A/C Electrical System 2 Reviewer PDF

A/C ELECTRICAL SYSTEM 2 REVIEWER FREQUENCY AND PERIODIC TIME The frequency of a repetitive waveform is the number of cycles of the waveform that occur in unit time. Frequency is expressed in hertz (Hz). The periodic time (or period) of a waveform is the time taken for one complete cycle of the wav...

A/C ELECTRICAL SYSTEM 2 REVIEWER FREQUENCY AND PERIODIC TIME The frequency of a repetitive waveform is the number of cycles of the waveform that occur in unit time. Frequency is expressed in hertz (Hz). The periodic time (or period) of a waveform is the time taken for one complete cycle of the wave. The relationship between periodic time and frequency is thus: t = 1/f or f=1/t ALTERNATING CURRENT AND TRANSFORMERSVIONICS A graph showing the variation of voltage or current present in a circuit is known as a waveform. ALTERNATING CURRENT AND TRANSFORMERS Direct currents are currents which essentially flow only in one direction. In other words, direct currents are unidirectional. Alternating currents, on the other hand, are bidirectional and continuously reversing their direction of flow. INDUCTORS Inductors provide us with a means of storing electrical energy in the form of a magnetic field. Typical applications include chokes, filters, and frequency. selective circuits. The electrical characteristics of an inductor are determined by a number of factors including the material of the core (if any), the number of turns, and the physical dimensions of the coil. SELF-INDUCTANCE AND MUTUAL INDUCTANCENIONIS When two inductors are placed close to one another, the tux generated when a changing current flows in the first inductor will cut through the other inductor. This changing flux will, in turn, induce a current in the second Inductor. This effect is known as mutual inductance and it occurs whenever two inductors are inductively coupled. This is the principle of a very useful component, the transformer. SELF-INDUCTANCE AND MUTUAL INDUCTANCENIONICS An induced e.m.t. l.e. a back e.m.f. is produced by a tiux change in an inductor. The back e mf. is proportional to the rate of change of current trom Lenz's law). This effect is called self-inductance (or just inductance) which has the sumbol I. The unit of inductance is the henry (H) and a coil is said to have an inductance of 1H if a voltage of 1Vis induced across it when a current changing at the rate of 1 As Is flowing in it. FARADAY'S AND LENZ'S LAWS Faraday's law tells us that the magnitude of the induced em. is dependent on the relative velocity with which the conductor cuts the lines of !! magnetic flux. Lenz's law states that the current induced in a conductor opposes the changing field that produces it. The induced current always acts in such a direction so as to oppose the change in flux. Example A closed conductor of length 15 cm cuts the magnetic flux field of 1.25 T with a velocity of 25 m/s. Determine the induced e.m.f. when: (a) the angle between the conductor and field lines is 60° (b) the angle between the conductor and field lines is 90°. ELECTROMAGNETIC INDUCTION he magnitude of the induced (generated) e.m.t., e, is proportional to the flux density, length of conductor and relative velocity between the tield and the conductor. The magnitude of the induced e.m.f. Also depends on: o the length of the conductor / in m o the strength of the magnetic field, B, in tesla (T) o the velocity of the conductor, v, in m/s. ELECTROMAGNETIC INDUCTION In order to generate electricity we require movement in to get electricity out. In fact we need the same components to generate electricity as those needed for the electric motor, namely a closed conductor, a magnetic field and movement. Whenever relative motion occurs between a magnetic field and a conductor acting at right angles to the field, an e.m.f. is induced, or generated in the conductor. The manner in which this e.m.f. is generated is based on the principle of electromagnetic induction. ELECTROMAENETISM AND INDUCTORS Key points A magnetic field of flux is the region in which the fire meated by theraped hit he noth and south poles of the magnet. Whenever an electric current flows in a conductor a magnetic field is set up in the space surrounding the conductor. The field spreads out around the conductor in concentric circles with the greatest density of magnetic flux nearest to the conductor. ELECTROMAGNETISM AND INDUCTORS Magnetism is an elfect created by moving the elementary atomic particles in certain materials-such as iron, nickel and cobalt. Iron has outstanding magnetic properties, and materials that behave magnetically, in a similar manner to iron, are known as ferromagnetic materials. These materials experience forces that act on them when placed near a magnet. POWER AND ENERGY Power, P, is the rate at which energy is converted from one form to another and it is measured in watts (W). The larger the amount of power the greater the amount of energy