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EET 412
Electrical Machine Design
Dina Maizana
E-mail :dina @ kukum.edu.my
Electrical Machine
Design
Topic
Magnetism
By: DINA MAIZANA
Introduction
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
Magnetism is essential to the generation of electricity
by rotating apparatus and the creation of mechanical
power by the electric motor.
Scientists have found magnetism to be a property of
the atom and it is the result the normal electronic
activity.
Electron
Direction of Line of
Magnetic Force
Direction of spin
of electron
Orbit of electron
Figure 1 Shows an orbiting electron.








A moving electron is accompanied by a line magnetic force.
Forming a complete circle ellipse.
Direction of this force is parallel to the axis of spin .
An electron spinning in a counter-clockwise direction.
Its magnetic force clockwise when observed from the right.
This direction of spin may be term positive.
If the electron caused to turn over, it will appear to spin in the opposite or negative direction
when observed from above, its magnetic force now counter-clockwise from the right.
When an atomic shell is filled, or complete, the number of positively-spinning electrons is
exactly equal to the number of negatively-spinning electrons, so that their magnetic effects
cancel.
Classes of Materials
Found in Nature
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Nonmagnetic substances are some
materials no magnetic effect, ex. Vacuum.
Paramagnetic substances possess a low
order magnetic quality, ex. Aluminum, or
Tungsten.
Ferromagnetic substances is a high order
magnetic quality, ex. iron, cobalt, or nickel.
Diamagnetic substances are certain metals
, however, exhibit more or less negative
reaction to the presence of magnetic lines.
ex. Bismuth.
Iron filings
Magnetized bar
The filings arrange themselves in paths which are
highly concentrated at the two ends, or poles, and
extend from one end of the strip to the other. The form
assumed by these paths is dictated by that of the
endless magnetic lines of force established by
spinning electrons in the material of which the strip is
composed
Patterns assumed by iron filings
around a bar magnet
Powder pattern
of domain
(a)
(b)
(a), distinct groupings visible in many random directions,
a few in end to end formation.
(b), more of the groupings assuming an end to end state,
the higher the degree of magnetization, the greater the
number of such groups.
Finally, when the iron has been magnetized as strongly
as possible, substantially the whole surface will show a
continuous structure of this kind. Under this condition,
the piece of iron is said to be magnetically saturated.
Powder patterns formed by magnetic domains
Scientific investigations show these surface groupings to be characteristic of the atomic inside the
material. These groupings are termed domains, and are found in all ferromagnetic substances. A domain
is formed by a drawing together of atoms whose magnetic forces are all in one direction, that is, either
negative or positive.
N -p o le
of
com pa ss
a ttr a c te d to w a r d s S p o le o f m a g n e t
B ar m agnet
F o rce
of
a ttr a c tio n
(a )
S
N
N
S
N
F o rce o f
r e p u ls io n
S
(b )
N
S
The S pole
repels the S
pole of the
second, but
attracts its N
pole.
N
(c )
If a bar of soft iron is magnetized, it tends to remain strongly magnetic only so long as the magnetizing force
is present, quickly reverting to its original state. Such a piece is called temporary magnet.
If a bar of hard steel is magnetized, it tends to remain so longer after the force has been removed. This piece
is called a permanent magnet.
S
N
Bar
magnet
N
S
S
N
Nail
N
(a)
S
(b)
The method of inducing magnetism by contact with magnetized piece,
which is called magnetic induction.
(a), the nail will cling to the magnetic pole. If the pole happens to be the
N pole of the magnet, an N pole will be found at the lower end of the nail.
If the nail is touched to the S pole of the magnet
(b), the lower end of the nail will have an S pole. If taken away from the
magnet, the nail will lose its magnet properties. In other words, the nail is
only a temporary magnet.
S
Magnets are produced in a variety of shapes
and sizes. The pattern assumed by lines of
force is shown by iron filings attracted to the
poles. These magnets are employed in
numerous small devices, in direct current
measuring instruments, and in tachometer
generators for indicating speed of rotation.
Some of them have accompanied satellites
into orbit.
N
Some of the various forms of the magnets and the iron
filings indicate the field pattern.
