L-18(NKD)(ET) ((EE)NPTEL) - Module 5 Three-phase AC...

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Module 5 Three-phase AC Circuits Version 2 EE IIT, Kharagpur
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Lesson 18 Three-phase Balanced Supply Version 2 EE IIT, Kharagpur
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In the module, containing six lessons (12-17), the study of circuits, consisting of the linear elements – resistance, inductance and capacitance, fed from single-phase ac supply, has been presented. In this module, which may also be termed as an extension of the previous one, containing three lessons (18-20), the solution of currents in the balanced circuits, fed from three-phase ac supply, along with the measurement of power, will be described. In this (first) lesson of this module, the generation of three-phase balanced voltages is taken up first. Then, the two types of connections (star and delta), normally used for the above supply, followed by line and phase quantities (voltages and currents) for the connections, in both supply and load sides (both being balanced), are described. Keywords : Three-phase balanced voltage, star- and delta-connections, balanced load. After going through this lesson, the students will be able to answer the following questions: 1. How to generate three-phase balanced voltages? 2. What are the two types of connections (star and delta) normally used for three-phase balanced supply? 3. What are meant by the terms – line and phase quantities (voltages and currents), for the two types of connections in both supply and load sides (both being balanced)? Generation of Three-phase Balanced Voltages In the first lesson (No. 12) of the previous module, the generation of single-phase voltage, using a multi-turn coil placed inside a magnet, was described. It may be noted that, the scheme shown was a schematic one, whereas in a machine, the windings are distributed in number of slots. Same would be the case with a normal three-phase generator. Three windings, with equal no. of turns in each one, are used, so as to obtain equal voltage in magnitude in all three phases. Also to obtain a balanced three-phase voltage, the windings are to be placed at an electrical angle of with each other, such that the voltages in each phase are also at an angle of with each other, which will be described in the next section. The schematic diagram with multi-turn coils, as was shown earlier in Fig. 12.1 for a single-phase one, placed at angle of with each other, in a 2-pole configuration, is shown in Fig. 18.1a. The waveforms in each of the three windings (R, Y & B), are also shown in Fig. 18.1b. The windings are in the stator, with the poles shown in the rotor, which is rotating at a synchronous speed of (r/min, or rpm), to obtain a frequency of ° 120 ° 120 ° 120 s N ( ) 120 / ) ( s N p f = (Hz), p being no. of poles [ 2 = p ] (see lesson no. 12). Version 2 EE IIT, Kharagpur
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R R' Y' B Y B' N S 120º 120º 120º Fig. 18.1 (a) Schematic diagram of three windings of stator for the generation of three phased balanced voltage (2-pole rotor).
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L-18(NKD)(ET) ((EE)NPTEL) - Module 5 Three-phase AC...

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