L-41(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 41 Problem Solving on D.C Machines Version 2 EE IIT, Kharagpur
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Contents 41 Problem solving on D.C machines (Lesson-41) 4 41.1 Introduction ............................................................................................................... 4 41.2 Shunt motor problems ............................................................................................... 4 41.3 Problems on Series Motor …………………………………………………………. 8 41.4 D.C generator problems ............................................................................................ 12 41.5 Operation of d.c machine connected to d.c bus (fixed d.c voltage) ………………. .. 13 Version 2 EE IIT, Kharagpur
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Introduction In this lecture some typical problems on D.C machines are worked out not only to solve the problems only but also to bring out important features of the motors involving its performance. To begin with few problems on d.c motors have been solved and then problems on generator are taken up. For a beginner, it is suggested to follow the following guidelines for successful completion of the problem. 1. Read the problem and note down the informations provided about the motor. 2. In general, some initial steady operating conditions are given in terms of armature current, field current, speed etc. 3. Draw a circuit diagram showing the initial variable values. 4. Write down the back emf equation and the torque equation for the initial operating point, it is reiterated here that these two equations hold the key for the gateway of correct solution. 5. In the statement of the problem, with respect to the initial operating point, some variable (like, armature resistance, field resistance, load torque, armature applied voltage etc.) will be changed and you will be asked perhaps to calculate new armature current, speed etc. 6. Assume the variables to be calculated and write down once again the back emf equation and the torque equation for the new operating point. 7. You have now four equations – 2 corresponding to initial operating point and the other 2 corresponding to the new operating point. 8. Take the ratio of the torque equations and the ratio of back emf equations. 9. Solve from the above equations the unknown. Let us now solve some problems on shunt, separately excited and series motors. 41.2 Shunt motor problems 1. A 220 V shunt motor has armature and field resistance of 0.2 and 220 respectively. The motor is driving a constant load torque and running at 1000 rpm drawing 10 A current from the supply. Calculate the new speed and armature current if an external armature resistance of value 5 is inserted in the armature circuit. Neglect armature reaction and saturation. Version 2 EE IIT, Kharagpur
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Solution The circuit diagram of the shunt motor is: + I L I a For initial operating point: I L 1 = 10 A, r a = 0.2 and supply voltage V = 220 V. Field current I f 1 = 220/220 A = 1A Armature current I a 1 = 10A – 1A = 9A Now we write down the expressions for the torque and back emf.
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L-41(TB)(ET) ((EE)NPTEL) - Module 9 DC Machines Version 2...

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