# Adoption of Multiphase and Variable Flux Motors in Automotive Applications
## 1 Introduction  
The transition towards electric transportation has been driven by concerns about climate change and global warming. This shift has resulted in a surge in the sales of electric vehicles (EVs), stimulating a wealth of research in this field. The motor of an electric vehicle has a direct impact on autonomy, which is a key hurdle in the transition to electric transportation. Thus, the development of efficient and power-dense motors can contribute to reducing costs and enhancing vehicle autonomy. The trend towards higher speeds is evident in the evolution of the Toyota Prius, as discussed by [PERSON] et al. [1]. Higher speeds result in lower torque requirements for a given power, enabling a reduction in motor size [2]. Additionally, increased speeds demand higher switching frequencies, making wide band-gap semiconductors promising candidates for electric vehicle inverters [3]. Regarding motor topologies, permanent magnet motors are predominantly used due to their superior power densities and efficiencies, as shown in the market analysis by [PERSON] et al. [4]. However, induction motors (e.g., Tesla Model S) and electrically excited motors (e.g., Renault ZOE) are also used [5]. In recent investigations, multiphase (MP) permanent magnet motors have been suggested as a feasible option for electric vehicles, offering several advantages over the standard three-phase motors. These include higher torque density, lower torque ripple, power segmentation, and improved fault tolerance [6; 7; 8]. Moreover, several studies demonstrate that higher efficiency and power density can be obtained with MP motors compared to their three-phase counterparts [7; 9]. MP motors are already being implemented in market EVs, but their presence remains limited due to the simplicity and cost-effectiveness of the well-established 3-phase motors [10]. DANA TM4 offers 6- and 9-phase motors ranging from 100 kW to 430 kW of continuous power, which are used in EMOSS trucks (6-phase200 kW/950 Nm and 9-phase 250 kW/3400 Nm) [10; 11]. The BMW iX M60 is another example, featuring a 6-phase motor with an electrically excited rotor [4]. Additionally, companies such as Mercedes are dedicating part of their research to MP motors [10; 12]. Although 3-phase motors currently dominate the EV market, MP motors are gaining interest due to their high potential. Variable flux (VF) motors have also been shown to improve efficiency and torque-speed capability. These motors have several mechanisms for adjusting the permanent magnet flux, which can maximise efficiency in each working point. In fact, a number of authors have found that high levels of efficiency are obtained in the whole working region [13; 14; 15]. Although the academic interest in these motors is increasing, no commercial examples were found. It is clear that the interest in MP and VF motors is increasing, and [PERSON] et al. [16] recently reviewed multiphase motors together with hybrid excitation ones, these last being one type of VF motors. However, no published comparison between both motor types has been found. Therefore, the improvement in performance of both topologies has not been clearly quantified. Moreover, although studies such as [17; 18] propose multiphase hybrid excited variable flux motors, up to now, far too little attention has been paid to multiphase variable flux permanent magnet motors, which could combine the advantages of both. This article, therefore, identifies and compares the benefits and drawbacks of each topology. To this end, a literature review of MP and VF motors is conducted, in which the main characteristics and design challenges are addressed. Then, to quantify improvements in motor performance, an automotive three-phase motor is taken as a reference, and it is modified to obtain equivalent MP, VF, and MP-VF motors. Then, these three design alternatives are compared to the reference three-phase motor in terms of torque and power capability, efficiency, and torque ripple, using the Finite Element Method (FEM).