**1. Introduction**

A traction Linear Induction Motor (LIM) has been deployed worldwide in numerous transit systems and in driverless, elevated guideway systems requiring all weather operations under very short headways. LIM-based urban transport has proven to be, by far, 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 material handling and amusement park roller-coaster propulsion to very high-output military aircraft launchers; advanced research is underway to investigate LIMs as potential power conversion devices for ocean wave energy recovery [1].

Because of low operating costs and extremely high reliability, LIM-propelled systems have become an ever more frequent part of the public transport offering. LIMbased 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. References for the applications are readily available by any browser search using such keywords as "Linear Metro". For economic reasons, the operation of these systems as well as most other LIM-based systems has been based on a single-sided LIM [1,2]. Although, in comparison to its rotary counterpart, a traction LIM has necessarily a large air gap between the stator and its rotor equivalent and thus is less efficient on the motor-tomotor comparison bases, the LIM-based systems show better system-level performance resulting from several characteristic features specific to the LIMs. First, in comparison

**Citation:** Palka, R.; Woronowicz, K. Linear Induction Motors in Transportation Systems. *Energies* **2021**, *14*, 2549. https://doi.org/10.3390/ en14092549

Academic Editors: Frede Blaabjerg and Andrea Mariscotti

Received: 10 March 2021 Accepted: 26 April 2021 Published: 29 April 2021

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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 multimodal 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 completed, 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 conclusions summarize the authors' experience in the subject matter and highlight the areas of advancement for future research.
