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Electricity
& Magnets
A prerequisite
for studying this topic is to have completed basic
electricity-1 and basic electricity-2.
This topic is built upon the knowledge gained in those two topics.
Magnetism
plays an integral part in almost every electrical device used
today in industry. In fact, if you complete this topic, you will
probably have an electromagnet personality .... or something like
that. Generators, motors, televisions, radios, and telephones
all operate with magnetic fields. In Trucks, magnetic fields are
what make generators and alternators work. They are what make
the starter motors and windshield wiper motors perform their work.
They are what make horn relays and starter solenoids function.
Magnetic fields are also what make those electric gauges, such
as fuel gauges work. The following concepts are covered in this
topic:
Magnetism
comes in two basic forms. Permanent magnets require no energy
to hold their magnetism forever. Electromagnets are pieces of
iron which are momentarily turned into magnets with the aid of
electricity. Both type are used in truck electrical devices such
as electric motors.
Permanent
Magnet Principles
Our planet
Earth is the biggest permanent magnet that we have. Our Earth
has a magnetic North pole at what we call the North end of our
Earth's axis, and a South pole at the other end of our Earth's
axis. Explorers learned that loadstone (a natural bar magnet made
of stone) would always point to the same direction, and that direction
was later named North.
Another permanent
magnet example is the common bar magnet which you may have played
with or studied in school. All magnets have one thing in common.
They all emit lines of flux from their North pole,  and
receive the same lines of flux into their South pole. Notice how
the lines of flux are more heavily concentrated within the bar
of the magnet. In the figure to the left, there are only 3 lines
of flux passing through the same cross-sectional area of air,
whereas, there are many more line of flux passing through the
same cross-sectional area of the bar magnet itself. Lines of flux
always seek the path of least resistance to get from the bar magnet
North pole to the bar magnet South pole.
In the next
example, two bar magnets are held in close proximity, and oriented
with opposite poles near  each
other. Do you remember from being a kid, just what happens next?
The two magnets slam together because the lines of flux from both
magnets see the other magnet as an easier path than air. Lines
of flux can add together, so the lines of flux merge and pass
through both magnets. Because the lines of flux like the magnet
better than the air, the magnets slam together to reduce the air
path. Now the two smaller magnets have become one big magnet.
Remember to keep your fingers out of the way for this experiment!
If you orient
these same two bar magnets so that the same poles are touching,
what happens? The two poles push away from each other with great
force. In  fact,
you would be hard pressed to hold two magnets together with the
same poles actually touching (even with small magnets). The reason
you get this great reaction is because lines of flux can not cross
over each other. Notice in this figure, how the lines of flux
turn at sharp angles to prevent crossing. This effect of lines
of flux not crossing, is what makes motors work. The repelling
force of same magnetic poles, is what does the work of electric
motors.
We mentioned
earlier that lines of flux prefer the bar magnet path over a path
of air. This is why unlike poles attract. This next figure shows
that lines of flux pass more easily through soft iron than through
air or glass. This characteristic of lines of flux will be  used
to concentrate lines of flux through soft iron, so that the effect
of the lines of flux will be more concentrated. By concentrating
the lines of flux, we are able to get much more force out of the
magnetic effect. Motors and relays use a very small air gap to
concentrate the lines of flux heavily into a small area to get
great force at that area.
Electromagnet
Principles
Ok, so we
all have a pretty good idea of how permanent magnets work. Now
lets find out how electromagnets work. A physicist name Hans Oersted
discovered quite by accident back in 1820, that a compass needle
would deflect if brought near a current-carrying conductor. He
reasoned that the compass needle was aligning with some force
emitted by the current-carrying conductor. For the first time,
it was demonstrated that electricity and magnetism were related
in some way.
 For
many years after that, men such as Mikey Faraday, Karl Gauss,
and Jimmy Maxwell conducted many experiments to prove the basic
concepts of electromagnetism. The first thing they learned was
that magnetic lines of flux are generated which circle a current-carrying
conductor.
After much
consternation, they finally agreed that these electric lines of
flux were acting like the Earth's lines of flux, and were causing
the loadstone to align with the electric lines of flux, when they
were greater than the Earth's lines of flux. They invented what
is called the right-hand rule. If you wrap the fingers of your
right-hand around a conductor, and you pass current through the
conductor in the direction pointed to by your thumb, then your
finger tips emanate electric lines of flux. Hold your left hand
up to the screen and compare with this figure to see for yourself.
