Thursday, January 15, 2009

Magnetism.

Introduction to Magnetism

Magnetism is another form of force that causes electron flow or current. A basic understanding of magnetism is necessary to study electricity. Magnetism provides a link between mechanical energy and electricity. The use of magnetism causes an alternator to convert some of the mechanical power developed by an engine to electromotive force (EMF). Magnetism will allow a starter motor to convert electrical energy from a battery into mechanical energy for cranking the engine.

The Nature of Magnetism

Most electrical equipment depends directly or indirectly upon magnetism. There are a few electrical devices that do not use magnetism.

There are three basic types of magnets:

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Natural
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Man-made
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Electromagnets

Natural Magnets

The Chinese discovered magnets about 2637 B.C. The magnets that were used in the primitive compasses were called lodestones. Lodestones were crude pieces of iron ore. The lodestones are known as magnetite. Since magnetite has magnetic properties in the natural state, lodestones are classified as natural magnets.

Man-made Magnets

Man-made magnets are typically produced in the form of metal bars. These bars have been subjected to very strong magnetic fields. All man-made magnets are produced. These man-made magnets are sometimes referred to as artificial magnets.

Electromagnets

A Danish scientist named Oersted discovered a relation between magnetism and electric current. Oersted discovered that an electric current flowing through a conductor produced a magnetic field around the conductor.

Magnetic Fields

Every magnet has two points opposite each other which attract pieces of iron. These points are called the poles of the magnet: the north pole and the south pole. Just like electric charges repel each other and opposite charges attract each other, like magnetic poles repel each other and unlike poles attract each other.

A magnet clearly attracts a bit of iron because of some force that exists around the magnet. This force is called a magnetic field. Although it is invisible to the naked eye, the force can be shown by sprinkling small iron filings on a sheet of glass or paper over a bar magnet. In Illustration 1, a piece of glass is placed over a magnet and iron filings are sprinkled on the glass. When the glass cover is gently tapped the filings will move into a definite pattern, which shows the field force around the magnet.

The field seems to be made up of lines of force that appear to leave the magnet at the north pole. The lines of force travel through the air around the magnet. The lines of force continue through the magnet to the south pole to form a closed loop of force. The stronger the magnet is, the greater the lines of force are, and the larger the area covered by the magnetic field.

Lines of Force

To better visualize the magnetic field without iron filings, the field is shown as lines of force in Illustration 3. The direction of the lines outside the magnet shows the path a north pole would follow in the field, repelled away from the north pole of the magnet and attracted to the south pole. Inside the magnet, which is the generator for the magnetic field, the lines are from south pole to north pole.

Lines of Magnetic Flux

The entire group of magnetic field lines is called magnetic flux. The flux density is the number of magnetic field lines per unit of a section perpendicular to the direction of flux. The unit is lines per square inch in the English system. The unit is lines per square centimeter in the metric system. One line per square centimeter is called a gauss.

Magnetic Force

Magnetic lines of force pass through all materials. There is no known insulator against magnetism. Flux lines pass more easily through materials that can be magnetized than through those that cannot be magnetized. Materials that do not readily pass flux lines are said to have high magnetic reluctance. Air has high reluctance. Iron has low reluctance.

An electric current flowing through a wire creates magnetic lines of force around the wire. Illustration 4 shows lines of small magnetic circles that form around the wire.

Because such flux line are circular, the magnetic field has no north pole or no south pole. Individual circular fields merge when the wire is wound into a coil. The result is a unified magnetic field with north and south poles, as shown in Illustration 5.

As long as current flows through the wire, it behaves just like a bar magnet. The electromagnetic field remains as long as current flows through the wire. The field that is produced on a straight wire does not have enough magnetism in order to do work. To strengthen the electromagnetic field, the wire can be formed into a coil. The magnetic strength of an electromagnet is proportional to the number of turns of wire in the coil, and the current flowing through the wire. Whenever electrical current flows through the coil of wire, a magnetic field, or lines of force, build up around the coil. If the coils are wound around a metal core, like iron, the magnetic force strengthens considerably.

