MIT OpenCourseWare http://ocw.mit.edu Electromechanical Dynamics For any use or distribution of this textbook, please cite as follows: Woodson, Herbert H., and James R. Melcher. Electromechanical Dynamics. 3 vols. (Massachusetts Institute of Technology: MIT OpenCourseWare). http://ocw.mit.edu (accessed MM DD, YYYY). License: Creative Commons Attribution-NonCommercial-Share Alike For more information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms_ Chapter 1INTRODUCTION1.0 INTRODUCTIONThe human is first of all a mechanical entity who exists in a mechanicalenvironment. The day-by-day habits of man are dictated largely by suchconsiderations as how rapidly he can transport or feed himself. Communica-tion with his environment is geared to such mechanical operations as thetime required for his eye to scan a page or the speed with which he can speakor typewrite. Furthermore, his standard of living is very much a function ofhis ability to augment human muscle for better transportation and for thediverse industrial processes needed in an advanced society.There are two major conclusions to be drawn from these thoughts. First,the unaided human faculties operate on limited time and size scales. Thus themechanical effects of electric and magnetic forces on ponderable bodies wereobserved and recorded by the Greeks as early as 500 B.c., and electricity andmagnetism were developed largely as classical sciences in the nineteenthcentury, on the basis of unaided human observations. Coulomb enunciatedhis inverse square law from measurements taken with an electrical torsionbalance; magnetic forces, as they influenced ponderable objects such asmagnetized needles, were the basis of experiments conducted by Oersted andAmpere. These electromechanical experiments constituted the origins of themodern theories of electricity and magnetism. Faraday and Maxwell unifiedthe subjects of electrostatics and magnetostatics into a dynamical theory thatpredicted phenomena largely beyond the powers of direct human observation.Thus today we recognize that electromagnetic theory encompasses not onlythe electromechanical effects that first suggested the existence of electric andmagnetic fields but also numerous radiation effects, whether they involveradio frequency waves or x-rays. Nonetheless, when man controls thesephenomena, detects their existence, and puts them to good use, he most oftendoes so by some type of electromechanical interaction-from the simple act ofturning a switch to the remote operation of a computer with a teletypewriter.____I__IntroductionThe second major conclusion to be drawn from our opening remarks isthat man's need for motive power for transportation and industrial processesis satisfied largely by conversion of electric energy to mechanical energy.Energy in electric form is virtually useless, yet the largest and fastest growingsegment of our economy is the electric utility industry, whose source ofincome is the sale of electric energy. This is eloquent testimony to the factthat electric energy can be converted easily into a variety of forms to aid manin his mechanical environment. It is remarkable that the same 60-Hz powerline can supply the energy requirements of a rolling mill, a television station,a digital computer, a subway train, and many other systems and devicesthat provide a fuller and more comfortable life. In the vast majority of theseexamples electromechanical energy conversion is required because of man'sbasic need for mechanical assistance.As long as engineers are concerned with making the electrical sciencesserve human needs, they will be involved with electromechanical phenomena.1.0.1 Scope of ApplicationBecause they serve so many useful functions in everyday situations,transducers are the most familiar illustration of applied electromechanicaldynamics. These devices are essential to the operation of such diverseequipment as automatic washing machines, electric typewriters, and powercircuit breakers in which they translate electrical signals into such usefulfunctions as opening a switch. The switch can be conventional or it can opena circuit carrying 30,000 A while withstanding 400,000 V 2 msec later. Thetelephone receiver and high-fidelity speaker are familiar transducers; lessfamiliar relatives are the high-power sonar antenna for undetwater communi-cation or the high-fidelity shake tables capable of vibrating an entire spacevehicle in accordance with a recording of rocket noise.Electromechanical transducers play an essential role in the automatic con-trol of industrial processes and transportation systems, where the ultimategoal is to control a mechanical variable such as the thickness of a steelsheet or the speed of a train. Of course, a transducer can also be made totranslate mechanical motion into an electrical signal. The cartridge of aphonograph pickup is an example in this category, as are such devices astelephone transmitters, microphones, accelerometers, tachometers anddynamic pressure gages.Not all transducers are constructed to provide mechanical input or output.The (electro)mechanical filter is an example of a signal-processing devicethat takes advantage of the extremely high Q of mechanical circuits atrelatively low frequencies. Filters, delay lines, and logic devices capable ofIntroductionperforming even above 30 MHz are currently the object of research onelectromechanical effects found in piezoelectric and piezomagnetic materials.Primary sources of energy are often found in mechanical form in thekinetic energy of an expanding heated gas and in the potential energy ofwater at an elevation. Electromechanics has always played a vital role inobtaining large amounts of electric power from primary sources. This isaccomplished by using large magnetic field-type devices called rotatingmachines. Today a single generator can produce 1000 MW (at a retail priceof 2 cents/kWh this unit produces an income of $20,000/h), and as electricutility systems grow larger generating units (with attendant problems of anunprecedented nature) will be needed. This need is illustrated by the fact thatin 1960 the national peak load in the United States was 138,000 MW, whereasit is expected that in 1980 it will be 493,000 MW, an increase of more than250 per cent in 20 years.A large part of this electric power will be used to drive electric motors ofimmense variety to do a multitude of useful tasks, from moving the hands ofan
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