I -~, I I TECHNICAL BACKGROUND ~ J l ON LINEAR INDUCTION MOTORS j IN TRANSPORTATION June 1970 .-I I i ~ i OFFICE OF HIGH SPEED GROUND TRANSPORTATION I I I FEDERAL RAILWAY ADMINISTRATION U. S. DEPARTMENT OF TRANSPORTATION - ' I I - _j Prepared by The Garrett Corporation, Torrance, California j - j CONTENTS -) j I Foreword Linear Motion Electrical Machines by E. R. Laithwaite and S. A. Nasar Linear Induction Motor Research in the U.S.A. by K. M. Chirgwin Application of Linear Motors to High Speed Transport Systems by E. R. Laithwaite and F. T. Barwell Application of the Linear Motor to Transport by D. S. Armstrong Linear Induction Motor for High Speed Tracked Vehicles by K. M. Chirgwin, C. H. Lee, and P. J. Larsen Electric Propulsion System for Linear Induction Motor Test Vehicle by G. P. Kalman, D. Irani and A. U. Simpson Control and Instrumentation for a High Speed Rail Vehicle Propelled by a Linear Induction Motor by J. Chapa I --' FOREWORD In March 1966 the office of High Speed Ground Transportation awarded a contract to study the linear induction motor and its feasibility for high speed ground transportation. In June 1967 the report resulting from this study was pub] ished.* It concluded that a 1 inear motor was feasible for this '1 application I and recommended that a full scale motor be built and tested. At the time of writing (June 1970) this full scale motor and a vehicle in which to test it has been completed and has been operated at low speeds on a short test track. Operation on the Department of Transportation's high speed test track near Pueblo Colorado is planned to begin in a few months. i i In addition to this activity a second generation I inear induction motor is now in the design phase. This 8000 horse power motor, together with an on­ board power conditioning unit is intended to be used as an all electric propul­ sion system for a 300 mph tracked Air Cushion Research Vehicle which is also in the design phase. All of this work has been sponsored by the Office of High Speed Ground Transportation. Seven technical papers have been selected to form a technical background on Linear Induction Motors applied to ground transportation. Three of these papers come from Britain and four from the United States. The national origin of the papers is largely fortuitous and no comparisons are intended between the work that is going on in these two countries and in other countries such as France, Germany, Japan and Russia. For those not familia~ with the subject, it is recommended that the papers be read in the order presented. The first paper forms a convenient introduction to the Linear Induction Motor. It is a survey type paper which 1 ists different configurations of the 1 inear induction motor that are possible and compares these configurations with each other and with other types of I inear electrical machines. The second paper gives some of the background of the U.S. 1 inear motor test program and its status in early 1968. The third paper is a more general treatment of I inear induction motors in high speed ground transportation applications. ~~study of Linear Induction Motor, Its Feasibility for High Speed Ground Transportation Report No. P.B. 174866 available for $3.00 from The Clearinghouse U.S. Dept of Commerce 5285 Port Royal Road Springfield, Virginia 22151 The fourth paper deals with many factors involved when considering the application of linear motor propulsion to an existing railway system. It ~, concludes that for the British Rail System, linear motor propulsion is not 1I J recommended at the present time. The three remaining papers deal in more detail with the U.S. Linear Motor Raii Test Vehicie program. K. M. Chirgwin Torrance, California June 1970 Jt I i PROCEEDINGS OF THE IEEE. VOL. 58, NO. 4, APRIL 1970 531 Linear-Motion Electrical Machines E. R. LAITHWAITE, FELLOW, IEEE, AND s. A. NASAR, SENIOR MEMBER, IEEE 'l Abstract-A survey of linear-motion electrical machines is pre­ sented. Although various types of de and ac linear machines are briefly mentioned, linear induction motors are the main concern of the paper and they are discussed in considerable detail. Based on topological considerations. a classification of these machines is pre­ sented and their development through the last 70 years is reviewed. A brief qualitative description of the newly developed hybrid machine Fig. l. Imaginary process of unrolling· a conventional motor is also included. Analysis and design problems, and some solutions. as to obtain a linear induction motor. unique to linear machines are discussed. Several possible applications of these machines are included. boaooaoaooqoaoopaooaq3 Aooaaooooooooaooooaood F~S'S' INTRODUCTION ~7 I I I I f. HE great majority of electrical machines are designed r"' ~ ____, 7 ~ to produce