Transcript electromagnetic induction - Pearson Higher Education

OBJECTIVES
After studying Chapter 13, the reader should be able to:
1. Prepare for ASE Electrical/Electronic Systems
(A6) certification test content area “A” (General
Electrical/Electronic Systems).
2. Explain magnetism.
3. Describe how magnetism and voltage are related.
4. Describe how an ignition coil works.
5. Explain how an electromagnet works.
Automotive Electricity and Electronics, 2/e
By James D Halderman
© 2009 Pearson Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
FUNDAMENTALS OF MAGNETISM
• Magnetism is a form of
energy that is caused
by the motion of
electrons in some
materials.
• It is recognized by the
attraction it exerts on
other materials.
Automotive Electricity and Electronics, 2/e
By James D Halderman
FIGURE 13-1 A freely suspended
natural magnet will point toward
the magnetic north pole.
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A CRACKED MAGNET BECOMES TWO
MAGNETS
• Magnets are commonly
used in vehicle
crankshaft, camshaft,
and wheel speed
sensors.
• A typical problem
occurs when a
magnetic crankshaft
sensor becomes
cracked, resulting in a
no-start condition.
Automotive Electricity and Electronics, 2/e
By James D Halderman
FIGURE 13-2 If a magnet breaks or is
cracked, it becomes two weaker magnets.
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FUNDAMENTALS OF MAGNETISM
Lines of Force
• The lines that create a
field of force around a
magnet are believed to
be caused by the way
groups of atoms are
aligned in the magnetic
material.
FIGURE 13-3 Magnetic lines of force
leave the north pole and return to the
south pole of a bar magnet.
Automotive Electricity and Electronics, 2/e
By James D Halderman
© 2009 Pearson Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
FUNDAMENTALS OF MAGNETISM
Lines of Force
• The more lines of force that exist, the
stronger the magnet.
• The magnetic lines of force, also called
magnetic flux or flux lines, form a magnetic
field.
• Flux density refers to the number of flux
lines per unit of area.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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FUNDAMENTALS OF MAGNETISM
Lines of Force
FIGURE 13-4 Iron filings on a compass can be
used to observe the magnetic lines of force.
Automotive Electricity and Electronics, 2/e
By James D Halderman
© 2009 Pearson Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
FUNDAMENTALS OF MAGNETISM
Magnetic Induction
• If a piece of iron or steel is placed in a
magnetic field, it will also become
magnetized.
• This process of creating a magnet by using a
magnet field is called magnetic induction.
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FUNDAMENTALS OF MAGNETISM
Attracting or Repelling
• The poles of a magnet are called north (N) and south (S)
because, when a magnet is suspended freely, the poles tend
to point toward the North and South poles of the Earth.
FIGURE 13-5 Magnetic poles behave like electrically charged
particles—unlike poles attract and like poles repel.
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FUNDAMENTALS OF MAGNETISM
Permeability
• Magnetic flux lines cannot be insulated.
• There is no known material through which
magnetic force does not pass, if the force is
strong enough.
• However, some materials allow the force to
pass through more easily than others.
• This degree of passage is called
permeability.
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FUNDAMENTALS OF MAGNETISM
Reluctance
• Although there is no absolute insulation for
magnetism, certain materials resist the
passage of magnetic force.
• This can be compared to resistance without
an electrical circuit.
• Air does not allow easy passage, so air has a
high reluctance.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETISM
• The interaction and relationship between
magnetism and electricity is known as
electromagnetism.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETISM
Creating an Electromagnet
• A magnet can be created by magnetizing a
piece of iron or steel or by using electricity to
make an electromagnet.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETISM
Straight Conductor
• The magnetic field
surrounding a straight,
current-carrying
conductor consists of
several concentric
cylinders of flux that are
the length of the wire.
FIGURE 13-6 A magnetic field surrounds a
straight current-carrying conductor.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETISM
Left and Right Hand Rules
• Magnetic flux cylinders have direction, just as the
flux lines surrounding a bar magnet have direction.
• The left-hand rule is a simple way to determine this
direction.
• When you grasp a conductor with your left hand so
that your thumb points in the direction of electron
flow (- to +) through the conductor, your fingers curl
around the wire in the direction of the magnetic flux
lines.
