Dimensionamento e analisi di diverse configurazioni di rotore per macchine sincrone a campo avvolto = Design and analysis of different rotor configurations for wound field synchronous machines

In the last three decades, due to a more stringent regulation of pollution issue, Electric Vehicles (EV) have gradually gained attention. Permanent magnet (PM) motors and induction motors (IM) have been widely used and investigated as the traction motor of commercial EV, while less research can be found on the wound rotor synchronous machine (WR-SM). This type of motor presents some characteristics that finely fit to the requirements of the electric traction; therefore, it can be a valid alternative to dominant technologies, as its employment by some major car manufacturers proves. The WR-SM comes in two possible configura- tions depending on the rotor construction: salient pole and non-salient pole (also known as "cylindrical"). Of the two solutions, only the first has been investigated for automotive ap- plications. Nevertheless, the cylindrical machine is generally more suitable for high speed applications, since the rotor is more robust, and the conductors are placed on slots. This implies that the rotor winding is less affected by mechanical vibrations, which can cause the deterioration of insulation. Hence, the aim of this study is to understand if a cylindrical pole rotor can be a beneficial choice in electric traction. To reach the goal, a comparative analysis of the two machines in terms of performances and efficiencies is carried out. An already existing salient pole EV motor is taken as a reference for the study. By keeping the same stator and the same number of poles, the rotor is redesigned in its new cylindrical shape, respecting electrical and magnetic constraints of the original machine. Then, by means of a finite element analysis (FEA), torque, efficiency and losses are computed for different work- ing points. Results show that the torques of the cylindrical solution are about 15% lower than those of the salient pole machine. The reason has been investigated, and it turned out that the cylindrical machine is more affected by cross saturation. Moreover, it presents a higher torque ripple. This is due to the necessity of sacrificing the optimal pole shape in order to host the slots, resulting in a worse distribution of magnetic flux density in the air gap. One possible solution to this issue is the machine skewing. Therefore, the torque of the skewed motor has been computed and its ripple has been reduced from 46% to 5%. For what concerns the efficiency of the motor, three working points have been simulated. The non-salient pole machine efficiencies are only 1% lower than the reference. For further development of the study, some future works are suggested. First, a reduction of the number of poles could be investigated, since cylindrical machines are usually 2- or 4-pole machines. This can al- low to increase the number of rotor slots, resulting in a better distribution of magnetic flux density in the air gap. Moreover, the stator can be redesigned considering different slot/pole combinations. Finally, multiple-phase solution can be adopted for additional improvement.