rotary motion, thereby exploiting the ---ACTIVE LENGTH/ Tblessings of circularity which man has enjoyed since Fig. 2. Active length of primitive form of linear motor the discovery of the wheel. The forces of electromagnetism is reduced once motion takes place. may, of course, also be employed to produce linear motion, as for example, in a linear induction machine in which the loooo9o~~~og~oES&r~~ogooooooool · ~~SSS~STATOR primary member consists of a row of coils carrying currents ROTOR SHORT STATOR MACHINE in phase progression. A simple method of introducing (a) linear machines is that the primary member resembles a conventional rotary machine stator which has been cut by ROTOR~ STATOR a radial plane and subsequently unrolled, as shown in Fig. 1. ~~~ A number of different types of linear machines may be de­ SHORT ROTOR MACHINE veloped in this way although, as will be seen later, the linear (b) machine family does not consist only of fiat machines which Fig. 3. Basic types of linear motor. result from such an unrolling process. It is almost a general principle that when an engineer in the airgap. That the splitting and unrolling process is makes a device in a different size, or of a different shape, or likely to modify the characteristics is evident from the fact with a new material, he changes the whole operating condi­ that any linearly traveling field must now have a start and tions and the new product may have such different charac­ a finish. Moreover, it is apparently unnecessary for the teristics as to change basically its field of application. In the primary unit to be designed to have an even number, in­ case of linear electrical machines the effect of linearization is deed even an integral number of poles. to introduce new phenomena which generally reduce their Perhaps even more fundamentally, a linear machine performance below that of corresponding conventional which consisted of an exact copy of the result of mentally rotary machines. The history of linear machines tells the "unrolling" a conventional squirrel-cage motor, as shown in story first of the struggle against the factors which detract Fig. 2, could only be used in a limited number of cases; for from performance and of increasing willingness to accept to allow the secondary member to move is to lose an reduced performance for specific applications in which the ever-increasing amount of the motor as the "cage" emerges linear machine offers advantages in other ways. at one end, laying primary coils bare at the other. It is clear The changes in operating conditions imposed by changes that where motion over a considerable distance is required in shape will first be discussed, using the induction machine with a limited amount of power, either the primary or the to illustrate the processes. secondary member must be elongated. Such elongation leads at once to two major classes of ToPO LOGICAL CoNSIDERAnoNs linear machine which may be designated "short primary" Many characteristics of ac machines and of induction and "short secondary." An example of each is shown in motors in particular, are explained in terms of the concept Fig. 3. In general, the short primary is by far the cheaper to that the primary member sets up a rotating magnetic field build and to run. The secondary member can be simplified in form, often to a simple sheet of conductor and the whole Manuscript received November 13, 1969. This work was supported system is only fed with current over a small proportion of in part by the National Science Foundation Grant GK-10989. its length. In certain situations, however, a compromise E. R. Laithwaite is with the Imperial College of Science and Tech­ nology, London, England. arrangement may consist of a long sectionalized primary in S. A. Nasar is with the University of Kentucky, Lexington, Ky. 40506. which only the sections actually in use are energized. 532 PROCEEDINGS OF THE IEEE, APRIL 1970 The "Sheet-Rotor Motor" CONVENTIONAL MOTOR UNROLLED f;oooo~oooooal The arrangements so far described have assumed both members to consist of electrical conductors in slots in a STAGE (I) REMOVE ROTOR CONDUCTORS FROM SLOTS, SHORTEN ROTOR IRON laminated steel core, which is the usual arrangement in a [ -- - - -~ -_ -~ rotary machine. With such a structure, however, there \111)d!lO~~ooooorlnl exists, in addition to the tangential electromagnetic thrust STAGE (2) MERGEc_---=:J ROTOR BARS INTO A SHEET which the machine is designed to give, a purely magnetic pull between the oppositely magnetized surfaces. In a ==;i~~~G~~~~~~~== cylindrical machine only the out-of-balance pull resulting MOTOR from any asymmetry which may exist is observable. The fact that even this amount may be sufficient to worry the designer of rotary motors indicates the size of the problem Fig.