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ELECTROMAGNETISM
Left and Right Hand Rules
FIGURE 13-7 The left-hand rule for magnetic field direction
is used with the electron flow theory.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETISM
Left and Right Hand Rules
• Most automotive
circuits use the
conventional theory of
current (+ to -) and,
therefore, the righthand rule is used to
determine the direction
of the magnetic flux
lines.
Automotive Electricity and Electronics, 2/e
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FIGURE 13-8 The right-hand rule for
magnetic field direction is used with the
conventional theory of electron flow.
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ELECTROMAGNETISM
Field Interaction
• The cylinders of flux surrounding current-carrying
conductors interact with other magnetic fields.
FIGURE 13-9 Conductors with opposing magnetic fields
will move apart into weaker fields.
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ELECTROMAGNETISM
Motor Principle
• Electric motors, such as automobile starter motors,
use this field interaction to convert electrical energy
into mechanical energy.
FIGURE 13-10 Electric motors use the
interaction of magnetic fields to produce
mechanical energy.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETISM
Coil Conductor
• If several loops of wire are made into a coil, then the
magnetic flux density is strengthened.
• Flux lines around a coil are the same as the flux
lines around a bar magnet.
FIGURE 13-11 The magnetic lines of flux
surrounding a coil look similar to those surrounding
a bar magnet.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETISM
Coil Conductor
• They exit from the north pole and enter at the south
pole.
• Use the left-hand thread rule to determine the north
pole of a coil.
FIGURE 13-12 The left-hand rule for coils is shown.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETISM
Electromagnet Strength
• The magnetic field surrounding a currentcarrying conductor can be strengthened
(increased) three ways.
– Place a soft iron core in the center of the coil.
– Increase the number of turns of wire in the coil.
– Increase the current flow through the coil
windings.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETISM
Electromagnet Strength
• The magnetic field
strength is often
expressed in the units
called ampere-turns.
• Coils with an iron core
are called
electromagnets.
FIGURE 13-13 An iron core concentrates the
magnetic lines of force surrounding a coil.
Automotive Electricity and Electronics, 2/e
By James D Halderman
© 2009 Pearson Education, Inc.
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ELECTROMAGNETISM
Relays
• A relay is a control
device which allows a
small amount of current
to control a large
amount of current in
another circuit.
• A simple relay contains
an electromagnetic coil
in series with a battery
and a switch.
FIGURE 13-14 An electromagnetic relay.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETISM
Relays
• A contact point, made of a good conductor, is
attached to the free end of the armature.
• Another contact point is fixed a small distance
away.
• The two contact points are wired in series
with an electrical load and the battery.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETISM
Relays
• When the switch is closed, the following
occurs.
– Current travels from the battery through the
electromagnet.
– The magnetic field created by the current attracts
the armature, pulling it down until the contact
points meet.
– Closing the contacts allows current in the second
circuit from the battery to the load.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETISM
Relays
• When the switch is open, the following
occurs.
– The electromagnet loses its current and its
magnetic field.
– Spring pressure brings the armature back.
– The second circuit is broken by the opening of
the contact points.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETISM
Relays
FIGURE 13-15 In this electromagnetic switch, a light
current (low amperes) produces an electromagnet and
causes the contact points to close. The contact points then
conduct a heavy current (high amperes) to an electrical
unit.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETIC INDUCTION
• Magnetic flux lines create an electromotive
force, or voltage, in a conductor if either the
flux lines or the conductor is moving.
• This movement is called relative motion.
• This process is called induction, and the
resulting electromotive force is called induced
voltage.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETIC INDUCTION
FIGURE 13-16 Voltage can be induced by the relative motion
between a conductor and magnetic lines of force.
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By James D Halderman
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ELECTROMAGNETIC INDUCTION
Voltage Strength
• Voltage is induced when a conductor cuts across
magnetic flux lines.
• There are four ways to increase induced voltage.
– Increase the strength of the magnetic field, so there are
more flux lines.
– Increase the number of conductors that are breaking the
flux lines.
– Increase the speed of the relative motion between the
conductor and the flux lines so that more lines are broken
per time unit.
