Effects and applications of electric currents 635 Effects of an electric current 636 Heating effect of a current 636 Chemical effect of a current 637 Magnetic effect of a current 638 Theory of magnetism 638 Electromagnetism 640 Solenoids and electromagnets 641 Electromagnetic switches (relays) 643 Electrical measuring instruments 644 Electromagnetic induction 645 Electric motors 648 Direct and alternating current 649 Technical terms 649 Review questions 649
Trang 1Effects and applications
of electric currents
Chapter 36
Effects of an electric current
Heating effect of a current
Chemical effect of a current
Magnetic effect of a current
Theory of magnetism
Electromagnetism
Solenoids and electromagnets
Electromagnetic switches (relays)
Electrical measuring instruments
Electromagnetic induction
Electric motors
Direct and alternating current
Technical terms
Review questions
Trang 2This chapter deals with the effects that are produced by
an electric current and shows how these apply to
various electrical components of a motor vehicle.
One of these is magnetism This has applications in
many parts of the electrical system, so an appreciation
of magnetism is particularly helpful in understanding
the operation of various electrical components and
devices.
Effects of an electric current
Three different effects can be produced by an electric
current, and all three of these have automotive
applications Generally, the effects are used to
advantage, but sometimes they are unwanted and
have to be removed, or precautions are taken to
prevent them from becoming excessive The three
effects are:
1 heating
2 chemical
3 magnetic.
■ Of these three effects, the magnetic effect is the
most extensively used in automotive electrical
systems.
Heating effect of a current
Any conductor that carries an electric current will
become heated as a result of the current flow.
The heat produced and the temperature rise in the
conductor will depend on the material of which the
con-ductor is made, the size of the concon-ductor and the rate at
which current flows in the conductor.
The conductors in an automotive electrical system
do not normally produce enough heat to cause
problems This is because the system is designed with
conductors (cables) of a suitable size for the particular
current that they will have to carry.
However, problems can arise if part of the system is
overloaded or if connections and terminals are not
clean and tight A loose or dirty electrical connection
can cause a high resistance, which will produce
unwanted heat and also cause voltage drop.
Figure 36.1 shows a number of cables Where large
currents are used, such as in starter motor circuits,
large cables are required These prevent excessive
voltage drop and are able to carry the starter current
without overheating In other circuits, which have
much lower currents, small cables are adequate.
Applications of heating effect
Applications of the heating effect of a current are outlined under the headings that follow.
Lamps
The heating effect of an electric current is used in light bulbs Current passing through the bulb filament causes it to become white hot and this produces light.
Fuses
Excess current passing through a fuse will cause it to overheat to the extent that it ‘blows’ by melting the fuse wire Fuses and fusible links are used as protec-tive devices to prevent overheating of circuits There are a number of designs of fuses; one type is shown in Figure 36.2.
figure 36.1 A range of electrical cables – heavy cables
are used for high currents to prevent over-heating and voltage drop
figure 36.2 A fuse provides protection by melting when it
is overheated by excess current
Trang 3Glow plugs
In some diesel engines, glow plugs and heater plugs
are used as starting aids These contain elements that
are electrically heated by passing a current through
them In turn, they heat the air or fuel mixture around
them.
Switches
Heat-operated controls and switches function by means
of the heating effect of the current that they carry.
Switches of this type have one of their contacts
mounted on a bimetal blade which normally holds the
contacts closed (Figure 36.3) The contacts are opened
by the heating effect of the current passing through the
blade which causes it to distort This occurs whenever
the blade reaches a predetermined temperature As the
blade cools, it resumes its normal shape and this closes
the contacts.
Switches and controls of this type, which are
sensitive to heat and current flow, are used as circuit
breakers to protect circuits from excessive current The
principle is also used in some flasher lights where an
intermittent current is required.
positive ions move through the electrolyte to the cathode, which is the negative electrode, and the negative ions move to the anode, which is the positive electrode.
Action of electrolysis
The action of electrolysis not only enables current to flow in a liquid, but it can also deposit material from the anode onto the cathode The extent to which this occurs will depend on the material of the electrodes and the type of electrolyte.
The process of electroplating uses this principle to deposit plating material from the anode onto the article being plated, which is arranged as the cathode In this process, the material of the anode gradually erodes away as it is being deposited on the cathode.
Electroplating is a controlled process, but electrolysis can exist where it is not wanted, taking place wherever there are two dissimilar metals and moisture or impure water The dissimilar metals have different electrical potential, so one becomes the anode and the other the cathode The moisture acts as an electrolyte, and over a period of time, material is gradually removed from the anode.
