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Definition
Application of electrical machines
Electromagnetism: review
Analogies between electric and magnetic
circuits
Faraday’s Law
Electromagnetic Force
Motor action
Generator action
Types and parts of machines
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1.
Definition
Electric machines are energy conversion devices
that convert electrical energy to mechanical energy
and vice versa through the medium of magnetic
field. An electric machine is called a generator when
it is used to convert mechanical energy to electrical
energy. On the contrary, when an electric machine is
operated to convert electrical energy to mechanical
energy, it is called a motor.
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Application of Machines
Electric motors are used to operate washing
machines, elevators, cranes etc while electric
generators are used to generate electricity for power
generation and alternator for charging car battery.
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Applications of Machines
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Applications of Machines
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Applications of Machines
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Electromagnetism
Magnetic fields are the fundamental mechanism by which
energy is conserved from one form to another in motors,
generators and transformers.
A magnetic field is a condition resulting from electric
charges in motion. For convenience in visualization and
analysis, magnetic fields are represented on diagrams by
close loops. Theses loops, called magnetic flux lines
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Magnetic Circuit
Magnetic circuit showing an arrangement of
ferromagnetic materials (core) that forms the path and
guide the magnetic flux.
The flux always takes the shortest path
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Megnetomotive force
It is the driving force that causes the magnetic filed to
appear in a magnetic circuit.
Magnetic Field Intensity
It is the mmf per unit length of the magnetic circuit and it
may vary from point to point throughout the circuit.
Flux Density
It’s a measure of the concentration of lines of flux in
a particular section of magnetic circuit
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Reluctance
It’s a measure of the opposition the magnetic circuit
offers to the flux.
The reluctance of a magnetic circuit is related to its
length, cross sectional area, and permeability.
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Relative permeability and magnetization curves
Relative permeability is the ratio of the permeability
of a material to the permeability of free space. It is
very useful for comparing the magnetizability of
different magnetic materials whose relative
permeabilities are known.
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The ratio µ=B/H is called magnetic permeability and
has different values for different magnetic materials.
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Analogies between Electric and
Magnetic Circuits
The relationship between mmf, flux, and reluctance in a
magnetic circuit is an analog of the relationship bwteen emf,
current and resistance respectively, in an electric circuit.
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Magnetic Circuit and its Electrical Analog
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Magnetic Circuit and its Electrical Analog
A ferromagnetic core is shown in figure below . The depth of
the core is 5 cm. The other dimensions of the core are as
shown in the figure. Find the value of the current that will
produce a flux of 0.005 Wb. With this current, what is the flux
density at the top of the core? What is the flux density at the
right side of the core? Assume that the relative permeability of
the core is 1000.
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Magnetic Hysteresis and hysteresis losses
Hysteresis means lagging behind. In magnetic hysteresis,
flux density B lags behind the field intensity H
Hysteresis loop shows the characteristics of magnetic
material. It is obtained by plotting values of flux density
B for periodically reversing field intensity
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Magnetic Field play an important role in
electric machines.
Magnetic fields are produced when current
flows in coils of wire.
Magnetic fields have 2 funtions:
- produce torque for motor operation
- generate voltage for generator
operation
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If a conductor is moved by an external force, D in a magnetic
field, an electromagnetic force is produced between the two ends
of the conductor. This is the basis for the operation of a
generator.
According to Faraday’s Law, a magnetic field will induce a
voltage, e in a coil. This forms the basis for the operation of a
generator. The voltage, e is given by,
dNΦ NdΦ
e

dt
dt
N … Number of turns of each coil
Φ … Magnetic flux (Weber)
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Force F is produced and it
acts on the conductor. Force
is given by, F = BI 
(Newtons)
Current flows in a conductor
and produces a force and a
mechanical output i.e. conductor
motion. This serves as the basis
for motor operation.
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Michael Faraday showed that passing a current through a
conductor freely suspended in a fixed magnetic field creates a
force which causes the conductor to move through the field.
The force created by the current, now known as the Lorentz force,
acts between the current conductor and the magnetic field, or the
magnet creating the field.The magnitude of the force acting on the
conductor is given by:
F = BLI where F is the force on the conductor, L is the length of
the conductor and I is the current flowing through the conductor
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Faraday also showed that the converse is true moving a conductor through a magnetic field, or
moving the magnetic field relative to the conductor,
causes a current to flow in the conductor.
The magnitude of the EMF generated in this way is
given by:
E = BLv
where E is the generator EMF (or back EMF in a
motor) and v is the velocity of the conductor through
the field
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In practice, both the motor and the generator effects take place at
the same time.
Passing the current through a conductor in the magnetic field
causes the conductor to move through the field but once the
conductor starts moving it becomes a generator creating a current
through the conductor in the opposite direction to the applied
current. Thus the motion of the conductor creates a "back EMF "
which opposes the applied EMF.
Conversely moving the conductor through the field causes a current
to flow through the conductor which in turn creates a force on the
conductor opposing the applied force.
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1.
3 types of rotating machines:
- Synchronous AC Machines
- Asynchronous Machines or Induction
Machines
- DC Machines
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- Shaft is fasten to the rotor so that both rotate at the same angular
speed
- Connection of the machine to load is through the shaft
- 3 main parts for both rotor and stator:
core
winding
insulation
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