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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