Electrolysis and cooling systems
The conditions outlined above exist in an engine’s cooling system, where cast iron of the cylinder block, aluminium alloy of the cylinder head, and water are present Electrolysis can occur in the cooling system and cause corrosion of the water-jackets and passages.
For this reason, distilled or deionised water, which
is practically free of chemicals, is used in cooling systems, together with special chemical additives.
figure 36.3 Thermal switch with a bimetal arm
figure 36.4 Electrolysis – a chemical effect of an electric
current
Chemical effect of a current
Some liquids, such as water with a small quantity of
acid, will conduct an electric current, and this will
produce a chemical action The conductors that are in
contact with the liquid are known as electrodes, and
the liquid is known as an electrolyte The chemical
action that occurs is called electrolysis (Figure 36.4).
With electrolysis, conduction in the electrolyte is
by movement of ions, which are atoms carrying
positive or negative charges When a current flows, the
Trang 4The chemical effect of current flow can also be related
to batteries Automotive batteries consist of a number
of cells These are referred to as lead-acid cells
because they consist basically of lead plates immersed
in an acid solution (electrolyte) The cells store
electrical energy in chemical form.
When current is flowing from the battery, chemical
energy is being converted to electrical energy When
current is supplied to the battery, the battery is being
charged and electrical energy is being converted to
chemical energy.
■ Batteries are covered separately in Chapter 38: The
battery.
Magnetic effect of a current
Current flowing in any conductor produces a magnetic
effect and this is used in many ways in automotive
components In fact, magnetism is so closely
associated with electricity that a knowledge of the
principles of magnetism is essential in order to
understand the operation of many of the electrical
devices which are fitted to motor vehicles.
Some of the electrical components that involve
magnetism in their operation are the starter motor,
alternator, ignition, horn, windscreen wipers, and
instruments.
Magnets
A permanent magnet is a piece of special alloy steel
that has the property of attracting other magnetic
materials Magnets can be made in a variety of shapes,
the common types being bar magnets and horseshoe
magnets (Figure 36.5).
■ Magnetic materials are those containing iron.
Metals such as aluminium and copper are
non-magnetic.
Magnetic polarity
The compass needle is a familiar example of a magnet.
This is a small permanent magnet, lightly suspended
on a pivot, which always locates itself in a direction N
and S, being attracted in this direction by the magnetic
north pole of the earth Because of this, the end of the
magnetic needle pointing to the north is referred to as
the north pole and the other end as the south pole.
The attractive force of a magnet is concentrated at
its ends This can be seen by dipping the magnet in iron filings The filings will be attracted to the ends, and not to the other parts of the magnet (see Figure 36.5) This demonstrates the magnetic force at the poles.
Attraction and repulsion
Magnets not only attract other magnetic materials, but they will also attract and repel other magnets If two magnets are placed with their unlike poles opposite (N facing S), then they will be attracted to each other as shown in Figure 36.6 When magnets are placed with like poles opposite (N facing N, or S facing S) they will repel each other.
■ The rule for magnetic attraction is – unlike poles attract, like poles repel.
figure 36.5 The poles at the ends of the magnets attract
iron filings
figure 36.6 Unlike poles of a magnet attract each other
whereas like poles repel
Theory of magnetism
A suitable theory for magnetism supposes that every magnet is made up of a number of minute magnetic particles In an unmagnetised piece of iron or steel, these particles are arranged in such a way that their
Trang 5poles form closed magnetic loops, thereby neutralising
one another, so that no magnetic effect is evident
(Figure 36.7).
When a piece of steel is magnetised, the magnetic
particles are rearranged, so that their poles are all
aligned in the same direction to produce the effect of
N and S poles at the ends of the bar, which becomes a
magnet.
To continue this idea a little further, it can also be
presumed that the particles are easy to arrange in some
materials and more difficult in others Iron and mild
steel are easy to magnetise because their particles will
arrange easily, while hard and special steels are
difficult to magnetise because their particles are hard
to disturb.
On the other hand, iron and mild steel, which
magnetise easily, also lose their magnetism quickly.
Hard steels, once magnetised, will retain their
magnetism.
A material that can be easily magnetised is referred
to as being susceptible to magnetism, while a material
which, once magnetised, retains its magnetism is said
to have the property of retentivity.
A material cannot have both these properties Iron
has the property of susceptibility which makes it most
useful as an electromagnet, or as a temporary magnet,
while special steels have the property of retentivity
which makes them suitable for permanent magnets.
