Transcript ENT 163 06a-08 - UniMAP Portal

FUNDAMENTALS OF ELECTRICAL
ENGINEERING
[ ENT 163 ]
LECTURE #6a
MAGNETISM AND ELECTROMAGNETISM
HASIMAH ALI
Programme of Mechatronics,
School of Mechatronics Engineering, UniMAP.
Email: [email protected]
CONTENTS
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•
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Introduction
The Magnetic Field
Electromagnetism
Electromagnetic Devices
Magnetic Hysteresis
Electromagnetic Induction
ELECTROMAGNETISM
Electromagnetism is the production of a magnetic field by current in
a conductor.
Magnetic field around a currentcarrying conductor
• The right hand rule is used to determine the direction of the lines of
force.
ELECTROMAGNETISM
Conductor is a material which contains movable electric charges in
which an electric current can be placed.
• Principle – when an electric potential difference is impressed across
separate points on a conductor, the mobile charges within the
conductor are forced to move, and electric current between those
points appears in accordance with Ohm’s law; metallic and nonmetallic.
.
ELECTROMAGNETISM
Magnetic lines of force
are continuous along
wire
Magnetic field around a current-carrying conductor
• Insulators refer to non-conducting materials.
• Current – produces an electromagnetic field around a conductor.
• Although magnetic field cannot be seen, but its capable of
producing visible effects.
ELECTROMAGNETISM
• When current passes through a conductor, an electromagnetic
field is created around the around.
A current-carrying wire –
inserted through a sheet of
paper in perpendicular
direction, iron filings placed on
the surface of the paper arrange
themselves along the magnetic
lines of force in concentric
rings.
The north pole of a compass
placed in the electromagnetic
field will point in the direction
of the line of force.
ELECTROMAGNETISM
• The field is stronger closer to the conductor and becomes
weaker with increasing distance from the conductor.
• Direction of the lines of force can be determine from the left-hand
rule.
• For parallel conductors:
• The conductors repel with each other when the currents are
in opposite direction.
• The conductors attract with each other when the currents are
in the same direction.
ELECTROMAGNETISM
Several important electromagnetic properties:
•
Permeability,µ
i.
Mechanical property of a material
ii. Higher the permeability, the more easily a magnetic field can be
established.
 o  4 10 7

Relative permeability of material,  
r
o
iii. Permeability of a vacuum,
iv.
ELECTROMAGNETISM
•
Reluctance, 
i.
Opposition to the establishment of a magnetic field in a material.

•
l
A
l=length of magnetic path
µ=permeability
A=cross-sectional area of the material
Electromagnetic coil:
 The magnetic field is produced by a straight wire
but not very strong.
 Stronger field is produced by coiling wires around
a piece of soft iron
 Also known as solenoid.
 Shape of magnetic field is same as bar magnet.
 The soft iron inside the coil makes the magnetic
field stronger because it becomes a magnet itself
when the current is flowing.
ELECTROMAGNETISM
 Soft iron is used because it loses its magnetism as soon as the current
stops flowing (temporary magnet).
 In this way, the electromagnet can be switched on and off by turning the
electricity on and off.
 The strength of the magnetic field around the coil can be increased by:
 Using a soft iron core.
 Using more turns of wire on the coil.
 Using a bigger current.
 Reversing the direction of the current will reverse the magnetic field
direction.
ELECTROMAGNETISM
Magnetomotive force (mmf) is a force that produces the magnetic field.
Unit as ampere-turn(At)
Equation:
•
Fm  NI
Fm= magnetomotive force
N=number of turns of wire
I= current in amperes
Ohm’s law for magnetic circuits:
Fm


V
I
R
Where flux(ø), is analogous to current, the mmf (Fm) is
analogous to voltage and the reluctance,( ) is analogous to
ELECTROMAGNETIC DEVICES
 Electromagnets are used in devices such as magnetic disk, electric motors,
speakers, solenoids and relays.
 Read/write function on a magnetic surface.
ELECTROMAGNETIC DEVICES
 Solenoid.
 The solenoid is used for applications such as opening and closing
valves and automobile door locks.
 Solenoid is a type of electromagnetic device that has a movable iron
core called plunger.
 The movement of this iron core depends on both an electromagnetic
field and a mechanical spring force.
 Basic structure – consists of a cylindrical coil of wire wound around a
nonmagnetic hollow form; a stationary iron core is fixed in position at
the end of the shaft, and a sliding iron core is attached to the
stationary core with a spring.
Sliding core
(plunger)
Stationary core
Spring
Coil
ELECTROMAGNETIC DEVICES
 Basic solenoid operation:




At rest (unenergized) state – plunger – extended.
Solenoid is energized by the current through the coil.
The current sets up an electromagnetic field that magnetizes both iron
cores.
The south pole of the stationary core attracts the north pole of the movable
core, which causes it to slid inward, thus retracting the plunger and
compressing the spring.
ELECTROMAGNETIC DEVICES