– Increase the angle between the flux lines and the
conductor to a maximum of 90 degrees.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETIC INDUCTION
Voltage Strength
FIGURE 13-17 No voltage is induced if the conductor is
moved in the same direction as the magnetic lines of
force (flux lines).
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETIC INDUCTION
Voltage Strength
FIGURE 13-18 Maximum voltage is induced when
conductors cut across the magnetic lines of force (flux
lines) at a 90 degree angle.
Automotive Electricity and Electronics, 2/e
By James D Halderman
© 2009 Pearson Education, Inc.
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ELECTROMAGNETIC INDUCTION
Voltage Strength
• An induced current moves so that its
magnetic field opposes the motion which
induced the current.
• This principle is called Lenz’s law.
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By James D Halderman
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ELECTROMAGNETIC INDUCTION
Self Induction
• When current begins to flow in a coil, the flux
lines expand as the magnetic field forms and
strengthens.
• As current increases, the flux lines continue
to expand, cutting across the wires of the coil
and actually inducing another voltage within
the same coil.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETIC INDUCTION
Mutual Induction
• When two coils are close together, energy
may be transferred from one to the other by
magnetic coupling called mutual induction.
• Mutual induction means that the expansion
or collapse of the magnetic field around one
coil induces a voltage in the second coil.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETIC INDUCTION
Mutual Induction
FIGURE 13-19 Mutual induction occurs when the expansion or collapse of a
magnetic field around one coil induces a voltage in a second coil.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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IGNITION COILS
• The heart of any
ignition system is the
ignition coil.
• The coil creates a highvoltage spark by
electromagnetic
induction.
FIGURE 13-20 Internal construction of an oilcooled ignition coil. Notice that the primary
winding is electrically connected to the
secondary winding. The polarity (positive or
negative) of a coil is determined by the
direction in which the coil is wound.
Automotive Electricity and Electronics, 2/e
By James D Halderman
© 2009 Pearson Education, Inc.
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IGNITION COILS
FIGURE 13-21 Typical air-cooled
epoxy-filled E coil.
FIGURE 13-22 Cutaway of a General
Motors Type II distributorless
ignition coil. Note that the primary
windings are inside of the secondary
windings.
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By James D Halderman
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WHAT IS A “MARRIED”AND
“DIVORCED”COIL DESIGN?
FIGURE 13-23 A tapped (married) type of ignition coil
where the primary winding is tapped (connected) to
the secondary winding.
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By James D Halderman
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ELECTROMAGNETIC INTERFERENCE SUPPRESSION
• Until the advent of the onboard computer,
electromagnetic interference (EMI) was not
a source of real concern to automotive
engineers.
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ELECTROMAGNETIC INTERFERENCE SUPPRESSION
• There are four ways of transmitting EMI, all of which
can be found in a vehicle.
– Conductive coupling is actual physical contact through
circuit conductors.
– Capacitive coupling is the transfer of energy from one
circuit to another through an electrostatic field between
two conductors.
– Inductive coupling is the transfer of energy from one
circuit to another as the magnetic fields between two
conductors form and collapse.
– Electromagnetic radiation is the transfer of energy by the
use of radio waves from one circuit or component to
another.
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By James D Halderman
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ELECTROMAGNETIC INTERFERENCE SUPPRESSION
EMI Suppression Devices
• There are four general ways in which EMI is
reduced.
– By the addition of resistance to conductors, which
suppresses conductive transmission and radiation
– By the use of capacitors and radio choke coil
combinations to reduce capacitive and inductive coupling
– By the use of metal or metalized plastic shielding, which
reduces EMI radiation in addition to capacitive and
inductive coupling
– By an increased use of ground straps to reduce
conductive transmission and radiation by bypassing the
unwanted signals to ground
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By James D Halderman
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ELECTROMAGNETIC INTERFERENCE SUPPRESSION
Suppression Capacitors and Coils
• Capacitors are installed across many circuits
and switching points to absorb voltage
fluctuations.