■ If a piece of unmagnetised steel is brought within the influence of the magnetic field, it also will become magnetised, but not as readily as the soft iron.
Residual magnetism
When the piece of soft iron or steel is removed from the influence of the magnet, it becomes demagnetised, but there is a very weak magnetic effect that remains This is called residual magnetism Almost every magnetic material has the property of residual magnetism to some degree and it is made use of in some electrical devices.
Magnetic field
The magnetic field is the space surrounding the magnet
in which a magnetic force may be detected It is considered to be made up of a number of magnetic lines of force, which represent the field by showing the direction in which a north pole would move if it were free to do so.
The magnetic field can be demonstrated in various ways, such as by plotting the positions of the north end
of a compass needle as it is moved within the field, and also by the use of iron filings scattered over a sheet of glass or paper placed on top of a magnet.
From various experiments, the magnetic lines of force are found to have the following properties (Figure 36.9):
1 They go from the north to south outside the magnet, and from south to north within the magnet.
2 They repel each other sideways, and do not cross.
3 They tend to crowd through magnetic materials placed in the field.
4 They are not affected if non-magnetic materials are placed in the field.
■ Magnetic flux is a term that can be used instead of lines of force It is used to represent all the lines
of force in a magnetic field.
figure 36.7 Arrangement of magnetic particles in a steel
bar
figure 36.8 Magnetic induction – a piece of soft iron
becomes an induced magnet only while it is within the magnetic field of the permanent magnet
Magnetic induction
Magnetism can be induced from a magnet into an
unmagnetised piece of soft iron to make a temporary
magnet This is called magnetic induction.
If a piece of soft iron is brought close to a magnet,
it will become an induced magnet, as long as it
remains within the influence of the magnetic field of
the magnet It will lose its magnetism the instant that it
is removed from the magnetic field (Figure 36.8).
Trang 6Whenever a current flows in a conductor, a magnetic
field is set up around the conductor This effect is
known as electromagnetism.
This means that any wire, or conductor of any type,
will have a magnetic field if current is flowing through
it In most cases, this is only a weak field and is of
little importance, but when a strong magnetic field is
required, the wire can be wound into a coil with many
turns to concentrate the effect and so produce a very
strong magnetic field.
To demonstrate this basic principle, on which
further principles depend, a small experiment can be
used If a piece of wire is connected to a battery (with
a resistance in series to prevent excessive current), the
current flow will produce magnetic lines of force
around the wire as shown in Figure 36.10.
If a compass is brought close to the wire, the
compass needle will be deflected by the lines of force, and will assume a position at right angles to the wire as shown in Figure 36.11 From this, it can be concluded that magnetic lines of force do exist and that they are
in a circular direction around the wire.
figure 36.9 Magnetic fields of a bar magnet
figure 36.10 Magnetic lines of force around a conductor
figure 36.11 Compass needles are deflected by lines of
force – this is the basic principle of electrical measuring instruments
magnetic needle turns at right angles to wire when current flows.
magnetic needle
Simple measuring instrument
The arrangement of a compass needle, in conjunction with a conductor carrying a current, is the basic principle of electrical measuring instruments When the current flows, the needle deflects; when the current
is stopped, the needle returns to its original position This indicates that the magnetic field is present only when the current is flowing.
If the compass needle was provided with a spring
to limit its deflection, then it could also be used to indicate the strength of the magnetic field It would be found that the field strength would increase and decrease in proportion to current flow.
Current flow therefore provides a means of varying the strength of an electromagnet and this can be summarised as follows:
1 No current – no magnetic field.
2 Small current – weak magnetic field.
3 Large current – strong magnetic field.
Effect of current direction
The direction of current flow in a conductor will affect the lines of force With the current flowing from left to right, as shown in Figure 36.10, the lines of force will
be clockwise around the conductor.
The north end (shaded end) of the compass needle
in Figure 36.11 also indicates this by pointing in the direction in which a north pole of the field would move.
Trang 7If the direction of the current in the conductor was
reversed, then the lines of force would also reverse
their direction around the conductor The compass
needle would swing around and point in the opposite
direction.
Figure 36.12 shows two conductors with current
flowing in opposite directions In Figure 36.12(a), the
direction of current is shown by a cross, representing
current flowing away from us to produce a clockwise
field In Figure 36.12(b), the direction of current is
shown by a dot, representing current flowing towards
us to produce an anticlockwise field.
■ The current direction is shown by an arrow The
cross (x) represents its tail and the dot (•)
represents its point.