that is converted in a given period of time. P = W/t Electrical energy is the capacity to do work. Energy can be converted from one form to another. Energy can only be transferred when a difference in energy levels exists. The unit of energy is the joule (U). Then, from the definition of power, W= Pt Thus joules are measured in watt-seconds. If the power was to be measured in kilowatts and the time in hours, then the unit of electrical energy would be the kilowatt- hour (kWh) POWER AND ENERGY Thus joules are measured in watt-seconds. If the power was to be measured in Kllowatts and the time in hours, then the unit of electrical energy would be the kilowatt- hour (kWh) (commonly known as a unit of electricity The electricity meter in your home records the amount of energy that you have used expressed in kilowatt-hours. The power in an electrical circuit is eauivalent to the product of voltage and current. where P is the power in watts (W), I is the current in amps (A), and V is the voltage in volts (V). POWER AND ENERGY We can combine the Ohm's law relationship that we met earlier with the formula for power to arrive at two further useful relationships: P = IV = 1 x (IR) = 1/2 R P = IV = (V/R) x V = V^2/R SERIES AND PARALLEL CIRCUIT In a series circuit, the components are connected in a line and the same current flows through all of them. In a parallel circuit, the components are connected so that each component has its own separate branch and the same voltage is applied to each component. SERIES AND PARALLEL CIRCUIT Figure shows a simple battery test circuit which is designed to draw a current of 2 A from a 24 V DC supply. The two test points, A and B, are designed for connecting a meter. Determine: (a) the voltage that appears between terminals A and B (without the meter connected); (c) the value of resistor, R. DIRECT CURRENT o Direct current (DC) is current that flows in one direction only. Because of their negative charge, electrons will flow from a point of negative potential to a point with more positive potential. However, when we indicate the direction of current in a circuit we show it as moving trom a point that has the greatest positive potential to a point that has the most negative potential. We call this conventional current and, itflows in the opposite direction to that of the motion of electrons. o The most commonly used method of generating direct current is the electrochemical cell. A cell is a device that produces a charge when a chemical reaction takes place When several cells are connected together, they form a battery. o There are two types of cell: primary and secondary. Primary cells produce electrical energy at the expense of the chemicals from which they are made and once these chemicals are used up, no more electricity can be obtained from the cell. In secondary cells, the chemical reaction is reversible. This means that the- chemical energy is converted into electrical energy when the cell is discharged whereas electrical energy - is converted into chemical energy when the cell is being charged DIRECT CURRENT When removing and replacing batteries, it is essential to observe the, guidance given in the aircratt maintenance manual (AMM when removing, charging or replacing aircraft batteries. The AMM will describe the correct procedures for isolating the battery trom the aircraits electrical system prior to its physical removal. ELECTRIC FIELD The force exerted on a charged particle is a manifestation of the existence of an electric Tield. The electric field defines the direction and magnitude of a force on a charged object. The field itself is invisible to the human eye but can be drawn by constructing lines which indicate the motion of a tree positive charge within the field; the number of field lines in a particular region being used to indicate the relative strength of the field at the point in question. ELECTRIC FIELD The field that exists between two charged parallel metal plates which forms a charge storage device known as a capacitor. The strength of an electric field (E) is proportional to the applied potential difference and inversely proportional to the distance between the two conducting surfaces ELECTROSTATICS AND CAPACITORS Charged bodies with the same polarity repel one another whilst charges with opposite polarity will attract one another. Static charges can be produced by friction. In this case, electrons and protons in an insulator are separated from each other by rubbing two materials together in order to produce opposite charges. These charges will remain separated for some time until they eventually eak away due to losses in the insulating dielectric material or in the air surrounding the materials. Static electricity is something that can cause particular problems in an aircraft and special measures are taken to ensure that excessive charges do not build upon the aircraft's structure. The aim is that of equalizing the potential of all