• Among other applications, they are used
across the following:
– The primary circuit of some electronic ignition
modules
– The output terminal of most alternators
– The armature circuit of electric motors
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETIC INTERFERENCE SUPPRESSION
Ground Straps
• Ground or bonding straps between the
engine and chassis of an automobile help
suppress EMI conduction and radiation by
providing a low-resistance circuit ground
path.
• Such suppression ground straps are often
installed between rubber-mounted
components and body parts.
Automotive Electricity and Electronics, 2/e
By James D Halderman
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ELECTROMAGNETIC INTERFERENCE SUPPRESSION
Ground Straps
FIGURE 13-24 To help prevent under-hood electromagnetic devices
from interfering with the antenna input, it is important that the hood be
grounded to the body to form one continuous metal covering around the
engine compartment. This is particularly important if the vehicle has a
front fender-mounted antenna. This braided ground strap is standard
equipment on this Dodge Caliber and helps eliminate radio interference.
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SUMMARY
1. Most automotive electrical components use
magnetism, the strength of which depends on
both the amount of current (amperes) and the
number of turns of wire of each electromagnet.
2. The strength of electromagnets is increased by
using a soft-iron core.
3. Voltage can be induced from one circuit to
another.
Automotive Electricity and Electronics, 2/e
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SUMMARY
4. Electricity creates magnetism and
magnetism creates electricity.
5. Radio-frequency interference (RFI) is a part
of electromagnetic interference (EMI).
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REVIEW QUESTIONS
1. What is the relationship between electricity
and magnetism?
2. What is the difference between mutual
induction and selfinduction?
3. What is the result if a magnet cracks?
4. How can EMI be reduced or controlled?
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CHAPTER QUIZ
1. Technician A says that magnetic lines of force can
be seen by placing iron filings on a piece of paper
and then holding them over a magnet. Technician
B says that the effects of magnetic lines of force
can be seen using a compass. Which technician is
correct?
a)
b)
c)
d)
Technician A only
Technician B only
Both Technicians A and B
Neither Technician A nor B
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CHAPTER QUIZ
2. Unlike magnetic poles _____ and like
magnetic poles _____.
a)
b)
c)
d)
Repel; attract
Attract; repel
Repel; repel
Attract; attract
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CHAPTER QUIZ
3. The conventional theory for current flow is being
used to determine the direction of magnetic lines
of force. Technician A says that the left-hand rule
should be used. Technician B says that the righthand rule should be used. Which technician is
correct?
a)
b)
c)
d)
Technician A only
Technician B only
Both Technicians A and B
Neither Technician A nor B
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CHAPTER QUIZ
4. Technician A says that a relay is an
electromagnetic switch. Technician B says
that a solenoid uses a movable core. Which
technician is correct?
a)
b)
c)
d)
Technician A only
Technician B only
Both Technicians A and B
Neither Technician A nor B
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CHAPTER QUIZ
5. Two technicians are discussing electromagnetic
induction. Technician A says that the induced
voltage can be increased if the speed is increased
between the conductor and the magnetic lines of
force. Technician B says that the induced voltage
can be increased by increasing the strength of the
magnetic field. Which technician is correct?
a)
b)
c)
d)
Technician A only
Technician B only
Both Technicians A and B
Neither Technician A nor B
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CHAPTER QUIZ
6. An ignition coil operates using the principle
of _____.
a)
b)
c)
d)
Electromagnetic induction
Self-induction
Mutual induction
All of the above
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CHAPTER QUIZ
7. Electromagnetic interference can be
reduced by using a _____.
a)
b)
c)
d)
Resistance
Capacitor
Coil
All of the above
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CHAPTER QUIZ
8. An ignition coil is an example of a _____.
a)
b)
c)
d)
Solenoid
Step-down transformer
Step-up transformer
Relay
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CHAPTER QUIZ
9. Magnetic field strength is measured in
_____.
a)
b)
c)
d)
Ampere-turns
Flux
Density
Coil strength
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CHAPTER QUIZ
10. Two technicians are discussing ignition coils.
Technician A says that some ignition coils have
the primary and secondary windings electrically
connected. Technician B says that some coils
have totally separate primary and secondary
windings that are not electrically connected.
Which technician is correct?
a)
b)
c)
d)
Technician A only
Technician B only
Both Technicians A and B
Neither Technician A nor B
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END
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