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Electrical Machines IWeek 1: Overview, Construction and EMF equation
Course Contents
Definition of the magnetic terms, magnetic materials and the
B-H curve.
Magnetic circuits principles.
Electromechanical Energy Conversion Principles.
Force and torque equations in magnetic circuits.
Construction of a DC machine.
EMF and torque equations in dc machines.
Armature windings and commutator design.
Armature reaction and compensation techniques.
Self excitation of dc generators.
External characteristics of dc generators.
Kinds of losses and efficiency of dc machines.
Torque and speed characteristics of dc motors.
Speed control of dc motors.
Starting of dc motors.
DC Motor electrical braking techniques.
Electrical Machines
I
Study
Understand
Lab
work
ReportsExamples
Ask
Read
Course Work:
Course work:
1- Every week assignment (solve questions related to the lecture): to behanded in every week
for points
2- points are transformed to marks if you are consistent in delivering your reports
3- NO late submission are allowed
Lab reports:
1- Contribute to almost 10 marks – related to your physical presence in lab
Quizes:
1- 7th, 12th, ….etc.
Final
Its not about marks
in tests.. Its about
continuously working
hard all semester!
Introduction
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.
Ele
ctr
ical M
achin
es
DC machines
Motor
Generator
AC MachinesTransformers
Induction motor
Synchronous generator
Special Machines
Faraday's Law
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. Not all machines have this feature
except for the DC machine
Lets formulate this in a more “scientific way”
Applications of DC Motors:
1. D.C Shunt Motors: It is a constant speed motor. Where the speed is required to remain almost constant
from no-load to full load. Where the load has to be driven at a number of speeds and any
one of which is nearly constant.
• Lathes
• Drills
• Boring mills
• Shapers
• Spinning and Weaving machines.
2. D.C Series motor:It is a variable speed motor. The speed is low at high torque. At light or no load ,the
motor speed attains dangerously high speed.
• Electric traction
• Cranes
• Elevators
• Air compressor
• Vacuum cleaner
• Hair drier
• Sewing machine
LETS BRAIN STORM!!!!
WHAT DO YOU THINK IS
INSIDE THE MACHINE????
Construction of DC machine
Rotor: rotating part of the
machine
Stator: Stationary part of the machine
Two electrical circuits present in
the dc machine:
1- Field circuit
2- Armature circuit
Stator
1- Stator:
Frame: provides physical support
Poles: projects
inwards and
provides a
path for the
magnetic flux
Poles: the end of the poles
that are close to rotor
“spread out” over the rotor
surface to distribute flux
evenly over the rotor
surface. We call the end as
“pole shoe”. Due to their
spread out they are often
called Salient Poles.
Field windings: windings responsible for
magnetic flux production
Air gap
Air gapAir gap
Inter Poles: located
between poles and
used to overcome
armature reaction
THE STATOR COULD BE
LAMINATED OR MADE OF
SINGLE CAST PIECE OF
METAL
Armature
2- Rotor: Rotating part of machineRotor of dc machine is often called “armature” as it holds the armature windings
THE ROTOR IS COMPOSED OF MANY LAMINATIONS
STAMPED FROM A STEEL PLATE.
Commutator: built on the shaft of the rotor at one end of
the core. Made of copper bars insulated by mica (ورنيش ).
Mica is very hard and is harder than the commutator
material itself for good sticking. Serves as a “mechanical
rectifier”.
Brushes: made of carbon, graphite or a mixture
of both. They have high CONDUCTIVITY and low
friction coefficient to reduce the wear but they
are softer than commutator to avoid
commutator wear. It is very much affected by
the current flowing in them and how they are
adjusted.
Armature winding: carries current crossing the
field, thus creating shaft torque in a rotating
machine or force in a linear machine as well as
generate an electromotive force (EMF). Some
call it “The power-producing component” of an
alternator, generator, dynamo or motor.
Faraday Laws
1- If a flux passes through a turn of coil of a wire, a voltage will be
induced in the turn of wire that is directly proportional to the rate of
change in flux with respect to time.
tN
D
DF-=e
e= average emf (V)
N= number of turns
ф = flux passing through the turn
t= time
-ve sign is an expression of Lenz’s law: The direction of
the voltage buildup in a coil is such that if the coil end
were short cct, it would produce current that would
cause a flux opposing the original flux changeф𝐵
Opposing flux
I
e
+
-
If a flux is increasing in strength, then the voltage
built up in the coil will tend to establish a flux that
will oppose the increase
HOW CAN MAGNETIC FIELD AFFECT THE
SURROUNDING
في حاله وجود ملف في مجال مغناطيسي،سيلاحظ وجود فرق جهد حثي علي اطراف الملف و هذا الجهد سيؤدي
لتوليد مجال اخر عكس اتجاه المجال الاساسي
FLUX ALREADY
PRESENT
FLUX CREATED BY
EMF
Faraday Laws HOW CAN MAGNETIC FIELD AFFECT THE
SURROUNDING
2- Magnetic field induces a force on a current carrying wire within the
field.
