap.
Rotor
c'
b'
a
Stator
Stator
• Stator:
Does not move and normally is
the outer frame of the machine.
R
B’
N
Y
S
Y’
Rotor
B
R’
• Rotor:
Is free to move and normally is
the inner part of the machine.
• Both rotor and stator are made
of ferromagnetic materials.
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DC Machines
Construction
8
4
Induction Machines
Construction
9
Synchronous Machines
Construction
10
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Contents
■  Basic construction and principles
■  DC machines
■  Synchronous machines
■  Induction machines
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Direct Current (DC)
Machines Fundamentals
• Generator action: An emf (voltage) is induced
in a conductor if it moves through a magnetic
field.
• Motor action: A force is induced in a conductor
that has a current going through it and placed in a
magnetic field
• Any DC machine can act either as a generator or
as a motor.
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6
■  DC Machine is most often used for a motor.
■  The major advantages of dc machines are the easy speed and
torque regulation.
■  However, their application is limited to mills, mines and trains.
As examples, trolleys and underground subway cars may use dc
motors.
■  In the past, automobiles were equipped with dc dynamos to
charge their batteries.
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■  Even today the starter is a series dc motor
■  However, the recent development of power electronics has
reduced the use of dc motors and generators.
■  The electronically controlled ac drives are gradually replacing
the dc motor drives in factories.
■  Nevertheless, a large number of dc motors are still used by
industry and several thousand are sold annually.
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DC Generator Fundamentals
e =(B × v). l
e =B l v sinα cosβ
e = induced voltage, v = velocity of the conductor, B
= flux density and l is the length of the conductor
α - angle between the direction in
which the conductor is moving and the
flux is acting.
β - smallest possible angle the
conductor makes with the direction of,
the vector product, ( v × B) and for
maximum induction,
b = 0. Hence, e = Blv for most cases.
( v × B) indicates the direction of the
current flow in the conductor, or the
polarity of the emf.
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DC machine Construction
• Stator: Stationary part of the
machine. The stator carries a field
winding that is used to produce the
required magnetic field by DC
excitation. Often know as the field.
• Rotor: The rotor is the rotating
part of the machine. The rotor
carries a distributed winding, and is
the winding where the emf is
induced. Also known as the
armature.
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8
DC motor stator with poles
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Rotor of a dc motor.
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9
Details of the commutator of a dc motor.
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DC Motors Equivalent circuit
The equivalent circuit of DC Motors (and Generators)
has two components:
• Armature circuit: it can be represented by a voltage
source and a resistance connected in series (the
armature resistance). The armature winding has a
resistance, Ra.
• The field circuit: It is represented by a winding that
generates the magnetic field and a resistance connected
in series. The field winding has resistance Rf.
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10
Classification of DC Motors
• Separately Excited and Shunt Motors
Field and armature windings are either connected
separate or in parallel.
• Series Motors
Field and armature windings are connected in series.
• Compound Motors
Has both shunt and series field so it combines
features of series and shunt motors.
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Separately Excited DC Motors
– The armature winding supplies the load.
– The field winding is supplied by a separate DC source whose
voltage is variable.
– Good speed control.
Ea
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Shunt DC Motors
– The armature and field windings are connected in parallel.
– Constant speed operation.
– Applications: centrifugal pump, machine tools, blowers fans,
reciprocating pumps.
R
a
Ia
Rf
Ea
Pout
m
It
Vt
If
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Series DC Motors
– The armature and field winding are connected in
series.
– High starting torque.
- Applications: hoists, electric trains, conveyors,
Ra
elevators, electric cars.
Rf
Ea
It
Vt
Pout
m
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Compound dc motors
Variable speed motors.
It has a high starting torque and the no-load speed is controllable
unlike in series motors.
Applications: Rolling mills, sudden temporary loads, heavy
machine tools, punches.
Ra
–
–
–
Rfs
Ia
It
Ea
Pout
m
Vt
Rfp
Ifp
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Comparison of DC Motors
Shunt Motors: “Constant speed” motor (speed regulation is very
good). Adjustable speed, medium starting torque.
Applications: centrifugal pump, machine tools, blowers fans,
reciprocating pumps, etc.
Series Motors: Variable speed motor which changes speed drastically
from one load condition to another. It has a high starting torque.
Applications: hoists, electric trains, conveyors, elevators, electric
cars.
Compound motors: Variable speed motors. It has a high starting
torque and the no-load speed is controllable unlike in series motors.
Applications: Rolling mills, sudden temporary loads, heavy machine
tools, punches, etc
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13
Power Flow and Losses in
DC Motors
Protational
VtIt
VaIt
VaIa
EaIa
Pout
Pinput
It 2Rsr
It 2Rt
Ia 2Ra
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14
Speed Control of DC Motors
Speed can be controlled by varying:
1) Armature circuit resistance using an external
resistance Ra Ext.
2) IF can be varied by using an external resistance Radj
in series with Rf to control the flux, hence the speed.
3) The applied voltage to the armature circuit resistance,
if the motor is separately excited
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Contents
■  Basic construction and principles
■  DC machines
■  Synchronous machines
■  Induction machines
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Classification of
AC Rotating Machines
Synchronous Machines:
•Synchronous Generators: A primary source of electrical energy.
•Synchronous Motors: Used as motors as well as power factor
compensators (synchronous condensers).