Bar magnet
Stroke constantly in this
direction from one end to
the other end
Ferromagnetic substances can be magnetized in any way .
S
(a) Stroking a piece of unmagnetized iron with a magnet
N
(a)
Magnetizing
coil
(b)
Unmagnetized
bar of iron
(b) The unmagnetizd metal being placed inside a direct
current magnetizing coil
When current flows through an electrical
conductor so magnetic lines of force are set
up at right angles to the direction of current
flow. They form circles around the wire and
are called circles of force.
f
no
tio low
c
e
f
D i r re n t
r
cu
North Pole
of compass
Circle of
force
Circle of
force
f
no
tio w
ec t flo
r
i
D re n
r
cu
North Pole
of compass
The indication of a compass depends on the direction of current flow
Thumb indicates
direction of electron
flow
Fingers curve in same
direction of magnetic
lines set up by electron
flow in conductor
Circle of force
Left hand rules for determining direction of magnetic lines set up by
electron flow in conductors.
If the direction of the circles of force s given, the left hand may be
wrapped around the conductor, the fingers representing the circles
of force, and the thumb will then indicate the direction of current
flow
When there is no current in the conductor, there are no
circles of force surrounding it. When a small current
flows, there will be a circle of force at F1. If the current
increases, the circles will increase in size until it
occupies the position at F2. A further increase in
current will cause it to expand until it occupies the
position at F3.
Conductor
F1
F2
F3
There is another motion of the circle of force
N
S
Compass Needle
Compass Needle
Circle of
force
Wire
Circle of
force
Wire loop
Current
Current
Compass Needle
Compass Needle
N
S
(a)
(b)
(a) A piece of copper wire is bent
to form a loop. The loop is
connected to an electrical
generator or other source of
current so that current flows as
shown by the arrows. Applying
the left hand rule so that the
thumb indicates current flow, the
fingers will wrap around the wire
so that they are n the inside of
the loop at all times. Thus, the
direction of the circles of force
inside the loop will always be
downward.
(b) Should the current in the loop be reversed, the left hand rule shows the circles of force moving upward
inside it
N
Circle of
force
Circle of
force
S
Iron core
Iron core
Current
S
(a)
Wire
Current
N
The magnetizing effect of
wire loop may be greatly
increased by the used of
an iron core.
Wire
(b)
(a) shows a wire loop around a rectangular core of iron. Since the current in the loop flows clockwise
when viewed from the top an N pole would be expected there if it were a plain loop.
(b) When the directions of current flow are reversed, polarity of the iron core is reversed also. Since
the directions of the magnetic circles is downward inside the loop, domains arrange themselves so
magnetic lines emerge from the lower face of the core forming an N pole there, and an S pole on top.
Strength of electromagnet is determined by number of ampere turns.
Iron core
Four
turns of
wire
The coil surrounding the iron core has 4 turn. When current flows in the
coil, 4 times as many circles of force result as in the case of 1 turn. The
magnetic force is a result as in the case of 1 turn. The magnetic force
for a current of 1 ampere becomes 1 ampere x 4 turns, or 4 ampere
turns. The current of 1 ampere flowing through a coil of 4 turns
produces the same quantity of magnetism as 4 amperes flowing though
a coil of 1 turn. Eight times as much magnetism is will be produced by a
current of 2 amperes flowing through 4 turns as b a current of 2
amperes flowing through 4 turns as by a current of 1 amperes, and 20
times as much by a current of 5 amperes.
Questions
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
What activities of the electron are involved in producing magnetic effects?
State two characteristic of the lines of force emanating from a moving electron
Describe the easiest way for an electron to change its direction of spin.
How many positively spinning electrons will be found in a complete shell of eight?
Is iron paramagnetic?
Through what kind of material do magnetic lines pass with the greatest ease?
Which atomic shells are usually incomplete in ferromagnetic substances?
What is a domain?
Are domains visible in a pattern of iron filings?
Are domains present only at the surface of an iron bar?
State the law of magnetic attraction and repulsion.
What is the principle different between a temporary magnet and a permanent one?
Explain magnetic induction.
How does a diamagnetic substance behave when placed in a strong magnetic field?
Is wood diamagnetic substances?
Thank you for your attention.
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Good bye