tvnovellas.info History AIRCRAFT ELECTRICAL SYSTEMS PDF

AIRCRAFT ELECTRICAL SYSTEMS PDF

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•Produces alternating current. (AC) which is converted to direct current (DC). •Belt driven (engine-driven). Recharges battery while engine is running. • Creates. Aircraft Electrical and Electronic Systems continues the series of textbooks written for aircraft engineering students. This book addresses the electrical contents. PDF | This review paper summarizes state-of-the-art energy management methods applied to electrical systems of large aircraft. An electrical load management.


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aircraft's electrical system and their sequence sets a convenient pattern on which a study of principles and applications can be based. The material for tvnovellas.info book. Introduction. The satisfactory performance of any modern aircraft depends to a very great degree on the continuing reliability of electrical systems and. Aircraft Electrical Systems Pallett - Free ebook download as PDF File .pdf), Text File .txt) or read book online for free.

Views: Transcription 1 Chapter 9 Aircraft Electrical System Introduction The satisfactory performance of any modern aircraft depends to a very great degree on the continuing reliability of electrical systems and subsystems. Improperly or carelessly installed or maintained wiring can be a source of both immediate and potential danger. The continued proper performance of electrical systems depends on the knowledge and technique of the mechanic who installs, inspects, and maintains the electrical system wires and cables. Basically, Ohm s Law states that the current electron flow through a conductor is directly proportional to the voltage electrical pressure applied to that conductor and inversely proportional to the resistance of the conductor. The unit used to measure resistance is called the ohm. In mathematical formulas, the capital letter R refers to resistance. The resistance of a conductor and the voltage applied to it determine the number of amperes of current flowing through the conductor.

When a voltage is applied across the conductor, an electromotive force creates an electric field within the conductor, and a current is established. The electrons do not move in a straight direction, but undergo repeated collisions with other nearby atoms within a conductor. These collisions usually knock other free electrons from their atoms, and these electrons move on toward the positive end of the conductor with an average velocity called the drift velocity, which is relatively low speed.

To understand the nearly instantaneous speed of the effect of the current, it is helpful to visualize a long tube filled with steel balls. E I X R Figure Ohm's Law chart. The term free electron describes a condition in some atoms where the outer electrons are loosely bound to their parent atom. These loosely bound electrons are easily motivated to move in a given direction when an external source, such as a battery, is applied to the circuit.

These electrons are attracted to the positive terminal of the battery, while the negative terminal is the source of the electrons. So, the measure of current is actually the number of electrons moving through a conductor in a given amount of time. The internationally accepted unit for current is the ampere A. One ampere A of current is equivalent to 1 coulomb C of charge passing through a conductor in 1 second.

Aircraft Electrical System

One coulomb of charge equals electrons. Obviously, the unit of amperes is a much more convenient term to use than coulombs. The unit of coulombs is simply too small to be practical. When current flow is in one direction, it is called direct current DC. Later in the text, the form of current that periodically oscillates back and forth within the circuit is discussed.

The present discussion is concerned only with Figure Electron flow. It can be seen that a ball introduced in one end of the tube, which represents the conductor, immediately causes a ball to be emitted at the opposite end of the tube. Thus, electric current can be viewed as instantaneous, even though it is the result of a relatively slow drift of electrons. Conventional Current Theory and Electron Theory There are two competing schools of thought regarding the flow of electricity.

The two explanations are the conventional current theory and the electron theory. Both theories describe the movement of electrons through a conductor. They simply explain the direction current moves. Typically during troubleshooting or the connection of electrical circuits, the use of either theory can be applied as long as it is used consistently.

The conventional current theory was initially advanced by Benjamin Franklin, who reasoned that current flowed out of a positive source into a negative source or an area that lacked an abundance of charge.

Chapter 6 - Electrical Systems.pdf - Aircraft Electrical...

Later discoveries were made that proved that just the opposite is true. Both conventional flow and electron flow are used in industry. Electromotive Force Voltage Voltage is most easily described as electrical pressure force. It is the electromotive force EMF , or the push or pressure from one end of the conductor to the other, that ultimately moves the electrons. The symbol for EMF is the capital letter E. EMF is always measured between two points and voltage is considered a value between two points.