Then one of the scientists decided to coil the wire and see what
happens. What they found was that the compass needle was more
strongly deflected by the  same
current through the coiled wire. The figure on the right, shows
what they learned. They learned that the electric lines of flux
added together when the wire was coiled, and formed a greater
magnetic north field. Hold your right hand up to the right side
of this figure, with your thumb pointing up, and note that the
flux line are coming out of the screen in the center of the coil.
Now twist your hand and move it to the left side of the loop with
your thumb down, and you will see that the flux lines are still
coming out of the screen in the middle of the loop. All of these
little lines of flux add up when the wire is placed into a coil
shape.
They already
knew that lines of flux are more concentrated when passing through
soft iron, so they decided to wrap a current carrying conductor
around a bar of soft iron, and much to their surprise, they created
a very strong magnet out of the soft iron bar. When they stopped
the current in the wire, the soft iron magnet pretty much stopped
also. A  small
amount of residual magnetism was left in the soft iron bar, but
not nearly as much as when the current was passing through the
wire. They also found that if they reversed the current through
the wire in the opposite direction, that the North and South poles
of the soft iron bar also reversed. They also found that the residual
magnetism always held the last pole orientation the bar was exposed
to. Much later in time this residual magnetism would be referred
to as hysteresis, and is what make truck generators work.
Ohm's
Law for Magnetic Circuits
You may recall
that Ohm was not listed above as one of the scientists working
with magnetic theory. Those that were working with magnets soon
realized that they needed some organization and common names for
magnetic effects. They created a relationship between Effect,
Cause and Opposition. Sort of like E = I * R, but they used different
units of measure.
You could
spend hours at the library learning all these units of measure
for magnetism, but unless you are designing motors or studying
for an electrical engineering degree, you really don't need that
information. Since we are fixing trucks, we wont go into it either.
Electric
Relay Theory
OK, lets
do something with the results of all the above information. First
we will build a relay. A relay is a device where current is passed
through a coil, this coil forms an electromagnet, and the  electromagnet
pulls down an arm which closes some electrical contacts. This
figure shows the arm which is a spring which pulls down when current
is passed through the coil. Why does the arm pull down? Remember
how the flux doesn't like an air path? When coil current flows,
the relay body becomes an electromagnet. Lets say the flux created
flows down like the arrow shown in the figure. We now have a big
"G" shaped electromagnet, with the opposite poles separated
by this small air gap. What do opposite poles due? They attract,
and this is what pulls the spring arm down. This figure shows
the electric contacts which would be moved by the arm of the relay.
Notice that the relay arm does not move very far, usually about
a tenth of an inch or so.
Conductor
Current Electromagnetic Forces
If you place
a wire within a magnetic field, and then you pass current through
this wires, the old rule of lines of flux not crossing applies.
In the figure below, part (a shows the wire resting within the
magnetic flux field with no current passing through the wire.
Part (b) shows only the flux generated by the wire itself when
current
passes through
the wire and into the figure (the + represents the tail of current
flowing into the figure). Part (c) shows what happens when you
combine part (a) and part (b). Notice that the flux from the wire
aids the air gap flux at the top, and opposes the air gap flux
at the bottom. Lines of flux are like elastic rubber bands. These
flux lines are always attempting to contract to minimum length.
The tension in these lines above the conductor tends to force
it down as shown by the force arrow.
Part
(d) simply shows the force on the wire when the current is flowing
out of the wire (the dot in the middle represents the point of
the current arrow.
So, just
what does this all mean? If we place a wire within the air gap
field of a permanent magnet, and then we pass current through
that wire, that wire will be driven out of the permanent magnet's
flux field. The force which moves the wire out of the air gap
flux field, is what makes a motor run. The more current you force
through the wire, the more force that is generated. Of course,
one wire can not generate much force, but if you place 100 physically
connected wires in this air gap, then the force generated would
be 100 times stronger. And that is what motors do, they use many
loops of wire to get their powerful rotating force.
Now for the
million dollar question. If you just move a wire through the air
gap of flux lines, with no current passing through the wire, what
will happen? This is called a generator, and the left-hand rule
will show the direction of induced current that the wire will
carry due to the motion of the wire across the air gap. The same
magnetic principles apply to generators that apply to motors.
The current just flows in the opposite direction for the same
rotation.
