*3.4. Summary of the Comparison*

According to the loss comparison results, the 5L-HANPC was the first choice. Another advantage is that the 5L-HANPC may decrease the volume and weight of an AC inductor, which is necessary for rectifiers. However, according to the comparison in Table 3, more IGBTs were required to install, which in turn increased the surface area of the heatsink and required additional flying capacitors in the 5L-HANPC. On the other hand, by considering both the number of modules and the total loss, we found that the 3L-NPC and the 2L-FB could also be the favorable choices. However, the 2L-FB must connect to motors with three separated stator windings, and thus a specially designed motor was required. As a result, the 3L-NPC could be the best choice, especially for the inverter stage, where no AC-side inductor was required.

Based on the previous discussion, some possible back-to-back converter configurations were discussed. A very interesting fact is that the rectifier stage and the inverter stage did not have to apply converters with the same voltage level. It is because on the rectifier stage, increasing the voltage level can decrease the volume and weight of an AC inductor. On the contrary, no AC-side inductor was required on the inverter stage. Decreasing the voltage level can minimize the number of IGBT modules. Here come three potential solutions as well as one traditional solution used for comparison:


Among the three potential solutions, the 3L-NPC should be the only choice for inverter-stage circuits, since it had the highest power density, where no AC inductor was required. For the rectifier stage, a 5L-HANPC, a 3L-NPC, and a mature ATRU circuit are all possible choices. An asymmetrical back-to-back converter topology was proposed and given in Section 4. It aims to optimize the system's total weight on both the rectifier stage and the inverter stage. However, the loss calculation cannot tell which one is better, and only prototype design can clearly show the final winner.