For example, across the terminals of the typical aircraft battery, voltage can be measured as the potential difference of 12 volts or 24 volts. That is to say that between the two terminal posts of the battery, there is a voltage available to push current through a circuit. Free electrons in the negative terminal of the battery move toward the excessive number of positive charges in the positive terminal. The net result is a flow or current through a conductor.

There cannot be a flow in a conductor unless there is an applied voltage from a battery, generator, or ground power unit. The voltage at a second point in the circuit is To calculate the voltage drop, use the formula above to get a total voltage drop of 1. Figure illustrates the flow of electrons of electric current.

Two interconnected water tanks demonstrate that when a difference of pressure exists between the two tanks, water flows until the two tanks are equalized. Figure shows the level of water in tank A to be at a higher level, reading 10 pounds per square inch psi higher potential energy , than the water level in tank B, reading 2 psi lower potential energy.

Between the two tanks, there is 8 psi potential difference. If the valve in the interconnecting line between the tanks is opened, water flows from tank A into tank B until the level of water potential energy of both tanks is equalized.

It is important to note that it was not the pressure in tank A A Figure Difference of pressure. B that caused the water to flow; rather, it was the difference in pressure between tank A and tank B that caused the flow.

This comparison illustrates the principle that electrons move, when a path is available, from a point of excess electrons higher potential energy to a point deficient in electrons lower potential energy. The force that causes this movement is the potential difference in electrical energy between the two points.

This force is called the electrical pressure voltage , the potential difference, or the electromotive force electron moving force.

Resistance The two fundamental properties of current and voltage are related by a third property known as resistance. In any electrical circuit, when voltage is applied to it, a current results.

The resistance of the conductor determines the amount of current that flows under the given voltage. In general, the greater the circuit resistance, the less the current. If the resistance is reduced, then the current will increase.

This relation is linear in nature and is known as Ohm s Law. An example would be if the resistance of a circuit is doubled, and the voltage is held constant, then the current through the resistor is cut in half. There is no distinct dividing line between conductors and insulators; under the proper conditions, all types of material conduct some current. Materials offering a resistance to current flow midway between the best conductors and the poorest conductors insulators are sometimes referred to as semiconductors and find their greatest application in the field of transistors.

The best conductors are materials, chiefly metals, that possess a large number of free electrons. Conversely, insulators are materials having few free electrons. The best conductors are silver, copper, gold, and aluminum, but some nonmetals, such as carbon and water, can be used as conductors. Materials such as rubber, glass, ceramics, and plastics are such poor conductors that they are usually used as insulators. The current flow in some of these materials is so low that it is usually considered zero.

Factors Affecting Resistance The resistance of a metallic conductor is dependent on the type of conductor material. It has been pointed out that certain metals are commonly used as conductors because of the large number of free electrons in their outer orbits. Copper is usually considered the best available conductor material, since a copper wire of a particular diameter offers a lower resistance to current flow than an aluminum wire of the same diameter.

However, aluminum is much lighter than copper, and for this reason, as well as cost considerations, aluminum is often used when the weight factor is important. The longer the length of a given size of wire, the greater the resistance.

Figure shows two wire conductors of different lengths. If 1 volt of electrical pressure is applied across the two ends of the conductor that is 1 foot in length and the resistance to the movement of free electrons is assumed to be 1 ohm, the current flow is limited to 1 ampere.

If the same size conductor is doubled in length, the same electrons set in motion by the 1 volt applied now find twice the resistance. A chemical reaction exists between the metals which frees more electrons in one metal than in the other.

Heat used to produce electricity creates the thermoelectric effect. When a device called a thermocouple is subjected to heat, a voltage is produced.

A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. If the thermocouple is connected to a complete circuit, a current also flows. Thermocouples are often found on aircraft as part of a temperature monitoring system, such as a cylinder head temperature gauge.

As shown in Figure , when a conductor wire is moved through a magnetic field, an EMF is produced in the conductor. If a complete circuit is connected to the conductor, the voltage also produces a current flow. EMF Figure Resistance varies with length of conductor.

Electromagnetic Generation of Power Electrical energy can be produced through a number of methods. Common methods include the use of light, pressure, heat, chemical, and electromagnetic induction. Of these processes, electromagnetic induction is most responsible for the generation of the majority of the electrical power used by humans. Virtually all mechanical devices generators and alternators that produce electrical power employ the process of electromagnetic induction.

The use of light, pressure, heat, and chemical sources for electrical power is found on aircraft but produce a minimal amount of all the electrical power consumed during a typical flight. In brief, light can produce electricity using a solar cell photovoltaic cell.