Electric
Meter & Gauge Theory
The next
item of interest is the common electric meter. This figure shows
a typical meter such as one found in your truck dash. That large
horseshoe shaped iron with the flux lines passing through it is
a permanent magnet. Its flux lines are always present. This causes
concentrated lines of flux across the  circular
air gap at the bottom of the meter. This air gap contains a coil
of wire wound around a bobbin which is also made of soft iron.
This bobbin
is mounted on a delicate axle which is spring centered so it wont
easily rotate, and when the spring tension is overcome, the bobbin
will rotate and move the pointer which is attached to the bobbin.
As we pass current through the coil on the bobbin, the lines of
flux of the bobbin coil interact with the lines of flux from the
permanent magnet, and the resulting force overcomes the springs
on the bobbin axle, and the needle pointer moves. Refer back to
the conductor current forces above to review
the forces involved. If you reverse the current through the bobbin,
the needle pointer deflects in the opposite direction. With no
current in the bobbin, the needle pointer remains at rest in the
spring center of the gauge. Most gauges only indicate in one direction,
so the pointer is normally pointing to the left when the meter
is at rest. Please note that the bobbin only rotates about 30
- 45 degrees. The wires were attached through the centering springs
and this allows the springs to twist and still maintain electrical
connection to the bobbin.
Electric
Solenoid Theory
The
solenoid is a device which produces mechanical force in one direction
when current is applied to it. The electromagnetic lines of flux
are generated as shown below when the solenoid coil winding is
wrapped around a nonmagnetic tube (solenoid housing) such as brass,
aluminum, or bronze.
There is a soft iron slider, which can freely slide back and forth
inside of the tube. A spring inside the tube normally extends
the soft iron slider so that one end sticks out a short distance.
When power
is applied to the coil, the soft iron slider is sucked into the
center of the coil winding when the spring is overpowered by the
magnetic force. The mechanical device physically attached to the
slide is also jerked towards the tube.
A common
example of a power solenoid is an electric truck release for a
car. When
power is removed from the coil, the spring forces the slider back
to it's extended position.
The solenoid
force is generated because the lines of flux are like rubber bands,
and always want the shortest path possible. When the iron is sucked
into the tube, the shortest path through the soft iron has been
established. We have converted electrical power (watts) into mechanical
power (work done).
One last
thing for you to consider. Notice how the upper drawing shows
current entering into the left side of the coil. The lower drawing
shows the negative terminal on the right side of the coil, which
means that current would enter into the coil from the right side.
Does it make any difference? Would the slider go the wrong direction
if the current were reversed? The answer is "No, it doesn't
matter". Remember, the slider is seeking the center of the
coil winding to reduce the length of external lines of flux. Therefore,
current going in either direction will still suck the slider into
the center of the coil winding.
Electric
Motor Theory
Electric
motors have several uses in trucks. The starter motor is pretty
handy when you need to start the truck. The heater fan motor is
pretty handy when you are cold, and the windshield wiper motor
is pretty handy when you are driving in the beating rain.
Most battery
operated electric motors operate on the same principle. Those
of you who are paying attention, will notice that this picture
is labeled a generator. Motors  and
generators operate upon the same forces. The motor consumes current
to create physical force, and the generator creates current due
to external physical force.
As shown
in this figure, there is a large permanent magnet which completely
surrounds the motor. This large permanent magnet has a small air
gap in the center to concentrate the lines of flux. Within that
air gap, there is a device called an armature winding. Just like
the gauge described above, current is passed through the armature
and the armature is forced to rotate.
The reason
this is called an armature and not a bobbin is twofold. The armature
has more than one coil, and the armature continues to rotate in
the same direction. Remember, the gauge above only rotated one
coil about 30 - 45 degrees. The motor must rotate 360 degrees
(a full circle). Another problem with the motor, is that it keeps
rotating. The gauge example above had wires attached directly
to the bobbin because it rotated only a small distance, and then
returned. In fact, the wires were attached through the centering
springs. This allowed the springs to twist and still maintain
electrical connection to the bobbin. The motor armature solves
this problem.
Armature
& Commutator
 Here
is a diagram of a one coil loop motor. It has only one winding,
and therefore only one magnetic effect.
This drawing shows the single armature winding at its maximum
rotational force position. Refer back to the conductor
current forces drawing for a review of the direction of force.
