Instructor’s Manual to accompany Chapman Electric Machinery Fundamentals Fourth Edition Stephen J. Chapman BAE SYSTEMS Australia i www.net Instructor’s Manual to accompany Electric Machinery Fundamentals, Fourth Edition Copyright 2004 McGraw-Hill, Inc. All rights reserved. Printed in the United States of America. No part of this book may be used or reproduced in any manner whatsoever without written permission, with the following exception: homework solutions may be copied for classroom use. ISBN: ??? ii www.net TABLE OF CONTENTS CHAPTER 1: INTRODUCTION TO MACHINERY PRINCIPLES 1 CHAPTER 2: TRANSFORMERS 23 CHAPTER 3: INTRODUCTION TO POWER ELECTRONICS 63 CHAPTER 4: AC MACHINERY FUNDAMENTALS 103 CHAPTER 5: SYNCHRONOUS GENERATORS 109 CHAPTER 6: SYNCHRONOUS MOTORS 149 CHAPTER 7: INDUCTION MOTORS 171 CHAPTER 8: DC MACHINERY FUNDAMENTALS 204 CHAPTER 9: DC MOTORS AND GENERATORS 214 CHAPTER 10: SINGLE-PHASE AND SPECIAL-PURPOSE MOTORS 270 APPENDIX A: REVIEW OF THREE-PHASE CIRCUITS 280 APPENDIX B: COIL PITCH AND DISTRIBUTED WINDINGS 288 APPENDIX C: SALIENT POLE THEORY OF SYNCHRONOUS MACHINES 295 APPENDIX D: ERRATA FOR ELECTRIC MACHINERY FUNDAMENTALS 4/E 301 iii www.net PREFACE TO THE INSTRUCTOR This Instructor’s Manual is intended to accompany the fourth edition of Electric Machinery Fundamentals. To make this manual easier to use, it has been made self-contained. Both the original problem statement and the problem solution are given for each problem in the book. This structure should make it easier to copy pages from the manual for posting after problems have been assigned. Many of the problems in Chapters 2, 5, 6, and 9 require that a student read one or more values from a magnetization curve. The required curves are given within the textbook, but they are shown with relatively few vertical and horizontal lines so that they will not appear too cluttered. Electronic copies of the corresponding open- circuit characteristics, short-circuit characteristics, and magnetization curves as also supplied with the book. They are supplied in two forms, as MATLAB MAT-files and as ASCII text files. Students can use these files for electronic solutions to homework problems. The ASCII files are supplied so that the information can be used with non-MATLAB software. Please note that the file extent of the magnetization curves and open-circuit characteristics have changed in this edition. In the Third Edition, I used the file extent *.mag for magnetization curves. Unfortunately, after the book was published, Microsoft appropriated that extent for a new Access table type in Office 2000. That made it hard for users to examine and modify the data in the files. In this edition, all magnetization curves, open-circuit characteristics, short-circuit characteristics, etc. use the file extent *.dat to avoid this problem. Each curve is given in ASCII format with comments at the beginning. For example, the magnetization curve in Figure P9-1 is contained in file p91_mag. Its contents are shown below: % This is the magnetization curve shown in Figure % P9-1. The first column is the field current in % amps, and the second column is the internal % generated voltage in volts at a speed of 1200 r/min. % To use this file in MATLAB, type "load p91_mag. % The data will be loaded into an N x 2 array named % "p91_mag", with the first column containing If and % the second column containing the open-circuit voltage. % MATLAB function "interp1" can be used to recover % a value from this curve.950 To use this curve in a MATLAB program, the user would include the following statements in the program: % Get the magnetization curve. Note that this curve is % defined for a speed of 1200 r/min.dat if_values = p91_mag(:,1); ea_values = p91_mag(:,2); n_0 = 1200; Unfortunately, an error occurred during the production of this book, and the values (resistances, voltages, etc.) in some end-of-chapter artwork are not the same as the values quoted in the end-of-chapter problem text. I have attached corrected pages showing each discrepancy in Appendix D of this manual. Please print these pages and distribute them to your students before assigning homework problems. (Note that this error will be corrected at the second printing, so it may not be present in your student’s books.net The solutions in this manual have been checked carefully, but inevitably some errors will have slipped through. If you locate errors which you would like to see corrected, please feel free to contact me at the address shown below, or at my email address schapman@tpgi. I greatly appreciate your input! My physical and email addresses may change from time to time, but my contact details will always be available at the book’s Web site, which is http://www.com/engcs/electrical/chapman/. I am also contemplating a homework problem refresh, with additional problems added on the book’s Web site mid- way through the life of this edition. If that feature would be useful to you, please provide me with feedback about which problems that you actually use, and the areas where you would like to have additional exercises. This information can be passed to the email address given below, or alternately via you McGraw-Hill representative. Chapman Melbourne, Australia January 4, 2004 Stephen J. Chapman 278 Orrong