## 1 Introduction
### Literature Overview on the Use of Ferrite-Assisted Synchronous Reluctance Motors in Traction Drives  
Magnet-free synchronous reluctance motors (SynRMs) have lower losses than induction motors. However, in electric vehicles that require a wide CPSR, SynRMs have a reduced inverter utilization factor, which significantly increases the power rating and cost of the inverter power modules [6]. This drawback can be overcome, as well as the dimensions of the motor can be reduced by adding inexpensive ferrite magnets to the rotor [14]. Ferrite-assisted SynRMs (FaSynRMs) with high rotor magnetic anisotropy have proven themselves in pumping applications in which a wide CPSR is not required [18; 19; 20]. Moreover, FaSynRM is also an alternative to IPMSM in electric vehicle applications, with comparable torque capability, lower permanent magnet cost, and better field-weakening capability [21; 22]. For example, in [21], the characteristics of PMSM and FaSynRM are theoretically compared in a Toyota Prius 2010 drive with a rated power of 60 kW and CPSR of 3:5:1. It is concluded that the FaSynRM has characteristics similar to the original V-shaped interior PMSM with the same overall dimensions, while the cost of its active materials is much lower, and the maximum operating temperature of the rotor is much higher. In [22], a theoretical comparison is made between V-shape rare-earth assisted SynRM and FaSynRM for a BMW i3 2016 drive with a rated power of 60 kW and CPSR of 3:1. It is shown that the use of ferrite magnets makes it possible to obtain lower peak mechanical stress, better field-weakening capability, higher peak efficiency, and comparable efficiency over the operating range. It can be concluded that quite a lot of articles are devoted to the analysis of FaSynRM characteristics in light passenger vehicles. However, studies on optimizing FaSynRM design for a subway train drive have not been presented. Literature Overview on the Use of Synchronous Homopolar Motors in Traction Drives and on the Use of Ferrite-Assisted Synchronous Homopolar Motors  
Another alternative to PMSMs in electric vehicles is electrically excited wound rotor synchronous motors (WRSM), which solve many of the problems associated with rare earth magnets but require a solution to the problem of unreliable sliding contact [23]. By merging the advantages of permanent magnet synchronous machines (PMSMs) and wound rotor synchronous machines (WRSMs), synchronous homopolar machines (SHMs) eliminate the need for slip rings like in PMSMs and, similar to WRSMs, operate without magnets. They achieve a wide constant power speed range (CPSR) due to their adjustable excitation current. One advantage SHMs have over WRSMs is that the number of excitation coils is independent of the number of poles, whereas, in WRSMs, it increases with the number of poles, resulting in reduced excitation magnetomotive force (MMF) [14]. As a result, SHMs have lower mass and reduced losses in the excitation winding compared to WRSMs. Additionally, since SHMs have no losses in the rotor, there is no need for rotor cooling. SHMs find applications as highly reliable generators in various fields, including laser technology, pulse heating, welding units [24], automotive generators [25], railway passenger cars, ships, and aircraft [26]. In addition, the application of SHM in traction drives is being studied. In [23; 27], the utilization of SHMs as traction motors in mining trucks is discussed. Paper [27] presents and validates the computation method for the traction SHM, which relies on a set of 2D magnetostatic boundary problems. Furthermore, SHM optimization techniques for traction applications were developed [23] using the Nelder-Mead algorithm and the model proposed in [27]. Also, the performance of SHMs was compared with other types of motors. For example, in contrast to WRSMs, traction SHMs exhibit certain drawbacks, as highlighted in reference [27].