This is a demonstration motor and wouldn't work very well for
several reasons. The first problem is when the armature rotates
until the wires are at the top and bottom of the air gap as shown
in the figure below. In this position, the forces generated by
the winding, would try to move the armature up and down, and would
not rotate the armature. If the motor stopped in this position,
it would never start again, unless you gave it a push. Real motors
have two or more armature windings, which means that one of the
windings would always be in a position to exert rotational force.
 The
other problem with this motor has to do with the commutator and
brushes. Notice that this commutator has two sections, a dark
section and a light section. Each commutator section is connected
to one end of the coil loop. There is only a small gap which separates
the two commutator sections, and in this example, both sections
would touch the brushes at the same time. That would short out
the loop coil, and the short surge current would blow the motor
fuse (not shown) or burn up the motor wires which connect to the
power source.
In
a real motor, there are many armature windings and many commutator
sections in place, as shown in the picture below. This is a picture
of a heater fan motor armature on a truck. Notice the green armature
windings and the copper commutator sections. If you count the
commutator sections and divide by two, you can determine the number
of armature windings on this motor. There are twelve commutator
sections, so there must be six armature windings on this motor
armature.
The armature windings are all electrically insulated from each
other. Each winding repels for part of the motor rotation, and
then the next winding repels for another part of the motor rotation,
etc ...... When the motor completes one full rotation, the process
repeats again, with each winding doing its small share of the
motor rotation force generation.
Will
the brushes short out across the narrow gaps in this armature
also? Yes they will, but now you have simply applied current to
the next winding before you disconnect current from the present
winding. This causes no problem because the windings are insulated
from each other, therefore the brushes don't short out. In fact
this crossover between windings is a good thing to reduce commutator
arcing which would happen if each winding were completely disconnected
by itself. This arcing would be caused by inductive kick, and
is the result of collapsing magnetic fields, but we don't need
to understand that information to fix trucks.
 That
whole assembly of armature windings, commutator segments, and
brushes, also serves another purpose which we haven't mentioned
yet. If you look at this figure, it looks a lot like the figure
above, with one major difference. Can you see the difference?
The
current is entering the wire loop from the dark commutator segment
in this figure, and the current is entering the wire loop from
the light gray commutator segment on the figure
above. So the current through the wire loop has reversed directions
between the two figures. This is a major requirement for a direct
current (DC) motor. The commutator reverses the current through
the winding loop every 180 degrees, so that the magnetic force
keeps repelling in the same direction. This proves that I lied
to you above when I said the the whole process repeats for each
revolution of the motor. In reality, the whole process of each
winding segment generating rotational force repeats for each half
rotation of the motor.
Starter
Motor, Field Winding
When
high torque power is required, such as in a truck starter motor,
a permanent magnet is not powerful enough to generate the intense
flux field that is required. A permanent magnet which could do
the job, would be as large as your truck engine. The solution
was in creating a very powerful electromagnet. We have already
discussed how to make an electromagnet, and that is what is done
in the starter motors.
Earlier
we discussed how current through the armature loop created magnetic
lines of flux which forced the armature to rotate within the permanent
magnet flux field. Now we will place the loop current in series
with the field current as shown in this figure.
 The
starter motor current generates a very intense flux field across
the armature gap with the electromagnet, and the same motor current
also generates rotational force via the armature loop.
Let
us follow this motor current path. Start at the right post of
the battery (the negative battery terminal), and follow around
the north field winding, then across to the south field winding,
around the south field winding, then into the left hand armature
brush, through the loop, out the right hand armature brush, and
into the left post of the battery (positive battery terminal).
So this is in fact a series circuit, where both field windings
are in series with the armature loop. Once again, remember that
there are many electrically insulated loops on the starter armature,
for continued and smooth armature torque. In fact, most starter
motors have at least 12 armature loops, many more than your weak
little fan motor.
This
completes the electricity and magnets topic. We have covered
a lot of good information in this topic. This should provide you
with a solid background for learning the other electrical topics
that we offer. Further descriptions in other topics describe in
more physical detail how the relays, motors, and gauges work.
If you have
completed and understand this topic and it's prerequisites, then
you are now ready to study any of our remaining electrical topics
in any order. You now have the foundation to become a skilled
truck electrician by studying the remaining electrical topics
that we offer. Good luck, and don't blow a fuse when things don't
go your way. If you have any problems or comments, feel free to
contact webRider.
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