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5. energy conversion Week 11 5.1 Electro-mechanical energy conversion Energy is converted to electrical form because of the advantages listed in the introductory part of the note. It is seldom available or used in electrical form, but converted into electrical form at the input to a system and back to non-electrical form at the output of a system. A typical example is the processing of energy from and hydro generating plant. It is converted into electrical form at the power plant. Transmitted through transmission lines and distribution lines, and coverted to mechanical energy in an electric motor are the point use. A second example is in the conversion of the energy in sound pressure waves, and the transmission in electrical form from the taker to the listener in a telephone system. Few more energy conversion principles will be mentioned. 5.1.1 Major energy coversion principles Energy conversion between electrical and non- electrical forms includes 5.2 (i) Electrochemical eg battery (ii) Electrothermal eg. Thermocouple (iii) Photo electrical eg photo cell Energy coversion Theoretically, only a sourceless current is needed to develop a mechanical force magnetically. But in a machine the production of force is hardly enough: something must move in order to do useful work done demands a corresponding energy supply form somewhere. In a device energized only by a permanent magnet, the only energy source is the magnet itself. If the displacable part of the machine moves under force and does work, this can only be at the expense of the field energy of the permanent magnet, which must decrease. Such as 5. energy conversion Week 11 arrangement, has obvious limitations. It may also be inconvenient a permanent magnet” lifting magnet, for example would not e capable of releasing its load. Where the magnetic affected by the movement is produced by a current circuit, changes of field energy have to be supplied electrically from a source. This implies the appearance in the circuit of an electromotive force e, which, with the current I, represents the delivery or absorption by the source of energy at the rate ei consider the elementary system of fig 2.1 A sources of voltage is connected to a device (e.g a secondary battery or a machine) in which the energyconversion process results in the appearance of an e.f.m. The effective resistance of the circuit is represented by R. if current flows into the circuit form the positive terminal of the source, and the input power p = Vi Rl + el, has the direction as shown at (a). However¸ if e >, the current reverses and we can now call it –e. the power input from the now p = v (-i) = Rl2 + e (-l), which is negative, i.e it is an output from the device into the source, as at (b). to illustrate this simple but fundamental point, suppose that v = 10v d.c nad R = 1-2. then if e = 8vd.c. The current o = v-e R = 10-8 =2A 1 And the source provides an input power p = 10 x 2 = 20w The converting device accepts 8 x 2 W as a motor And power loss due to plissipation = 1 x RT2 = 4W Conversely, if e 12V, The current again is 10 -12 -2A (I.e reversed) The device produces 12 x 2 = 24 W as a generator of which RT2 22 x 1 = 4W is dissipated in R, and 10 x 20W is delivery as an output to the source. In the case of the electromagnetic machine, the relationship between the emf and the magnetic field is obtained from the faraday induction law (which had been mentioned in 1.1) 5. energy conversion 5.3 Week 11 Linked energy systems An electromechanical machine forms a coverting link between an electrical energy system ( such as a main power –supply network) and a mechanical one (such as a prime- mover or a train). In action a machine is not an isolated things, but has a behavious strongly influenced by its terminal systems. A relay, for instance, will be affected if its operating battery becomes discharged; a loudspeaker will behave very differently it enclosed in an evacuated vessels with the air loading thus removed; a hydro electric generator, suddenly shortcircuited, will react severely on the turbine and pipe-line. A machine can, of course, be studies initially in isolation, but the engineering interest begins in fact when the complete linked system is considered. Again, the steady-state behavious is informative up to a point, but operation in responses to change – i.e, the transient responses is fro move important and fundamental. Fig 2.1 Electro–mechanical linked energy system System analysis can be complicated. Fig 2.2 shows diagrammatically a typical electric supply system feeding a mechanical load through an electromechanical machine. In some cases we might simplify the analysis by assuming, say, that the terminal voltage and frequency of the machine were constant. This is good enough if the machine is a small contactor but if it is a 25MW motor the effects of its behaviour reach for back through even an extensive supply system. Methods are available for evaluating such a complex for any 5. energy conversion Week 11 given stimulus, such as the occurrence of a transmission- line fault or starting of a large motor. 5.4 Energy storage We now consider how a flux is established and energy is stored in simple toroidal magnetic Circuit of cross sectional area A, path length L, and of material of constant permeability u, The flux is to be established by a current i, in a uniformly wound coil of N-turns. In order To concentrate on energy storage we neglect the coil resistance. With i initially zero, let a Voltage V, be applied to the coil terminals, what happen thereafter depends on Faraday’s Law of electromagnetic induction.