Relays and Solenoids

Types of electromagnets that are typically used in Caterpillar machines are relays and solenoids. Relays and solenoids operate on the electromagnetic principle, but function differently. Relays are used as an electrically controlled switch. A relay is made up of an electromagnetic coil, a set of contacts, and an armature. The armature is a movable device that allows the contacts to open and to close. Illustration 6 shows the typical components of a relay.

When a small amount of electrical current flows in the coil circuit, the electromagnetic force causes the relay contacts to close. This process provides a much larger current path to operate another component, such as, a starter.

A solenoid is another device that uses electromagnetism. Like a relay, the solenoid also has a coil. Illustration 7 shows a typical solenoid. When current flows through the coil, electromagnetism pushes or pulls the core into the coil creating linear, or back and forth movements. Solenoids are used to engage starter motors, or control shifts in an automatic transmission.

Electromagnetic Induction

The effect of creating a magnetic field with current has an opposite condition. It is also possible to create current with a magnetic field by inducing a voltage in the conductor. This process is known as electromagnetic induction. Electromagnetic induction happens when the flux lines of a magnetic field cut across a wire or any conductor. It does not matter whether the magnetic field moves or the wire moves. When there is relative motion between the wire and the magnetic field, a voltage is induced in the conductor. The induced voltage causes a current to flow. When the motion stops, the current stops.

If a wire is passed through a magnetic field, voltage is induced. The voltage induced strengthens when the wire is wound into a coil. This method is the operating principle that is used in speed sensors, generators, and alternators. In some cases, the wire is stationary and the magnet moves. In other cases, the magnet is stationary and the field windings move.

Movement in the opposite direction causes current to flow in the opposite direction. This causes back and forth motion to produce AC voltage (current).

In practical applications, multiple conductors are wound into a coil. This concentrates the effects of electromagnetic induction and makes it possible to generate useful electrical power with a relatively compact device. In a generator, the coil moves and the magnetic field is stationary. In an alternator, the magnet is rotated inside a stationary coil.

The strength of an induced voltage depends on several factors:

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The strength of the magnetic field
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The speed of the relative motion between the field and the coil
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The numbers of conductors in the coil

Means of Induction

There are three ways voltage can be induced by electromagnetic induction:

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Generated Voltage
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Self-Induction
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Mutual Induction

Generated Voltage

Illustration 9 shows a simple DC generator that is used to show a moving conductor that passes a stationary magnetic field to produce voltage and current. A single loop of wire is rotating between the north and south poles of a magnetic field.

Self-Induction

Self-induction occurs in a current carrying wire when the current flowing through the wire changes. Since the current flowing through the conductor creates a magnetic field around the wire that builds up and collapses as the current changes, a voltage is induced in the conductor. Illustration 10 shows self-induction in a coil.

Mutual Induction

Mutual induction occurs when the changing current in one coil induces a voltage in an adjacent coil. A transformer is an example of mutual induction. Illustration 11 shows two inductors that are relatively close to each other. When an AC current flows through coil L1 a magnetic field cuts through coil L2 inducing a voltage and producing current flow in coil L2.



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Saturday, January 3, 2009

Electrical basic

Electrical System for All Caterpillar Products


Electricity - How It Works

Introduction to Electricity
Flashlights, electric drills, and motors are electric. Computers and televisions are refered as electronic.
Any component that works with electricity is electric, including both flashlights and electric drills, but not all electric components are electronic. The term electronic refers to semiconductor devices known as electron devices. These devices depend on the flow of electrons for operation.
To better understand electricity, it is necessary to have a basic knowledge of the fundamental atomic structure of matter. Matter has mass and occupies space. Matter can take several forms or states. The three common forms are a solid, a liquid and a gas. This course will provide a basic understanding of the theoretical principles needed to develop a foundation for studying and working with electrical circuits and components as a Caterpillar technician.