Using pressure to generate electrical power is commonly known as the piezoelectric effect. The piezoelectric effect piezo or piez taken from Greek: to press; pressure; to squeeze is a result of the application of mechanical pressure on a dielectric or nonconducting crystal. Chemical energy can be converted into electricity, most commonly in the form of a battery. A primary battery S Motion of conductor Figure Inducing an EMF in a conductor.

This produces a greater electrical output. In many cases, the magnetic field is created by using a powerful electromagnet. Remember voltage electrical pressure must be present to produce a current electron flow. Hence, the output energy generated through the process of electromagnetic induction always consists of voltage. Either the conductor or magnet can be moving or stationary. When a magnet and its field are moved through a coiled conductor, as shown in Figure , a DC voltage with a specific polarity is produced.

The polarity of this voltage depends on the direction in which the magnet is moved and the position of the north and south poles of the magnetic field. The generator left-hand rule can be used to determine the direction of current flow within the conductor. An application of the generator left-hand rule. Generally speaking, on all aircraft, a generator or alternator employs the principles of electromagnetic induction to create electrical power for the aircraft. Either the magnetic field can rotate or the conductor can rotate.

Voltage induced in a loop. Figure Inducing a current flow. Number of turns in the conductor coil more loops equals greater induced voltage 7 2.

Strength of the electromagnet the stronger the magnetic field, the greater the induced voltage 3. Speed of rotation of the conductor or magnet the faster the rotation, the greater the induced voltage Figure illustrates the basics of a rotating machine used to produce voltage. The simple generating device consists of a rotating loop, marked A and B, placed between two magnetic poles, N and S.

The ends of the loop are connected to two metal slip rings collector rings , C1 and C2. Current is taken from the collector rings by brushes. If the loop is considered as separate wires, A and B, and the left-hand rule for generators is applied, then it can be observed that as wire B moves up across the field, a voltage is induced that causes the current to flow towards the reader.

As wire A moves down across the field, a voltage is induced that causes the current to flow away from the reader. When the wires are formed into a loop, the voltages induced in the two sides of the loop are combined. Therefore, for explanatory purposes, the action of either conductor, A or B, while rotating in the magnetic field is similar to the action of the loop. Figure illustrates the generation of alternating current AC with a simple loop conductor rotating in a magnetic field.

As it is rotated in a counterclockwise direction, varying voltages are induced in the conductive loop. Position 1 The conductor A moves parallel to the lines of force. Since it cuts no lines of force, the induced voltage is zero. As the conductor advances from position 1 to position 2, the induced voltage gradually increases. Position 2 The conductor is now moving in a direction perpendicular to the flux and cuts a maximum number of lines of force; therefore, a maximum voltage is induced.

As the conductor moves beyond position 2, it cuts a decreasing amount of flux, and the induced voltage decreases. Position 3 At this point, the conductor has made half a revolution and again moves parallel to the lines of force, and no voltage is induced in the conductor.

As the A conductor passes position 3, the direction of induced voltage now reverses since the A conductor is moving downward, cutting flux in the opposite direction. As the A conductor moves across the south pole, the induced voltage gradually increases in a negative direction until it reaches position 4. Position 4 Like position 2, the conductor is again moving perpendicular to the flux and generates a maximum negative voltage. From position 4 to position 5, the induced voltage gradually decreases until the voltage is zero, and the conductor and wave are ready to start another cycle.

Position 5 The curve shown at position 5 is called a sine wave. Quarter turn completed Conductors cutting directly across the magnetic field as conductor A passes across the north magnetic pole and B passes across the S pole.

Position 4 Three quarters turn completed Conductors again moving directly across magnetic field A passes across south magnetic pole and B across N magnetic pole. The generator has generated one complete cycle of alternating voltage or current.

Figure Generation of a sine wave. The horizontal baseline is divided into degrees, or time, and the vertical distance above or below the baseline represents the value of voltage at each particular point in the rotation of the loop. The specific operating principles of both alternators and generators as they apply to aircraft is presented later in this text.

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AC is the same type of electricity used in industry and to power our homes. Direct current DC is used on systems that must be compatible with battery power, such as on light aircraft and automobiles. There are many benefits of AC power when selected over DC power for aircraft electrical systems. AC can be transmitted over long distances more readily and more economically than DC, since AC voltages can be increased or decreased by means of transformers.