As long as there is coil current, the plunger remains retracted by the
attractive force of the magnetic fields.
When the current is cut off, the magnetic field collapse; and the force of
the compressed spring pushes the plunger back out.
ELECTROMAGNETIC DEVICES
 Relay:
 Relays differ from solenoids in that the electromagnetic action used to
open/ close electrical contacts rather than to provide mechanical
movement.
 Basic structure of a relay:
ELECTROMAGNETIC DEVICES
 Relay – consists of two circuits
 Circuit 1 is a simple electromagnet which requires only a small current.
 When the switch is closed, current flows and the iron rocker arm is
attracted to the electromagnet.
 The arm rotates about the central pivot and pushes the contacts together.
Circuit 2 is now switched on.
 Circuit 2 may have a large current flowing through it, to operate powerful
motor or very bright lights.
 When the switched is opened the electromagnet release the rocker arm
and the spring moves the contacts apart. Circuit 2 is now switched off.
ELECTROMAGNETIC DEVICES
 The advantage of using a relay is that a small current (circuit 1) can
be used to switch on and off a circuit with a large current(circuit 2).
 This is useful for two reasons:
 Circuit 1 may contain a component which only uses small
currents,
 Only the high current circuit needs to be made from thick wire.
Application of relays is to operate the starter motor in cars and the
heating circuit in diesel engines.
MAGNETIC HYSTERESIS
Magnetizing force, H in a material is a magnetomotive force per unit length
of the material. Unit : ampere – turns permeter(At/m)
Equation:
Fm
H
l
Fm  NI
Fm= magnetomotive force
H= magnetizing force
I= length of material
NI
H 
l
H is depends on the number of turns of the coil of wire , current through
coil, length of material.
MAGNETIC HYSTERESIS
 Since,
Fm


 Recall that,
B
as

Fm , 
increase, the flux increases; therefore H
increases
, therefore B is also proportional to H.
A
 The B-H relationship- showed by a B-H curve, also known as the
hysteresis curve.
MAGNETIC HYSTERESIS
Hysteresis is a characteristic of a magnetic material whereby a change in
magnetization lags the application of a magnetizing force.
 Figure below illustrates the development of hysteresis curve:
 Start by assuming a magnetic core is unmagnetized (B=0). As the
magnetizing force (H) is increased from zero, the flux density (B)
increase proportionally. When H reached a certain value, the value
of B begins to level off.
MAGNETIC HYSTERESIS
 At H continuous to increase, B reaches a saturation value (Bsat) when H
reaches a value (Hsat). Once saturation is reached, a further increase in H
will not increase B.
(Bsat)
saturation
(Hsat).
MAGNETIC HYSTERESIS
 If H is decreased to zero, B will fall back along a different path to a residual
value (BR). This indicates that the material continues to be magnetized
even with the magnetizing force removed (H=0). The ability of the material,
once magnetized, to maintain a magnetized state without the presence of a
magnetizing force is called retentivity.
(BR).
(H=0).
MAGNETIC HYSTERESIS
 Reversal of the magnetizing force is represented b negative values of H on
the curve and is achieved of H on the curve and is achieved by reversing
the current in the coil of wire. An increase in H in negative direction causes
saturation to occur at a value (-Hsat) where the flux density is at its
maximum negative value.
(-Hsat)
saturation
(-Bsat)
MAGNETIC HYSTERESIS
 When the magnetizing force is removed (H=0), the flux density goes to its
negative residual values (-BR)
(H=0),
(-BR)
MAGNETIC HYSTERESIS
 From the (-BR) value, the flux density follows the curve back to its
maximum positive value when the magnetizing force equals Hsat in the
positive direction.
Bsat
Hsat
+Hc
MAGNETIC HYSTERESIS
 The complete B-H curve.called as the hysteresis curve. The magnetizing
force required to make the flux density zero is called the coercive force, Hc.
ELECTROMAGNETIC INDUCTION
 When a magnetic field is moved past a stationary conductor, there is a
relative motion, known as the induced voltage, vind ,which across the
conductor.
 Amount of induced voltage depends on the at which the conductor and
the magnetic field move with respect to each other (the faster the relative
motion, the greater the induced voltage).
ELECTROMAGNETIC INDUCTION
•
If the conductor is moved first one way and then another in the
magnetic field, a reversal of the polarity of the induced voltage will be
observed.
•
When the relative motion of the conductor is downward, a voltage is
induced with the polarity indicated in Figure below. When the relative
motion of the conductor id upward, the polarity is as indicated in part
(b) of the figure.
ELECTROMAGNETIC INDUCTION
•
When a load resistor is connected to the conductor, the voltage induced
by the relative motion between the conductor and the magnetic field will
cause a current on the load, called the induced current (iind).
Induced current (iind) in a load as the conductor moves through the
magnetic field.
ELECTROMAGNETIC INDUCTION
•
Figure below shows a current inward through a wire in a magnetic field:
ELECTROMAGNETIC INDUCTION
 The electromagnetic field set up by the current interacts with the permanent
magnetic field, as a result, the permanent lines of force above the wire tend
to be reflected down under the wore because they are opposite in direction
to the electromagnetic lines of force.
 Therefore the flux density above is reduced and the magnetic field is
weakened. The flux density below the conductor is increased and the
magnetic field is strengthened. An upward force on the conductor results,
and the conductor tends to move toward the weaker magnetic field.
 Figure (b) shows the current outward, resulting in a force on the conductor
in the downward direction.
ELECTROMAGNETIC INDUCTION
 Michael Faraday – discovered the principle of electromagnetic induction in
1831 (moving a magnet through a coil of wire induced a voltage across the
coil).
 Faraday’s observation:
 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.
 The amount of the voltage induced in a coil is directly
proportional to the numbers of turns of wire in the coil.
ELECTROMAGNETIC INDUCTION
Faraday’s Law
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.
As the magnet moves slowly to the
right, its magnetic field is changing
w. r. t coil, and a voltage is induced.
As the magnet moves rapidly to the
right, its magnetic field is changing
more rapidly w. r. t coil, and a
greater voltage is induced.
ELECTROMAGNETIC INDUCTION
Heinrich F. E. Lenz – defines the polarity or direction of the induced voltage
Lenz’s Law:
When the current through a coil changes, the polarity of the induced voltage
created by the changing magnetic field is such that it always oppose the
change in current that caused it.
Further Reading
Electric Circuit Fundamentals. (7th Edition), Floyd,Prentice Hall.
(chapter 7).