Matter and Elements.
Matter takes up space and has weight when matter is subjected to gravity. Matter consists of extremely tiny particles grouped together to form atoms. There are approximately 100 different naturally, occurring atoms called elements. An element is a substance that cannot be decomposed any further by chemical action. Most elements have been found in nature. The following are examples of some of the natural elements: copper, lead, iron, gold and silver. Other elements (approximately 14) have been produced in the laboratory. Elements can only be changed by an atomic reaction or nuclear reaction. Elements can be combined to make compounds. The atom is the smallest particle of an element that still has the same characteristics as the element. Atom is the greek word meaning a particle too small to be subdivided.

Atoms.
Although an atom has never been seen, the structure fits evidence that has been measured accurately. The size and electric charge of the invisible particles in an atom are indicated, by how much the atoms are deflected by known forces. The present atomic model, with a nucleus at the center was proposed by Niels Bohr in 1913. The atomic model is patterned after the solar system with the sun at the center and the planets revolving around it.
The center of an atom is called the nucleus. The nucleus is composed primarily of particles called protons and neutrons. Orbiting around every nucleus are small particles called electrons. These electrons are much smaller in mass than either the proton or neutron. Normally, an atom has an equal number of protons in the nucleus and electrons around the nucleus. The number of protons or electrons is called the atomic number. The atomic weight of an element is the total number of particles, protons, and neutrons that are in the nucleus.
Illustration 3 shows the structure of two of the simpler atoms. Illustration 3(a) is an atom of hydrogen, which contains 1 proton in its nucleus balanced by 1 electron in the orbit or shell. The atomic number for a hydrogen atom is 1. Illustration 3(b) shows a simple atom of helium, which has 2 protons in its nucleus balanced by 2 electrons in orbit. The atomic number for helium is 2 and the atomic weight would be 4 (2 protons + 2 neutrons).
Scientists have discovered many particles in the atom. In order to explain basic electricity, only three particles will be discussed: electrons, protons and neutrons. To better understand the basics of electricity, we will use an atom of copper as an example.
Illustration 4 shows a typical copper atom. The nucleus of the atom is not much bigger than an electron. In the copper atom the nucleus contains 29 protons (+), 35 neutrons. The copper atom has 29 electrons (-) orbiting the nucleus. The atomic number of the copper atom is 29 and the atomic weight is 64. When a length of copper wire is connected to positive and negative source, such as a dry cell battery, the following process occurs.
An electron (-) is forced out of orbit and attracted to the positive (+) end of the battery. The atom is now positive (+) charged because it now has a deficiency of electrons (-). The atom in turn attracts an electron from a neighbor. The neighbor in turn receives an electron from the next atom, and so on until the last copper atom receives an electron from the negative end of the battery.
The result of this chain reaction is that the electrons move through the conductor from the negative end to the positive end of the battery. The flow of electrons will continue as long as the positive charges and negative charges from the battery are maintained at each end of the conductor.

Electrical Energy.
There are two types of forces at work in every atom. Under normal circumstances, these two forces are in balance. The protons and electrons exert forces on one another. These are over and above gravitational or centrifugal forces. Besides mass, electrons and protons carry an electric charge. These additional forces are attributed to the electric charge that they carry. However, there is a difference in the forces. Between masses, the gravitational force is always one of attraction while the electrical forces both attract and repel. Protons and electrons attract one another, while protons exert forces of repulsion on other protons, and electrons exert repulsion on other electrons.
There appears to be two kinds of electrical charge. The protons are said to be positive (+). The electrons are said to be negative (-). The neutron carries a neutral charge. Polarity is based on directional flow of electricty. The type of change determines direction. This leads to the basic law of electrostatic which states, UNLIKE charges attract each other, while LIKE charges repel each other.

