# Emerging Technologies for Advanced Power Electronics and Machine Design in Electric Drives
## 2 Topologies of Electric Drives
### Multiphase Electric Drive Topologies
#### 2.3.3 Impact of Winding Layouts  
In multiple three-phase drives, there are many possible arrangements of three-phase winding sets along the stator circumference, each with its own advantages and disadvantages (Table 2). This degree of freedom makes it possible to choose a winding layout that is most suitable for a particular application.  
As an example, a PMSM drive with 12 stator slots and 10 poles with concentrated stator winding is considered. This combination of slots and poles enables the realization of a six-phase machine with two three-phase winding sets (Figure 10), whereby the coils with a reddish colour indicate Winding Set 1, and the coils with a greenish color indicate Winding Set 2. There are two fundamental ways to obtain a dual three-phase machine. The first option is to segment an original three-phase winding into two parts, whereas the second way is to design a dual three-phase winding arrangement using a six-phase machine star of slots (Figure 10).  
The multisector winding layout (MWL) places each winding set in its own machine sector. The main advantage of this layout is that it is easier to manufacture and there is virtually no magnetic coupling between the winding sets [103]. In case of an open-circuit fault in one winding set, a significant unbalanced magnetic pull (UMP) arises, whereas a short-circuit fault does not generate any braking torque. Highly coupled winding layout (HWL) results in a large mutual coupling between the sets [104]. An open-circuit fault can be easily handled as no UMP occurs and only derating of the drive is needed; however, short-circuit fault cannot be handled, and the drive must be stopped. In both cases, the magnetic axes of the two sets of windings are aligned, therefore they can be regarded identical to a symmetrical winding layout in terms of attainable electromotive force (EMF) per phase. The performance of MWL and HWL during healthy operation is equal to the original three-phase drive in terms of average torque and torque ripple.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Winding Layout**} \\ \cline{2-5}  & **HWL 1** & **MWL 2** & **SWL 3** & **AWL 4** \\ \hline Average torque & higher & moderate & moderate & moderate \\ Torque ripple & lower & moderate & moderate & moderate \\ Mutual coupling & strong & none & weak & moderate \\ UMP 5  & none & high & high & none \\ SC current 6  & high & none & low & moderate \\ \hline \hline \end{tabular}
\end{table}
Table 2: Performance of a dual three-phase machine with different winding layouts.  
Alternatively, the winding layout can be designed from scratch using a star of slots resulting in two possible configurations [105]. In the symmetric winding layout (SWL), groups of phase coils are arranged alternately along the stator circumference. The mutual coupling of winding sets is therefore weak, which indicates favorable behavior even in the event of short-circuit faults. However, since the coils of a particular phase are not arranged symmetrically, a considerable UMP occurs. In the asymmetric winding layout (AWL), the coils of the two winding sets alternate at every second tooth, resulting in geometrical symmetry of the coils of a given phase, thus ensuring negligible UMP in the event of a fault. The mutual coupling is moderate and can lead to considerable short-circuit currents. As can be seen from the star of slots (Figure 10), AWL has a higher winding factor and achieves a higher EMF. Therefore, the average torque is slightly higher and the torque ripple is lower compared to SWL. With AWL, the electrical shift between the magnetic axes is 30 degrees, which requires an out-of-phase current supply.