Magnetic field of a coil
The field around a wire can be concentrated by
winding the wire into a coil The small fields
surrounding each part of the wire will then combine to
form a strong field which can serve a useful purpose.
Coils are used in many practical electrical devices.
A section through a coil is shown in Figure 36.13.
The direction of current flow in the parts of the coil is
shown by crosses and dots The lines of force that
circle the wire are in opposite directions on each side
of the coil.
On the left-hand side, the current is flowing away
from us to produce a field in a clockwise direction On
the right-hand side, the current is flowing towards us,
so that the field is in an anticlockwise direction.
The fields on each side of the loop will combine because their lines of force are moving in the same direction Figure 36.14 shows this for the two loops
of the right-hand side of the coil, and how the fields of the two conductors have combined to form a stronger field.
If a number of loops of wire are used in the coil, then the fields of all these will combine in a similar manner to produce a strong field similar to a permanent bar magnet The lines of force will be in
a direction from N to S outside the coil, and S to N inside the coil (Figure 36.15).
■ If the direction of current in the coil is reversed, then the polarity of the electromagnet will also be reversed.
figure 36.12 Field surrounding two conductors
(a) current flowing away, field is clockwise (b) current flowing towards, field is anticlockwise
figure 36.13 Section through a coil showing the field
around each conductor
figure 36.14 Parallel conductors with the current in the
same direction have a combined field (this is the right-hand side of the coil of the previous figure)
figure 36.15 Field of a coil shown with iron filings – this is
similar to the field of a bar magnet
Solenoids and electromagnets Solenoids
A solenoid is a particular arrangement of an electro-magnet It is a coil consisting of a series of turns of
Trang 8insulated wire wound on a hollow cylindrical former
made of non-magnetic material.
When current passes through the solenoid, it
produces a magnetic field as previously described,
with a north and south pole at its ends.
The strength of the magnetic field of a solenoid can
be increased by:
1 Increasing the number of turns of the wire.
2 Increasing the current flowing through the wire.
This means that, to produce a magnetic field strong
enough for practical use, either a large number of turns
of wire are used or a heavy current is required In
practice, both these methods are used.
Electromagnets
If a coil is wound around a piece of soft iron, or a piece
of iron is placed in a solenoid, then the soft iron will
become a magnet while the current is flowing The
windings produce a magnetic field, and also arrange
the magnetic particles of the soft iron to form a
magnet.
When the current flow is stopped, the field around
the coil will collapse and the iron core will be
demagnetised because the magnetic particles will no
longer be under the influence of the electromagnetic
field.
Electromagnets can be found in various forms in
many parts of the motor vehicle and the starter,
alternator, ignition, and instruments are some of the
components that make use of the principle of an
electromagnet.
■ Where the coil is wound directly on to a soft-iron
core, this is known as an electromagnet Where the
coil is hollow, it is referred to as a solenoid.
Figure 36.16 shows a practical application of
electromagnetism This is a simplified arrangement
of an alternator rotor, which consists of a field coil
mounted inside a pair of claw-shaped iron poles The
coil has a large number of turns of wire and is able to
produce a strong magnetic field.
Because one end of the coil is a north pole and the
other a south pole, the rotor becomes an electromagnet
and the claws become alternate north and south poles
as shown.
The strength of the magnetic field can be varied by
altering the current flowing in the coil, and this is the
method that is used to control the output voltage of
the alternator.
Coils of electromagnets
Figure 36.17 illustrates two electromagnets of the type used in switches and relays The coil in Figure 36.17(a) has a few turns of heavy-gauge wire, while the coil in Figure 36.17(b) has many turns of light-gauge wire It is necessary to consider the manner in which these become electromagnets.
The strength of an electromagnet depends on the ampere-turns This is the product of the current (in amperes) and the number of turns in the coil There-fore, the two coils referred to above may be different
in design, but could still have the same number of ampere-turns, and the same field strength.
figure 36.16 Simplified alternator rotor – the field coil is
an electromagnet which magnetises the field poles
figure 36.17 Coils of electromagnets
Trang 9Series coil
The coil in Figure 36.17(a) has a few turns of heavy
wire As a result of the short length of wire and
comparatively large cross-section of the wire, this coil
has a very low resistance.
Such a coil cannot be connected directly across the
battery or alternator, as damage would be caused by
the excessively high currents that would result.
This coil requires some form of load or resistance
connected in series with it to limit the current, and is
therefore referred to as a series coil Because of its few
turns, a high current is needed to produce a useful
electromagnetic field.