Charges and Electrostatics
The attraction or repulsion of electrically-charged bodies is due to an invisible force called an electrostatic field, which surrounds the charged body. Illustration 7 shows the force between charged particles as imaginary electrostatic lines from the negative charge to the positive charge.
When two like charges are placed near each other, the lines of force repel each other as shown below.
Potential Difference
Because of the force of the electrostatic field, an electric charge has the ability to move another charge by attraction or by repulsion.
The ability to attract or repell is called potential. When one charge is different from the other, there must be a difference in potential between them.
The sum difference of potential of all charges in the electrostatic field is referred to as electromotive force (EMF). The basic unit of potential difference is the volt (V). The volt is named in honor of Alessandro Volta an Italian scientist. Volta invented the voltaic pile, the first battery cell. The symbol for potential is V, that indicates the ability to force electrons to move. Because the volt unit is used, the potential difference is called voltage.
The following conditions will produce voltage: friction, solar, chemical and electromagnetic induction. A photocell, such as on a calculator, would be an example of producing voltage from solar energy.
Coulomb
A need existed to develop a unit of measurement for electrical charge. A scientist named Charles Coulomb investigated the law of forces between charged bodies and adopted a unit of measurement that is called the Coulomb. When the Coulomb is written in scientific notation this measurement is expressed as One.
Coulomb = 6.28 ×1018 electrons or protons. Stated in simpler terms, in a copper conductor, one ampere is an electric current of 6.28 billion electrons passing a certain point in the conductor in one second.

Current
Another theory that needs to be explained is the theory of motion in a conductor. The motion of charges in a conductor is an electric current. An electron will be affected by an electrostatic field in the same manner as any negatively charged body. An electron is repelled by a negative charge and attracted by a positive charge. The drift of electrons or movement constitutes an electric current.
The magnitude or intensity of current is measured in amperes. The unit symbol is A. An ampere is a measure of the rate when a charge is moved through a conductor. One ampere is a coulomb of charge that moves past a point in one second.

Conventional versus Electron Flow
There are two ways to describe an electric current flowing through a conductor. Prior to the use of atomic theory to explain the composition of matter, scientists defined current as the motion of positive charges in a conductor from a point of positive polarity to a point of negative polarity. This conclusion is still widely held in some engineering standards and textbooks. Some examples of positive charges in motion are applications of current in liquids, gases, and semiconductors. This theory of current flow has been termed conventional current.
With the discovery of using atomic theory to explain the composition of matter, it was determined that current flow through a conductor was based on the flow of electrons (-), or negative charge. Therefore, electron current is in the opposite direction of conventional current and is termed electron current.
Either theory can be used, but the more popular conventional theory describing current as flowing from a positive (+) charge to a negative (-) charge will be used in this course.

Resistance
George Simon Ohm discovered that for a fixed voltage, the amount of current flowing through a material depends on the type of material and the physical dimensions of the material. In other words, all materials present some opposition to the flow of electrons. That opposition is termed resistance. If the opposition is small, the material is labeled a conductor. If the opposition is large, it is labeled an insulator.
The Ohm is the unit of electrical resistance. The symbol to represent an Ohm is the Greek letter omega, Ohms. A material is said to have a resistance of one ohm, if a potential of one volt results in a current of one ampere.
It is important to remember that electrical resistance is present in every electrical circuit, including components, interconnecting wires, and connections. Electrical circuits and the laws that relate to the electrical circuits will be discussed later in this unit.
As resistance works to oppose current flow, it changes electrical energy into other forms of energy, such as, heat, light, or motion. The resistance of a conductor is determined by four factors:
Atomic structure is the amount of free electrons. The more free electrons a material has, the less resistance that is offered to current flow.

1.
Length. The longer the conductor, the higher the resistance. If the length of the wire is doubled, as shown in Illustration 12(a), the greater the resistance between the two ends.
2.
Width (cross sectional area). The larger the cross sectional area of a conductor, the lower the resistance (a bigger diameter pipe allows for more water to flow). If the cross section area is reduced by half, as shown in Illustration 12(b), the resistance for any given length is increased by a factor of 4.