Asynchronous (Induction) Machines:
•Induction Motors: Most widely used electrical motors in both domestic
and industrial applications.
•Induction Generators: Due to lack of a separate field excitation, these
machines are rarely used as generators. 31
Synchronous Machine
■  Unlike induction machines, the rotating air gap field and the rotor
rotate at the same speed, called the synchronous speed.
■  Synchronous machines are used primarily as generators of electrical
power, called synchronous generators or alternators.
■  They are usually large machines generating electrical power at hydro,
nuclear, or thermal power stations.
■  Application as a motor: pumps in generating stations, electric clocks,
timers, and so forth where constant speed is desired.
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Synchronous Machines
Generator
Exciter
View of a two-pole round rotor generator and exciter.
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Synchronous Machine
Round Rotor Machine
•The stator is a ring shaped
laminated iron-core with
slots.
•Three phase windings are
placed in the slots.
•Round solid iron rotor with
slots.
•A single winding is placed in
the slots. Dc current is
supplied through slip rings.
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17
Round Rotor Machine
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Synchronous Machine
Salient Rotor Machine
•The stator has a laminated ironcore with slots and three phase
windings placed in the slots.
•The rotor has salient poles excited
by dc current.
•DC current is supplied to the rotor
through slip-rings and brushes.
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Salient Rotor Machine
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Synchronous Generator
Principle of Operation
1) From an external source, the field
winding is supplied with a DC
current -> excitation.
2) Rotor (field) winding is mechanically
turned (rotated) at synchronous
speed.
3) The rotating magnetic field produced
by the field current induces voltages
in the outer stator (armature) winding.
The frequency of these voltages is in
synchronism with the rotor speed.
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Parallel Operation of
Synchronous Generator
Generators are rarely used in isolated situations. More commonly,
generators are used in parallel, often massively in parallel, such as in
the power grid. The following steps must be adhered to:
•when adding a generator to an existing power grid:
1) RMS line voltages of the two generators must be the same.
2) Phase sequence must be the same.
3) Phase angles of the corresponding phases must be the same.
4) Frequency must be the same.
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Contents
■  Basic construction and principles
■  DC machines
■  Synchronous machines
■  Induction machines
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Induction Machine
■  The induction machine is the most rugged and the most widely used
machine in industry.
■  Both stator and rotor winding carry alternating currents.
■  The alternating current (ac) is supplied to the stator winding directly and
to the rotor winding by induction – hence the name induction machine.
■  Application (1φ): washing machines, refrigerators, blenders, juice mixers,
stereo turntables, etc.
■  2φ induction motors are used primarily as servomotors in a control
system.
■  Application 3φ: pumps, fans, compressors, paper mills, textile mills, etc.
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Induction Motors
■  The single-phase
induction motor is
the most frequently
used motor in the
world
■  Most appliances,
such as washing
machines and
refrigerators, use a
single-phase
induction machine
■  Highly reliable and
economical
Single-phase induction motor.
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Induction Motors
■  For industrial
applications, the
three-phase
induction motor is
used to drive
machines
■  Large three-phase
induction motor.
(Courtesy Siemens).
Housing
Motor
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Induction Machine
General
•The induction machine is used as the
most common motors in different
applications.
Construction of Induction Motor
•It has a stator and a rotor like other
type of motors.
•2 different type of rotors:
1-squirrel-cage winding,
2-Wound-rotor
•Both three-phase and single-phase
motors are widely used.
•Majority of the motors used by
industry are squirrel-cage
induction motors.
A typical motor consists of two parts:
1-An outside stationary stator having coils
supplied with AC current to produce a
rotating magnetic field,
2-An inside rotor attached to the output
shaft that is given a torque by the
rotating field.
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Induction Motor
Basic principles:
•An AC current is applied in the stator
armature which generates a flux in
the stator magnetic circuit.
•This flux induces an emf in the
conducting bars of rotor as they are
“cut” by the flux while the magnet
is being moved (E = BVL
(Faraday’s Law))
•A current flows in the rotor circuit
due to the induced emf, which in
term produces a force, (F = BIL )
can be changed to the torque as the
output. Induction motor components.
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Induction Motor
Stator construction
–The stator of an induction motor
is laminated iron core with slots
similar to a stator of a
synchronous machine
–Coils are placed in the slots to
form a three or single phase
winding.
Single-phase stator with windings.
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Induction Motors Magnetic Circuit
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Squirrel-cage Rotor
– Rotor is from laminated iron
core with slots.
– Metal (Aluminum) bars are
molded in the slots instead of
a winding.
– Two rings short circuits the
bars.–Most of single phase
induction motors have
Squirrel-Cage rotor.
– One or 2 fans are attached to
the shaft in the sides of rotor
to cool the circuit.
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Induction Motor
It is usually for large 3 phase induction
motors.
•Rotor has a winding the same as stator
and the end of each phase is
connected to a slip ring.
•Three brushes contact the three sliprings to three connected resistances
(3-phase Y) for reduction of starting
current and speed control.
Compared to squirrel cage rotors, wound
rotor motors are expensive and require
maintenance of the slip rings and
brushes, so it is not so common in
industry applications.
•Wound rotor induction motor was the
standard form for variable speed
control before the advent of motor
Rotor of a large
induction motor.
(Courtesy Siemens).
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