Shunt coil
The coil in Figure 36.17(b) has many turns of fine wire
(more than shown) As a result of the long length of
wire and its small cross-sectional area, this coil will
have a comparatively high resistance.
Because of its high resistance, this coil can be
connected directly across a battery or alternator It will
not burn out, as its resistance allows only a low current
flow However, because of the large number of turns
of wire and the fact that each turn provides some
magnetic lines of force, this coil will produce a strong
field It is often referred to as a voltage coil or a shunt
coil because it is connected in parallel with the
alternator or battery.
Being connected in this way makes the coil
responsive to any variation in voltage An increase in
the voltage applied to the coil will cause an increase
in current through the windings and a corresponding
increase in the strength of the electromagnet Any
decrease in the voltage will have the opposite effect.
Some voltage regulators use a coil in this way to
regulate the alternator voltage.
Electromagnetic switches (relays)
Figure 36.18 illustrates the principle of an
electro-magnetic switch This is also referred to as an
electromechanical switch, but is usually called a relay.
A relay consists of an electromagnet, with a
soft-iron armature pivoted to open and close a set of contact
points When a control switch is closed to connect the
battery to the coil, current energises the coil windings.
The magnetised core attracts the armature and the
contact points close When the control switch is open,
the coil is no longer energised and the contact points
are opened by the spring.
This arrangement could be used to operate the
headlamps The contact points are used to connect the headlamp to the battery by a much more direct circuit than through the headlamp switch on the steering column The relay is mounted close to the headlamps, with heavier and shorter wire, which reduces voltage drop The headlamp switch is used only to control a low current to operate the relay.
There are many relays used on a motor vehicle They are used in the circuits for horns, lights, fans and many other components.
■ Some relays are designed to close their points when energised, others are designed to open their points when energised.
Solenoid switch
The basic principle of a starter solenoid switch is illustrated in Figure 36.19 The coil is energised when the driver turns the ignition switch to the start position and this attracts the soft-iron plunger further into the hollow solenoid The copper disc bridges across the contacts to complete the starter circuit and operate the starter motor.
The solenoid switch is mounted on the starter (Figure 36.20) and a small current from the start switch closes the large switch contacts of the solenoid This provides a remote control where a small switch is able
to operate the starter.
The starter solenoid actually performs two functions As well as switching, it operates a lever that
figure 36.18 Diagram of an electromagnetic switch or
relay
control switch
Trang 10moves the starter pinion into mesh with the ring gear
on the flywheel.
Electrical measuring instruments
A compass needle placed in the magnetic field of a
conductor represents the basic principle of most
measuring instruments This was previously discussed
in relation to Figure 36.11 Now, the basic construction
of a workshop measuring instrument is illustrated in
Figure 36.21.
Ammeters and voltmeters both operate on the same
principle and their construction is basically the same as
far as the working parts of the instrument are
concerned The basic instrument is known as a
galvanometer and it is used to detect the flow of an
electric current The manner in which the
galvano-meter is arranged determines whether it will be a
voltmeter or an ammeter.
Moving-coil meter
The instrument shown (see Figure 36.21) is known as a moving-coil meter It has a permanent horseshoe magnet, with a small coil of fine wire suspended lightly between the poles of the magnet on two fine spiral springs A needle, or pointer, is attached to the coil, and a suitable scale is provided on the face of the instrument to show the deflection of the needle.
Operation
When there is no current passing through the coil, the spiral springs locate the coil so that the pointer registers with the zero on the scale.
When current passes through the coil, an electro-magnetic field is produced by the coil, which reacts with the field of the permanent magnet This causes the coil to rotate a few degrees against the tension of the springs, moving the pointer across the scale The amount of deflection will depend on the strength of the field of the coil The field, in turn, will depend on current flow The scale is graduated to suit this so that the pointer shows the current flow.
A galvanometer such as this will read only very small currents, and so cannot be used as a workshop instrument in this basic form.
■ Instruments with pointers operate magnetically and are known as analog meters, but there are digital meters which operate electronically.
Ammeter
An ammeter (Figure 36.22(a)) consists of a galvano-meter with a shunt The shunt, which has a very low resistance, is connected in parallel with the moving coil of the meter, so that most of the current being measured bypasses the moving coil via the shunt This
is a simple parallel circuit.
The current being measured will branch in the instrument and only a small current will pass through
figure 36.19 Principle of a solenoid switch
figure 36.20 The starter solenoid is mounted on top of the
starter motor where it acts as a switch and also moves the pinion into mesh
figure 36.21 Basic construction of an electrical measuring
instrument