1.
Temperature. For most materials, the higher the temperature, the higher the resistance. Illustration 12(c) shows the resistance increasing as the temperature rises. Please note, there are a few materials whose resistance decreases as temperature increases.

Electrical Circuits and Laws

An electrical circuit is a path or a group of interconnecting paths, that are capable of carrying electrical currents. The electrical circuit is a closed path that contains a voltage source or sources. There are two basic types of electrical circuits: series and parallel. The basic series and parallel circuits may be combined to form more complex circuits, but these combinational circuits may be simplified and analyzed as the two basic types. It is important to understand the laws that are needed to analyze electrical circuits and to diagnose electrical circuits. They are Kirchoff's Laws and Ohm's Law.

Gustav Kirchoff developed two laws for analyzing circuits. The two laws are stated below:

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Kirchoff's Current Law (KCL) states that the algebraic sum of the currents at any junction in an electrical circuit is equal to zero. Simply stated, all the current that enters a junction is equal to all the current that leaves the junction.
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Kirchoff's Voltage Law (KVL) states that the algebraic sum of the electromotive forces and voltage drops around any closed electrical loop is zero. Simply said, the addition of all differences of potential in a closed circuit will equal zero.

George Simon Ohm discovered one of the most important laws of electricity. The law describes the relationship between three electrical parameters: voltage, current and resistance. Ohms' law is stated as follows: The current in an electrical circuit is directly proportional to the voltage and inversely proportional to the resistance. The relationship can be summarized by a single mathematical equation:

Current = Electromotive Force/Resistance

or, stated in electrical units: I= Volts/Ohms

Single letters are used to represent mathematical equations that express electrical relationships. Resistance is represented by the letter R or the Omega symbol (Ohms). The voltage or the difference in potential is represented by the letter E or the letter V (electromotive force). Current is represented by the letter I (intensity of charge). Using these laws to calculate circuits will be discussed later in this course.

Electrical Conductors

In electrical applications, electrons travel along a path that is called a conductor or a wire. The electrons move by traveling from atom to atom. Some materials make it easier for electrons to travel.These are called good conductors.

The following materials are examples of good conductors :

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silver
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copper
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gold
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chromium
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aluminum
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tungsten

A material is said to be a good conductor if the material has many free electrons. The amount of electrical pressure or voltage it takes to move electrons through a material depends on how free the electrons are.

Although silver is the best conductor, silver is also expensive. Gold is a good conductor, but gold is not as good as copper. The advantage gold has is gold will not corrode like copper. Aluminum is not as good as copper, but is less expensive and lighter.

The conductivity of a material determines the quality of the material. Illustration 13 shows some of the common conductors and the relative conductivity to copper.

Other materials make it difficult for electrons to travel. These materials are called insulators. A good insulator keeps the electrons tightly bound in orbit. The following examples of insulators are: rubber, wood, plastics and ceramics. It is possible to make an electric current flow through every material. If the applied voltage is high enough, even the best insulators will break down and will allow current flow. The following chart, shown in Illustration 14, lists some of the more common insulators.

Dirt and moisture may serve to conduct electricity around an insulator. A dirty insulator or moisture could cause a problem. The insulator is not breaking down, but the dirt or moisture can provide a path for electrons to flow. It is important to keep the insulators and contacts clean.

Wires

A wire in an electrical circuit is made up of a conductor and an insulator. The conductor is typically made up of copper and the insulator (outside covering) is made of plastic or rubber. Conductors can be a solid wire or a stranded wire. In most earthmoving applications, the wire is stranded copper with a plastic insulation. This insulation covers the conductor.

There are many sizes of wire. The smaller the wire is, the larger the identification number. The numbering system is known as the American Wire Gage (AWG). Illustration 15 describes the AWG wire size standard.

Resistance can also be affected by other conditions, such as, corrosion. These conditions need to be considered when resistance measurements are made.
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