Magnetic and Electromagnetic DR. MOHD IRFAN HATIM MOHAMED
DZAHIR
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What you should know at the end of this chapter Magnetic field
Magnetic Flux Flux Density Permeability Different magnetic
materials Reluctance Ohms Law for Magnetic Circuits Magnetizing
Force Hysteresis Amperes Circuital Law Airgap Faradays Law Lenzs
Law
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Application of magnetic effects
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The shape of pulsating waveform of the input current is
determined by the sound to be reproduced by the speaker. The higher
the pitch of the sound pattern, the higher the oscillating
frequency between the peaks and valleys resulting higher frequency
of the vibration of the cone. Speaker
Hall Effect Sensor (a) Orientation of controlling (b) Effect on
electron flow Hall effect sensor is a semiconductor device that
generates an output voltage when exposed to the magnetic field. The
difference in potential is due to separation of charge established
by the Lorentz force. The direction of force can be determined by
left-hand rule.
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Bicycle Speed Indicator Use as sensor for alarm systems
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Magnetic Field
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The strength of a magnetic field in a particular region is
directly related to the density of flux lines in that region
Magnetic field strength at point a is twice that at point b since
twice as many magnetic flux lines are associated with the
perpendicular plane at point a than at point b.
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Magnetic Field Continuous magnetic flux line will strive to
occupy as small an area as possible. This results in magnetic flux
lines of minimum length between the unlike poles If unlike poles of
two permanent magnets are brought together, the magnets attract If
like poles are brought together, the magnets repel
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Magnetic Field If a nonmagnetic material, such as glass or
copper, is placed in the flux paths surrounding a permanent magnet,
an almost unnoticeable change occurs in the flux distribution if a
magnetic material, such as soft iron, is placed in the flux path,
the flux lines pass through the soft iron rather than the
surrounding air because flux lines pass with greater ease through
magnetic materials than through air.
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Magnetic Field If a nonmagnetic material, such as glass or
copper, is placed in the flux paths surrounding a permanent magnet,
an almost unnoticeable change occurs in the flux distribution if a
magnetic material, such as soft iron, is placed in the flux path,
the flux lines pass through the soft iron rather than the
surrounding air because flux lines pass with greater ease through
magnetic materials than through air.
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Magnetic Field The previously stated principle is used in
shielding sensitive electrical elements and instruments that can be
affected by stray magnetic fields
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Magnetic Field A magnetic field is present around every wire
that carries an electric current Right-hand rule can be used to
determine the direction of magnetic flux line
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Magnetic Field If the conductor is wound in a single-turn coil
the resulting flux flows in a common direction through the center
of the coil. A coil of more than one turn produces a magnetic field
that exists in a continuous path through and around the coil
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Magnetic Field The field strength of the coil can be
effectively increased by placing certain materials, such as iron,
steel, or cobalt, within the coil to increase the flux density
within the coil The whole concept electromagnetic
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2 type of magnets Permanent magnet A material such as steel or
iron that will remain magnetized for long periods of time without
the aid of external means. Magnetic Field Electromagnet Magnetic
effects introduce by the flow of charge or current. Flux
distribution is quite similar to permanent magnet Have north and
south pole Concentration of flux line is less than that of
permanent magnet Field strength may be increase by placing a core
made of magnetic materials (iron, steel, cobalt) Parameters
affecting field strength Currents Number of turn Material of the
core Without core With core Electromagnet
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Magnetic Field Right Hand Rule Case 1 Thumb : Direction of
current flow Other fingers : Direction of magnetic flux Case 2
Thumb : Direction of magnetic flux Other fingers : Direction of
current flow
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Magnetic Flux
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Flux Density The number of flux lines per unit area Use symbol
B Measured in Tesla (T) Magnitude of flux density If 1 weber of
magnetic flux passes through an area of 1 square meter, the flux
density is 1 tesla.
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Example 1 Find the flux and the flux density in the two
magnetic cores shown in following figure. The diagram represents
the cross section of a magnetized material. Assume that each dot
represents 100 lines or 1 Wb.
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Example 1 For figure a Flux is simply the number of lines
Finding flux density
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Example 1 For figure b Finding flux density Note : the core
with the largest flux does not necessarily have the highest flux
density.
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Example 2 If the flux density in a certain magnetic material is
0.23 T and the area of the material is 0.38 in 2, what is the flux
through the material? Convert the area to m 2 1 m = 39.37 inch
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The magnetomotive force (mmf), is proportional to the product
of the number of turns around the core (in which the flux is to be
established) and the current through the turns of wire
Magnetomotive force External force or 'Pressure' required to set up
the magnetic flux lines within the magnetic material. The cause of
a magnetic field Similar to the applied voltage in electric circuit
Measured in ampere-turns (At)
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Permeability Definition the measure of the ability of a
material to support the formation of a magnetic field within itself
degree of magnetization that a material obtains in response to an
applied magnetic field. Measure of the ease in which magnetic flux
lines can be established in the material Ability of magnetic
material to conduct flux
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Permeability Permeability of air (free space) Relative
permeability The ratio of the permeability of a material to that of
free space
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Permeability MaterialDescriptionExamplerr Nonmagnetic materials
Permeability same as that of free space copper, aluminium, glass,
air and wood r = 1 DiamagneticPermeability slightly less than that
of free space. Bismuth, pyrolitic carbon r < 1
ParamagneticPermeability slightly more than that of free space.
magnesium, molybdenum, lithium, and tantalum 1 < r < 100
Ferromagneticmaterials have a very high level permeability Iron,
nickel, steel and alloys of these materials r 100
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Reluctance The reluctance of a material to the setting up of
magnetic flux lines in the material Unit : Ampere-turns / Weber
Compare this to the resistance in electric circuit
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Ohms Law for Magnetic Circuit Recall Effect = Flux Cause =
Magnetomotive force Opposition = Reluctance For Magnetic
Circuit
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Example 3 Calculate the reluctance of a torus (a
doughnut-shaped core) made of low-carbon steel. The inner radius of
the torus is 1.75 cm and the outer radius of the torus is 2.25 cm.
