L-40(TB)(ET) ((EE)NPTEL) - Module 9 DC Machines Version 2...

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Module 9 DC Machines Version 2 EE IIT, Kharagpur
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Lesson 40 Losses, Efficiency and Testing of D.C. Machines Version 2 EE IIT, Kharagpur
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Contents 40 Losses, efficiency and testing of D.C. machines (Lesson-40) 4 40.1 Goals of the lesson ………………………………………………………………. . 4 40.2 Introduction ………………………………………………………………………. 4 40.3 Major losses……………….……………………………………………………… 4 40.4 Swinburne’s Test ……………. .…………………………………………………. . 6 40.5 Hopkinson’s test …………….…………………………………………………… 8 40.5.1 Procedure ………………………….…………………………………. . 9 40.5.2 Loading the machines ………………………………………………… 10 40.5.3 Calculation of efficiency ……………………………………………… 10 40.6 Condition for maximum efficiency ……………………………………………… 11 40.6.1 Maximum efficiency for motor mode ………………………………… 11 40.6.2 Maximum efficiency for General mode ………………………………. 12 40.7 Tick the correct answer …………………………………………………………. .. 13 40.8 Solve the following ……………………………………………………………… 14 Version 2 EE IIT, Kharagpur
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Chapter 40 Losses, efficiency and testing of D.C. machines (Lesson-40) 40.1 Goals of the lesson 1. To know what are the various power losses which take place in a d.c machine. 2. To understand the factors on which various losses depend upon. 3. To know how to calculate efficiency. 4. To know how to estimate and predict efficiency of a d.c shunt motor by doing simple tests. 40.2 Introduction In the previous sections we have learnt about the principle of operation of d.c. generators and motors, (starting and speed control of d.c motor). Motors convert electrical power (input power) into mechanical power (output power) while generators convert mechanical power (input power) into electrical power (output power). Whole of the input power can not be converted into the output power in a practical machine due to various losses that take place within the machine. Efficiency η being the ratio of output power to input power, is always less than 1 (or 100 %). Designer of course will try to make η as large as possible. Order of efficiency of rotating d.c machine is about 80 % to 85 %. It is therefore important to identify the losses which make efficiency poor. In this lesson we shall first identify the losses and then try to estimate them to get an idea of efficiency of a given d.c machine. 40.3 Major losses Take the case of a loaded d.c motor. There will be copper losses ( ) 22 and aa f f f I rI R V I = in armature and field circuit. The armature copper loss is variable and depends upon degree of loading of the machine. For a shunt machine, the field copper loss will be constant if field resistance is not varied.
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L-40(TB)(ET) ((EE)NPTEL) - Module 9 DC Machines Version 2...

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