𝐹
iL
XX
XX
XX
XX
XX
XX
XX
XX
𝐵B= magnetic flux density
(wb/m2)
i= current (A)
F= force induced (N)
L= length of conductor (m)
Force direction is
given by the left-
hand rule
𝐹 = 𝑖𝐿B sin Θ
Field into
the page
𝐹 = 𝑖 (𝐿 X B)
Θ = angle between the
wire and the flux density
vector
The induction of a force in a wire by a current in the presence of a magnetic field is the basis of the
motor action.
; = B BAA
F= F
+ تيار : بالعربي كدهقوة لتحريك = مجال
الملف
MOTOR
ACTION
Faraday Laws HOW CAN MAGNETIC FIELD AFFECT THE
SURROUNDING
3- If a wire moves through magnetic field, a voltage is induced in it
𝑣 = velocity of wire
B= magnetic flux density
(wb/m2)
L= length of conductor (m)
e= voltage induced
Force direction is given by the right-hand rule
𝑒 = (𝑣 X B) . L
𝑣
L
XX
XX
XX
XX
XX
XX
XX
XX
𝐵+
-
e
+ ++
- --
• A potential difference is maintained across the conductor as long as there is motion through
the field
• If motion is reversed, polarity of potential difference is also reversed
The induction of voltages in a wire moving in a magnetic field is the fundamental aspect of operation
of all types of generators. That’s why it is called generator action
: بالعربي كده= مجال + حركة
EMF
Force direction is
given by the right-
hand rule
GENERATOR
ACTION
The EMF equation :
Let,
ф= flux per pole in weber
Z = Total number of conductor
P = Number of poles
a = Number of parallel paths: This describes the way the machine's
armature conductors are connected relative to each other and to the
number of poles. The two basic ways of connecting these conductors are
called 'lap' and 'wave', but it gets more complicated.
n =armature speed in rpm
e = emf generated in any on of the parallel path
a=P
(lap)
a=2
(wave)
Assume one
coil only now
One coil = 2 conductor
EMF is induced in the conductor according to Faraday's law.
The average value of e.m.f. induced in each armature conductor is,
𝑒 = −𝑁𝑑ф
𝑑𝑡
Consider one revolution of conductor. In one revolution, conductor will cut total
flux produced by all the poles i.e. ф * P. ( الأقطابكلمنطالعالليالمجالكل )
• The time required to complete one revolution is 60/n seconds as speed is n
r.p.m. Hint: rpm (revolutions per minute)
The EMF equation :
n (rev) 1 min * 60 (sec)
1 rev ????? (sec)
𝑒𝑐𝑜𝑛𝑑 =ф𝑃60
𝑛
= ф P 𝑛
60
Now the conductors in one parallel path are always in
series. There are total Z conductor with a parallel paths,
hence Z/a number of conductors are always in series and
e.m.f. remains same across all the parallel paths.
EMF produced by
one conductor
𝑒𝑡𝑜𝑡𝑎𝑙 =ф P 𝑛
60x 𝑍
𝑎
Total EMF produced
by armature
conductors
P, Z, a: design parameters
N, ф: control parameters
𝑒𝑡𝑜𝑡𝑎𝑙 ∝ k ф nEMF is
proportional to the
field and speed of
rotation
MOST IMPORTANT
EQUATION IN DC
GENERATORS
𝒆 = −𝑵𝒅ф
𝒅𝒕= 1 * change of flux / time
Numerator=𝑵 𝒅𝝋
denomenator= 𝒅𝒕
Types of dc motor and generator:
• Separately excited dc
motor
• Shunt dc motor
• Permanent magnet dc
motor
• Series dc motor
• Compound dc motor
• Separately excited dc
generator
• Shunt dc generator
• Series dc generator
• Compound dc generator
MOTOR GENERATOR
1. Separately Excited: Field and armature windings are either connected separate.
2. Shunt: Field and armature windings are either connected in parallel.
3. Series: Field and armature windings are connected in series.
4. Compound: Has both shunt and series field so it combines features of series and shunt motors.
Questions
• Explain and describe using drawings the construction of dc machine
• What is the function of the following in dc machines:
a- armature winding
b- field winding
• Explain how dc machines can work as generator and motor
• State some applications and types of connections of dc machines
(generator and motor)
• Derive the EMF equation for dc machines
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