energies Review Linear Induction Motors in Transportation Systems Ryszard Palka * and Konrad Woronowicz Faculty of Electrical Engineering, West Pomeranian University of Technology, Sikorskiego 37, 70-313 Szczecin, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-91-449-4870 Abstract: This paper provides an overview of the Linear Transportation System (LTS) and focuses on the application of a Linear Induction Motor (LIM) as a major constituent of LTS propulsion. Due to their physical characteristics, linear induction motors introduce many physical phenomena and design constraints that do not occur in the application of the rotary motor equivalent. The efficiency of the LIM is lower than that of the equivalent rotary machine, but, when the motors are compared as integrated constituents of the broader transportation system, the rotary motor’s efficiency advantage diminishes entirely. Against this background, several solutions to the problems still existing in the application of traction linear induction motors are presented based on the scientific research of the authors. Thus, solutions to the following problems are presented here: (a) development of new analytical solutions and finite element methods for LIM evaluation; (b) comparison between the analytical and numerical results, performed with commercial and self-developed software, showing an exceptionally good agreement; (c) self-developed LIM adaptive control methods; (d) LIM performance under voltage supply (non-symmetrical phase current values); (e) method for the power loss evaluation in the LIM reaction rail and the temperature rise prediction method of a traction LIM; and (f) discussion of the performance of the superconducting LIM. The addressed research topics have been chosen for their practical impact on the advancement of a LIM as the preferred urban Citation: Palka, R.; Woronowicz, K. transport propulsion motor. Linear Induction Motors in Transportation Systems. Energies 2021, Keywords: linear induction motors; finite element analysis; end effect 14, 2549. https://doi.org/10.3390/ en14092549 Academic Editors: Frede Blaabjerg 1. Introduction and Andrea Mariscotti A traction Linear Induction Motor (LIM) has been deployed worldwide in numerous transit systems and in driverless, elevated guideway systems requiring all weather oper- Received: 10 March 2021 ations under very short headways. LIM-based urban transport has proven to be, by far, Accepted: 26 April 2021 Published: 29 April 2021 the least expensive in terms of operations and maintenance (including the energy costs). LIMs are also found in other various applications, ranging from small-power industrial Publisher’s Note: MDPI stays neutral material handling and amusement park roller-coaster propulsion to very high-output with regard to jurisdictional claims in military aircraft launchers; advanced research is underway to investigate LIMs as potential published maps and institutional affil- power conversion devices for ocean wave energy recovery [1]. iations. Because of low operating costs and extremely high reliability, LIM-propelled sys- tems have become an ever more frequent part of the public transport offering. LIM- based public transit systems have already been in operation for a few decades, and they are serving such cities as Yokohama, Vancouver, Toronto, Tokyo, Osaka, Seoul (Yongin), New York, Moscow (Moscow Monorail), Kuala Lumpur, Guangzhou, Fukuoka, and Beijing. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. References for the applications are readily available by any browser search using such This article is an open access article keywords as “Linear Metro”. For economic reasons, the operation of these systems as distributed under the terms and well as most other LIM-based systems has been based on a single-sided LIM [1,2]. Al- conditions of the Creative Commons though, in comparison to its rotary counterpart, a traction LIM has necessarily a large air Attribution (CC BY) license (https:// gap between the stator and its rotor equivalent and thus is less efficient on the motor-to- creativecommons.org/licenses/by/ motor comparison bases, the LIM-based systems show better system-level performance 4.0/). resulting from several characteristic features specific to the LIMs. First, in comparison Energies 2021, 14, 2549. https://doi.org/10.3390/en14092549 https://www.mdpi.com/journal/energies Energies 2021, 14, 2549 2 of 22 with rotary induction motors, a mechanical gear box that introduces high energy losses and accounts for a significant life cycle cost and is a potential source of serious reliability issues is eliminated. The LIM has no moving parts, and the propelling force is directly applied to the vehicle in the direction of motion, thus avoiding the losses introduced by the mechanical gear box. In most instances, rotary-motor-based transportation systems rely on adhesion between the wheels and the running rail. Relying on adhesion limits their acceleration and deceleration performance under wet or otherwise contaminated rail conditions. LIM systems do not suffer such disadvantages because their tractive effort is developed as a direct electrodynamic force between the LIM primary and the reaction rail, and this allows for an adhesion independent, reliable operation under all environmental conditions. This also means that LIM-propelled trains (objects) can accelerate at any rate and achieve their nominal speeds sooner, which limits the high current/tractive effort demand period and decreases the overall energy consumption. Because of their flat form, LIMs occupy significantly smaller vertical space, which enables a lower profile steerable bogie construction and, consequently, a lower vehicle cross-sectional area, thus decreasing the potential tunnel construction costs and energy consumption resulting from the motion air resistance [3,4]. In addition, LIM-based vehicles can run on steeper grades (due to direct forces) and negotiate sharper curves (due to steerable bogies), providing more flexibility in the elevated guideway structure design, which helps to reduce civil and land release costs. A large urban center allows for more targeted interconnection of the various multi- modal transportation systems, which is not always achievable with rotary-motor-based rail vehicles. The uniqueness of Linear Motor (LM) systems led to a heightened interest in LIM technology and resulted in a number of research projects aimed at improving LIM performance and further decreasing the operating cost. In this paper, an overview and categorization of linear transportation systems is com- pleted, in which LIMs are found to mainly be used for traction and braking. Next, the major characteristics of a LIM are described and associated with the system performance. Finally, a series of important practical research works carried out by the authors and aimed at advancing LIM system performance are reported, highlighting challenges associated with improving LIM performance in such areas as performance prediction, LIM adaptive control, LIM thermal protection, and the application of superconductivity. The conclu- sions summarize the authors’ experience in the subject matter and highlight the areas of advancement for future research. 2. Overview of Linear Transportation Systems Linear transportation systems (LTSs) can be divided into some fundamental groups according to their support, guidance, and drive solutions. The most important groups are listed below. 2.1. Levitated LTS Levitated LTSs mainly use electromagnetic suspension. These systems, Maglevs, can be divided into Permanent Magnet (PM)- and superconductor-based levitating systems. According to the postulate on the stability of bodies in various static force fields given in 1842 by Earnshaw [5], no object placed in an inverse square law force field (e.g., magnetic field) could be in a stable equilibrium; thus to achieve stability, PM flux should be modu- lated by the respective control coil currents that depend on the size of the air gap. Such controlled electromagnets are utilized in many different levitation systems, for both side and vertical stabilization [6]. Efficient operation of these systems depends on the advanced optimization of the magnetic field distribution within the air gap by means of proper geometry design and proper selection of material and power supply characteristics [7,8]. The first Maglev line to open to public traffic was the Birmingham Maglev in 1984 (propelled by a LIM), the second was the M-Bahn in Berlin (also 1984), and the third was the Shanghai Transrapid Maglev (the latter two using a long-stator synchronous linear-motor-based propulsion). Energies 2021, 14, 2549 3 of 22 Braunbeck [9] extended stability investigations to the systems containing diamagnetic materials. The characteristic feature of a diamagnetic material is that it opposes the external field variations, the feature exhibited by the superconductors. Since their inception, super- conducting levitation systems have been used in many different industrial applications. The principle of these systems is based on the interaction between the magnetic field and high-temperature superconductors [10,11]. Two examples of superconducting levitation systems are the Miyazaki and Yamanashi Maglevs. In the early stages of Maglev devel- opment, at the Miyazaki test track (1977), a purely repulsive electrodynamic suspension system was used [12]. A major advantage of the repulsive electrodynamic suspension system is its inherent stability—a decreasing