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Transcript

ELECTRICAL BASICS (Chapter 8) Electrical terms Electricity & magnetism Electricity Circuits Magnetism Electrical units Electric potential or eletromotive force Electric current Resistance resistance of a conductor is affected by (additional information): type of material length: directly proportional to the length of a conductor cross-section area or thickness temperature: directly proportional to the temperature of a conductor Electrical power Energy (additional information) The term energy is used to express work Energy = Power*Time Unit of energy is kilowatt-hour (kWh) ELECTRICAL BASICS Calculations and examples Ohm's Law (additional information) Current (I or A), electric potential (V or E), and resistance (R) are related to each other, and a variance in one will affect the others. This relationship is known as Ohm's Law. Current (I or A) is directly proportional to voltage (V or E) Current (I or A) is inversely proportional to resistance (R) Power formulas GENERATION AND DISTRIBUTION (Chapter 9) Power, work, and energy Demand and consumption Electric current: DC or AC Generating efficiency Cogeneration GENERATION AND DISTRIBUTION (Chapter 9) Direct current (DC) (additional information) In DC, electrons always move in the same direction Polarity of the generator remains always the same. Alternating current (AC) (additional information) polarity of the generator or alternator reverses periodically current varies periodically in value and directions, first flowing in one direction in the circuit and then flowing in the opposite direction Generation of AC (additional information) generated using a combination of physical motion and magnetism simplest form of AC generator is constructed using a single loop of wire placed between the poles of a permanent magnet and then rotating it by some suitable mechanical means. magnetic lines of force are interrupted with the rotation of loop an electromagnetic force is induced in the loop EMF thus produced exists between the two ends of the loop slip rings and brushes attached to each end of the loop apply the generated EMF to an external circuit. GENERATION AND DISTRIBUTION How the current alternates (additional information) Cycle of AC (additional information) Each complete rotation by the loop of wire (or armature coil) within the poles of magnet is called a cycle. Frequency of AC (additional information) When the loop is parallel to the magnetic lines of force, no magnetic flux is interrupted and no EMF is induced to the loop. As it begins to rotate, it cuts the magnetic flux at an increasing rate reaching a maximum when it has rotated a quarter turn (i.e. 90°). As the rotation continues, the EMF is still in the same direction but is decreasing in value. A half revolution (i.e. 180° ), the loop is again parallel to the magnetic lines and no magnetic flux is interrupted; EMF at this point equals 0. As the rotation continues further, the sides of the loop reverse position and the induced EMF reverses polarity; therefore, direction of the flow of current also reverses. The EMF increases to a maximum again at 3/4 turn (i.e. 270° ) and declines to 0 when the rotation is completed. The number of cycles per second is known as the frequency of the voltage or current The unit used to measure this frequency is Hertz In the United States, the frequency for alternating current is 60 Hertz. Use of AC results in the reduction of transmission loss. GENERATION AND DISTRIBUTION (Chapter 9) Example of transmission loss reduction Watt = Current2 x Resistance If current (A or I) = 20 and resistance (R) = 4, then transmission loss (W) = 202x4 = 1600 The loss can be reduced by decreasing R. If R were decreased by half to 2, then W = 202x2 = 800 The loss can also be reduced by decreasing current. If it were decreased by half to 10, then W = 102x4 = 400 It is obvious that if current is decreased by half, transmission loss would be reduced by a factor of 4. In case AC, the stepping up and stepping down of current and voltage can be achieved by using a transformer. GENERATION AND DISTRIBUTION (Chapter 9) Single-phase and three-phase power Single-phase (additional information) a single armature coil creates a complete cycle of voltage and current requires one 'hot' wire and a neutral wire Three phase (additional information) use of three separate coil conductors equally spaced (@ 120° ) around generator armature in order to obtain the value of line voltage, phase voltage (either neutral-to-phase or phase-to-phase) has to be multiplied by 3 or 1.73 (e.g. if single-phase voltage is 208, then the corresponding three-phase voltage would be 208* 3 = 360) a three-phase system with an Y-connection requires 3 'hot' wires and a neutral wire. GENERATION AND DISTRIBUTION (Chapter 9) Transformers Power distribution Transformers (additional information) Used for stepping up or stepping down voltages and current Consists of iron core surrounded by two circuit loops (windings) Capacity rated in kVA Power factor (PF) Grounding ELECTRICAL RATING OF EQUIPMENT (Additional information) Voltage rating Maximum voltage that can be safely applied continuously to an equipment. The rating is primarily determined by: Type and quantity of conductor insulation used Physical spacing between electrically energized parts of the equipment Current rating Determined by the maximum operating temperature at which the components of an equipment can operate continuously and properly maximum operating temperature is determined by the type of insulation of the conductor the maximum safe operating temperature of conductors having cotton braid as insulation would be 65° C Current which will be producing this temperature would be the maximum permissible current for the conductor the maximum safe operating temperature of conductors having silicone or glass compound would be 150° C Consequently, the maximum permissible temperature for the same conductor would increase