Road Caulfield North, VIC 3161 Australia Phone +61-3-9527-9372 vi www.net Chapter 1: Introduction to Machinery Principles 1-1. A motor’s shaft is spinning at a speed of 3000 r/min. What is the shaft speed in radians per second? SOLUTION The speed in radians per second is 1 min 2π rad ω = ( 3000 r/min ) = 314. A flywheel with a moment of inertia of 2 kg ⋅ m2 is initially at rest. If a torque of 5 N ⋅ m (counterclockwise) is suddenly applied to the flywheel, what will be the speed of the flywheel after 5 s? Express that speed in both radians per second and revolutions per minute. SOLUTION The speed in radians per second is: τ 5 N ⋅m ω =α t = t = ( 5 s ) = 12.5 rad/s J 2 kg ⋅ m 2 The speed in revolutions per minute is: 1 r 60 s n = (12.4 r/min 2π rad 1 min 1-3. A force of 5 N is applied to a cylinder, as shown in Figure P1-1. What are the magnitude and direction of the torque produced on the cylinder? What is the angular acceleration α of the cylinder? SOLUTION The magnitude and the direction of the torque on this cylinder is: τ ind = rF sin θ , CCW τ ind = ( 0.25 N ⋅ m, CCW The resulting angular acceleration is: τ 1. A motor is supplying 60 N ⋅ m of torque to its load. If the motor’s shaft is turning at 1800 r/min, what is the mechanical power supplied to the load in watts? In horsepower? SOLUTION The mechanical power supplied to the load is 1 min 2π rad P = τω = ( 60 N ⋅ m )(1800 r/min ) = 11,310 W 60 s 1r 1 www. A ferromagnetic core is shown in Figure P1-2. The depth of the core is 5 cm. The other dimensions of the core are as shown in the figure. Find the value of the current that will produce a flux of 0. With this current, what is the flux density at the top of the core? What is the flux density at the right side of the core? Assume that the relative permeability of the core is 1000. SOLUTION There are three regions in this core. The top and bottom form one region, the left side forms a second region, and the right side forms a third region. If we assume that the mean path length of the flux is in the center of each leg of the core, and if we ignore spreading at the corners of the core, then the path lengths are l1 = 2(27.5 cm) = 55 cm, l 2 = 30 cm, and l3 = 30 cm. The reluctances of these regions are: l l 0.05 m ) −7 The total reluctance is thus RTOT = R1 + R2 + R3 = 58.6 kA ⋅ t/Wb and the magnetomotive force required to produce a flux of 0.6 kA ⋅ t/Wb ) = 1008 A ⋅ t and the required current is F 1008 A ⋅ t i= = = 2.52 A N 400 t The flux density on the top of the core is φ 0.net The flux density on the right side of the core is φ 0. A ferromagnetic core with a relative permeability of 1500 is shown in Figure P1-3. The dimensions are as shown in the diagram, and the depth of the core is 7 cm. The air gaps on the left and right sides of the core are 0. Because of fringing effects, the effective area of the air gaps is 5 percent larger than their physical size. If there are 4001 turns in the coil wrapped around the center leg of the core and if the current in the coil is 1.0 A, what is the flux in each of the left, center, and right legs of the core? What is the flux density in each air gap? SOLUTION This core can be divided up into five regions. Let R1 be the reluctance of the left-hand portion of the core, R2 be the reluctance of the left-hand air gap, R3 be the reluctance of the right-hand portion of the core, R4 be the reluctance of the right-hand air gap, and R5 be the reluctance of the center leg of the core. Then the total reluctance of the core is RTOT = R5 + ( R1 + R2 ) ( R3 + R4 ) R1 + R2 + R3 + R4 l1 1.07 m ) −7 The total reluctance is 1 In the first printing, this value was given incorrectly as 300.3 The total flux in the core is equal to the flux in the center leg: φcenter = φTOT = F = (400 t )(1.8 kA ⋅ t/Wb The fluxes in the left and right legs can be found by the “flux divider rule”, which is analogous to the current divider rule.3 The flux density in the air gaps can be determined from the equation φ = BA : φleft 0. A two-legged core is shown in Figure P1-4. The winding on the left leg of the core (N1) has 400 turns, and the winding on the right (N2) has 300 turns. The coils are wound in the directions shown in the figure. If the dimensions are as shown, then what flux would be produced by currents i1 = 0.75 A? Assume µ r = 1000 and constant.net SOLUTION The two coils on this core are would so that their magnetomotive forces are additive, so the total magnetomotive force on this core is FTOT = N1i1 + N 2i2 = ( 400 t )( 0.75 A ) = 425 A ⋅ t The total reluctance in the core is l 2.15 m ) −7 and the flux in the core is: FTOT 425 A ⋅ t φ= = = 0. A core with three legs is shown in Figure P1-5. Its depth is 5 cm, and there are 200 turns on the leftmost leg. The relative permeability of the core can be assumed to be 1500 and constant. What flux exists in each of the three legs of the core? What is the flux density in each of the legs? Assume a 4% increase in the effective area of the air gap due to fringing effects. SOLUTION This core can be divided up into four regions. Let R1 be the reluctance of the left-hand portion of the core, R2 be the reluctance of the center leg of the core, R3 be the reluctance of the center air gap, and R4 be the reluctance of the right-hand portion of the core.