Because more and more units are being operated electrically in airplanes, the power requirements are such that a number of advantages can be realized by using AC especially with large transport category aircraft.

Space and weight can be saved since AC devices, especially motors, are smaller and simpler than DC devices. In most AC motors, no brushes are required, and they require less maintenance than DC motors.

Circuit breakers operate satisfactorily under loads at high altitudes in an AC system, whereas arcing is so excessive on DC systems that circuit breakers must be replaced frequently.

Finally, most airplanes using a volt DC system have special equipment that requires a certain amount of cycle AC current. For these aircraft, a unit called an inverter is used to change DC to AC. Inverters are discussed later in this book. AC is constantly changing in value and polarity, or as the name implies, alternating. Figure shows a graphic comparison of DC and AC. The polarity of DC never changes, and the polarity and voltage constantly change in AC. It should also be noted that the AC cycle repeats at given intervals.

With AC, both voltage and current start at zero, increase, reach a peak, then decrease and reverse polarity.

If one is to graph this concept, it becomes easy to see the alternating wave form. This wave form is typically referred to as a sine wave. These values help to define the sine wave and are called instantaneous, peak, and effective.

It should be noted that during the discussion of these terms, the text refers to voltage. But remember, the values apply to voltage and current in all AC circuits. Instantaneous An instantaneous voltage is the value at any instant in time along the AC wave. The sine wave represents a series of these values. The instantaneous value of the voltage varies from zero at 0 to maximum at 90, back to zero at , to maximum in the opposite direction at , and to zero again at Any point on the sine wave is considered the instantaneous value of voltage.

Peak The peak value is the largest instantaneous value, often referred to as the maximum value. The largest single positive value occurs after a certain period of time when the sine wave reaches 90, and the largest single negative value occurs when the wave reaches Although important in the understanding of the AC sine wave, peak values are seldom used by aircraft technicians.

Effective The effective values for voltage are always less than the peak maximum values of the sine wave and approximate DC voltage of the same value. For example, an AC circuit of 24 volts and 2 amps should produce the same heat through a resistor as a DC circuit of 24 volts and 2 amps. The effective value is also known as the root mean square, or RMS value, which refers to the mathematical process by which the value is derived. Most AC meters display the effective value of the AC.

In other words, the industry ratings are based on effective values. Peak and instantaneous values, used only in very limited situations, would be stated as such. In the study of AC, any values given for current or voltage are assumed to be effective values unless otherwise specified. In practice, only the effective values of voltage and current are used.

The effective value is equal to. Conversely, the peak value is 1. Thus, the volt value given for AC is only of the peak voltage of this supply. The frequency is typically measured in cycles per second CPS or hertz Hz. One Hz equals one CPS. The time it takes for the sine wave to complete one cycle is known as period P. Period is a value or time period and typically measured in seconds, milliseconds, or microseconds. It should be noted that the time period of a cycle can change from one system to another; it is always said that the cycle completes in related to the of rotation of an AC alternator.

The cycle repeats until the voltage is no longer available. So, the measure of current is actually the number of electrons moving through a conductor in a given amount of time. The internationally accepted unit for current is the ampere A. One ampere A of current is equivalent to 1 coulomb C of charge passing through a conductor in 1 second. One coulomb of charge equals 6. Obviously, the unit of amperes is a much more convenient term to use than coulombs.

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The unit of coulombs is simply too small to be practical. When current flow is in one direction, it is called direct current DC. Later in the text, the form of current that periodically oscillates back and forth within the circuit is discussed.

The present discussion is concerned only with It can be seen that a ball introduced in one end of the tube, which represents the conductor, immediately causes a ball to be emitted at the opposite end of the tube. Thus, electric current can be viewed as instantaneous, even though it is the result of a relatively slow drift of electrons.

Conventional Current Theory and Electron Theory There are two competing schools of thought regarding the flow of electricity. The two explanations are the conventional current theory and the electron theory. Both theories describe the movement of electrons through a conductor.

They simply explain the direction current moves. Typically during troubleshooting or the connection of electrical circuits, the use of either theory can be applied as long as it is used consistently. The conventional current theory was initially advanced by Benjamin Franklin, who reasoned that current flowed out of a positive source into a negative source or an area that lacked an abundance of charge. Later discoveries were made that proved that just the opposite is true.