points on the aircraft's external surfaces. The static charge that builds up during normal flight can be dissipated into the atmosphere surrounding the aircraft by means of small conductive rods connected to the aircraft's trailing surfaces. These are known as static dischargers or static wicks. ELECTROSTATICS AND CAPACITORS Stray static charges can very easily damage static-sensitive devices such as semiconductors, memory devices and other integrated circuits. Damage can be prevented by adopting the appropriate electrostatic sensitive device (ESD) precautions (described in the aircraft maintenance manual) when handling such devices. Precautions usually involve using wrist straps and grounding leads as well as using static-dissipative packaging materials. ELECTRON THEORY All electrons and protons carry an electrostatic charge but its value is so small that a more convenient unit of charge is needed for practical use which we call the coulomb. Conductor - a material which has many free electrons avallable to act as charge carriers, and thus allows current to flow freely. t.g. aluminium, copper, gold and iron. The figure a material with one outer electron that can become easily detached from the parent atom. A small amount of external energy is required to overcome the attraction of the nucleus. Sources of such energy may include heat, light or electrostatic fields. The atom once detached from the atom is able to move freely around the structure of the material and is called a free electron. It is these free electrons that become the charge carriers within a material. Materials that have large numbers of free electrons make good conductors of electrical energy and heat. ELECTRON THEORY Bohr model shows a single atom consisting of a central nucleus with orbiting electron's. Within the nucleus there are protons (positively charged and neutrons electrical neutral and have no charge). Orbiting the nucleus are electrons (negative charge), equal in magnitúdé (size) to the charge on the proton. These electrons are approximately two thousand times lighter than the protons and neutrons in the nucleus. In a stable atom the number of protons and electrons are equal, so that overall, the atom is neutral and has no charge. However, if we rub two particular materials together, electrons may be transferred from one to another. This alters the stability of the atom, leaving it with a net positive or negative charge. When an atom within a material loses electrons it becomes positively charged and is known as a positive ion, when an atom gains an electron it has a surplus negative charge and so is referred to as a negative ion. These differences in charge can cause electrostatic effects KIRCHHOFF'S LAWS Kirchhot's current law states that the algebraic sum of the curents present a a junction (or node ) in a circuit is zero. Kirchhoff's voltage law states that the algebraic sum of the potential drops present in a closed network (or mesh) is zero. OHM'S LAW For any conductor, the current flowing is directly proportional to the e mat applied. The current flowing will also be dependent on the physical dimensions (length and cross- sectional area) and material of which the conductor is composed. The amount of current that will flow in a conductor when a given e.m.f. is applied is inversely proportional to its resistance. Provided that temperature does not vary, the ratio of p.d. across the ends of a conductor to the current flowing in the conductor is a constant. This relationship is known as Ohm's law and it leads to the relationship: OHM'S LAW The most basic DC circuite o a cell (or battery) acting as a source of e.m.f o a resistor (or load) through which a current is passing. CURRENT, VOLTAGE AND RESISTANCE, Curent (I the rate of flow of charge and its unit is the ampere, A One ampere is equal to one coulomb C per second, or: One ampere of current | = Q/t Where t time in seconds So, for example: if a steady current of 3A flows for two minutes, then the amount of charge transferred will be: Potential difference (voltage) The force that creates the flow of current (or rate of flow of charge carriers) in. a circuit is known as the electromotive force (or e.m.f.) and it is measured in volts (V). The potential difference (or p.d. ) is the voltage difference, or voltage drop between two points. One volt is the potential difference between two points if one Joule of energy is required to move one coulomb of charge between them. V= W/Q Resistance All materials at normal temperatures oppose the movement of electric charge through them; this opposition to the flow of the charge carriers is known as the resistance, R i, of the material. This resistance is due to collisions between the charge carriers (electrons) and the atoms of the material. The unit of resistance is the ohm, with symbol o. Example: A 28 V DC aircraft supply delivers a charge of 5 C to a window heater every second. What is the resistance of the heater?