Assume the permeability of low- carbon steel is 2X10 -4 Wb/ At m
Solution: a b c The length is equal to the circumference of the
torus measured at the average radius
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Example 4 Mild steel has a relative permeability of 800.
Calculate the reluctance of a mild steel core that has a length of
10 cm and has a cross section of 1.0 cm X 1.2 cm. Solution:
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Magnetizing Force Magnetomotive force per unit length Also
called magnetic field intensity Symbol : H Independent of the type
of core material determined solely by the number of turns, the
current, and the length of the core.
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B-H Relationship Flux density (B) and magnetizing force are
related by the equation However, we know that So
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Hysterisis Hysteresis is a characteristic of a magnetic
material whereby a change in magnetization lags the application of
the magnetic field intensity. The magnetic field intensity (H) can
be readily increased or decreased by varying the current through
the coil of wire, and it can be reversed by reversing the voltage
polarity across the coil. In other word, hysteresis is the lagging
effect between the flux density, B of a material and the
magnetizing force, H applied.
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Hysterisis Series magnetic circuit used to define the
hysteresis curve.
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Hysterisis The entire curve (shaded) is called the hysteresis
curve.
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Hysterisis The flux density B lagged behind the magnetizing
force H during the entire plotting of the curve. When H was zero at
c, B was not zero but had only begun to decline. Long after H had
passed through zero and had equaled to H d did the flux density B
finally become equal to zero
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Hysteresis If the entire cycle is repeated, the curve obtained
for the same core will be determined by the maximum H applied
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Normal magnetization curve for three ferromagnetic
materials.
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Magnetic Equivalent Circuit Magnetic circuit Electric
circuit
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Amperes Circuital Law The algebraic sum of the rises and drops
of the mmf around a closed loop of a magnetic circuit is equal to
zero. Or The sum of the rises in mmf equals the sum of the drops in
mmf around a closed loop. Similar to KVL in electric circuit
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Amperes Circuital Law Steel Cobalt Iron 43
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Flux The sum of the fluxes entering a junction is equal to the
sum of the fluxes leaving a junction Similar to KCL in electric
circuit
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Series Magnetic Circuit 2 types of problem: is given, and the
impressed mmf, NI must be computed design of motors, generators and
transformers NI is given, and the flux of the magnetic circuit must
be found design of magnetic amplifiers B-H curve is used to find H
if B is given to find B if H is given
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Example 5: Series Magnetic Circuit
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Part a: Finding I
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Example 5 Use B-H curve to find H For cast steel When B=0.2
H=170 At/m 170 Part a: Finding I
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Example 5 Use Amperes circuital law Part a: Finding I
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Example 5 Part b: Finding and r
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Example 6
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Length of each material Area
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Example 6 Finding H for sheet steel
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Example 6 Finding H for cast iron
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Example 6 Use Amperes circuital law
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Example 7: NI is given, find flux
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Air Gaps Fringing The spreading of the flux lines outside the
common area of the core for the air gap. Only ideal case will be
covered in this course
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Air Gaps For ideal case so magnetizing force of air gap can be
determined by: Permeability of air is assumed to be equal to
permeability of free space Mmf drop across the air gap
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Example 8 : Air Gap
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Example 9 : Air Gap
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Application of magnetic effects
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Faradays law of electromagnetic induction Michael Faraday
discovered the principle of electromagnetic induction in 1831.
Basically he found that moving a magnet through a coil of wire
induced a voltage across the coil, Two observation: 1.The amount of
voltage induced in a coil is directly proportional to the rate of
change of the magnetic field with respect to the coil (d /dt).
2.The amount of voltage induced in a coil is directly proportional
to the number of turns of wire in the coil (N).
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Faradays law of electromagnetic induction First observation
Magnet is moved at certain rate and certain voltage is produced
Magnet is moved at faster rate and creating a greater induced
voltage. S N S N
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Faradays law of electromagnetic induction Second observation
Magnet is moved through a coil and certain voltage is produced
Magnet is moved at same speed through coil that has greater number
of turn and greater voltage is induced
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Faradays law of electromagnetic induction Faradays Law is
stated as follows: The voltage induced across a coil of wire equals
the number of turns in the coil times the rate of change of the
magnetic flux. Faraday's law is expressed in equation form as
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Example 10 : Faradays Law Apply Faraday's law to find the
induced voltage across a coil with 500 turns that is located in a
magnetic field that is changing at a rate of 8000 Wb/s.
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Lenzs Law Defines the polarity or direction of the induced
voltage. an induced effect is always such as to oppose the cause
that produced it. When the current through a coil changes, an
induced voltage is created as a result of the changing
electromagnetic field and the polarity of the induced voltage is
such that it always opposes the change in current.
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Lenzs Law The magnetic flux linking the coil of N turns with a
current I has the distribution shown in Fig. 11.30. If the current
through the coil increases in magnitude, the flux linking the coil
also increases. We just learned through Faradays law, however, that
a coil in the vicinity of a changing magnetic flux will have a
voltage induced across it. The result is that a voltage is induced
across the coil in Fig. 11.30 due to the change in current through
the coil. It is very important to note in Fig. 11.30 that the
polarity of the induced voltage across the coil is such that it
opposes the increasing level of current in the coil.