Both conventional flow and electron flow are used in industry. Electromotive Force Voltage Voltage is most easily described as electrical pressure force. It is the electromotive force EMF , or the push or pressure from one end of the conductor to the other, that ultimately moves the electrons.

The symbol for EMF is the capital letter E. EMF is always measured between two points and voltage is considered a value between two points. For example, across the terminals of the typical aircraft battery, voltage can be measured as the potential difference of 12 volts or 24 volts. That is to say that between the two terminal posts of the battery, there is a voltage available to push current through a circuit.

Free electrons in the negative terminal of the battery move toward the excessive number of positive charges in the positive terminal. The net result is a flow or current through a conductor. There cannot be a flow in a conductor unless there is an applied voltage from a battery, generator, or ground power unit. The voltage at a second point in the circuit is To calculate the voltage drop, use the formula above to get a total voltage drop of 1. Figure illustrates the flow of electrons of electric current.

Two interconnected water tanks demonstrate that when a difference of pressure exists between the two tanks, water flows until the two tanks are equalized. Figure shows the level of water in tank A to be at a higher level, reading 10 pounds per square inch psi higher potential energy , than the water level in tank B, reading 2 psi lower potential energy. Between the two tanks, there is 8 psi potential difference.

If the valve in the interconnecting line between the tanks is opened, water flows from tank A into tank B until the level of water potential energy of both tanks is equalized.

It is important to note that it was not the pressure in tank A that caused the water to flow; rather, it was the difference in pressure between tank A and tank B that caused the flow.

This comparison illustrates the principle that electrons move, when a path is available, from a point of excess electrons higher potential energy to a point deficient in electrons lower potential energy.

The force that causes this movement is the potential difference in electrical energy between the two points. This force is called the electrical pressure voltage , the potential difference, or the electromotive force electron moving force. Resistance The two fundamental properties of current and voltage are related by a third property known as resistance.

In any electrical circuit, when voltage is applied to it, a current results. The resistance of the conductor determines the amount of current that flows under the given voltage. In general, the greater the circuit resistance, the less the current.

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If the resistance is reduced, then the current will increase. An example would be if the resistance of a circuit is doubled, and the voltage is held constant, then the current through the resistor is cut in half. There is no distinct dividing line between conductors and insulators; under the proper conditions, all types of material conduct some current. Materials offering a resistance to current flow midway between the best conductors and the poorest conductors insulators are sometimes referred to as semiconductors and find their greatest application in the field of transistors.

The best conductors are materials, chiefly metals, that possess a large number of free electrons. Conversely, insulators are materials having few free electrons. The best conductors are silver, copper, gold, and aluminum, but some nonmetals, such as carbon and water, can be used as conductors.

Materials such as rubber, glass, ceramics, and plastics are such poor conductors that they are usually used as insulators. The current flow in some of these materials is so low that it is usually considered zero.

Factors Affecting Resistance A Figure Difference of pressure. B The resistance of a metallic conductor is dependent on the type of conductor material. It has been pointed out that certain metals are commonly used as conductors because of the large number of free electrons in their outer orbits. Copper is usually considered the best available conductor material, since a copper wire of a particular diameter offers a lower resistance to current flow than an aluminum wire of the same diameter.

However, aluminum is much lighter than copper, and for this reason, as well as cost considerations, aluminum is often used when the weight factor is important. The longer the length of a given size of wire, the greater the resistance.

Figure shows two wire conductors of different lengths. If 1 volt of electrical pressure is applied across the two ends of the conductor that is 1 foot in length and the resistance to the movement of free electrons is assumed to be 1 ohm, the current flow is limited to 1 ampere. If the same size conductor is doubled in length, the same electrons set in motion by the 1 volt applied now find twice the resistance.

When a device called a thermocouple is subjected to heat, a voltage is produced. A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. If the thermocouple is connected to a complete circuit, a current also flows. Thermocouples are often found on aircraft as part of a temperature monitoring system, such as a cylinder head temperature gauge.

Electromagnetic induction is the process of producing a voltage EMF by moving a magnetic field in relationship to a conductor.

As shown in Figure , when a conductor wire is moved through a magnetic field, an EMF is produced in the conductor. If a complete circuit is connected to the conductor, the voltage also produces a current flow. A chemical reaction exists between the metals which frees more electrons in one metal than in the other.

Resistance varies with length of conductor.