## 5 Future Directions in Research
### Noise and Vibration Improvement Through System-Level Design Optimization  
Noise and vibration are inherent drawbacks of SRMs due to radial electromagnetic forces, which should be minimized as much as possible [84]. The traditional approach to addressing these issues is through the optimization of the machine geometry and the reduction in pole saliency. In [85], there has been an effort to reduce these issues by modifying the geometry of the stator poles, introducing bridges between the poles to reduce the saliency ratio. Similarly, the stator pole bridge design in this study resulted in a 12.52 dBA noise reduction and shifted resonant frequencies, reducing vibration acceleration from 100 m/s\({}^{2}\) to 59.82 m/s\({}^{2}\). Unlike active control strategies, which require real-time sensors, controllers, and energy, this passive solution is robust and energy-free, offering consistent noise and vibration improvements across a wide operational range. For EVs and HEVs, electromagnetic noise in powertrains is particularly noticeable due to a phenomenon known as whistling, which stems from the interactions between the stator and rotor poles across the airgap [86]. In SRMs, this phenomenon is further amplified due to the high saliency of the poles, which intensifies the interactions in the airgap. As a result, noise mitigation becomes critically important for applications in EVs and HEVs, where noise mitigation is a key design constraint.  
On the other hand, one form of noise generation is the power electronics from the converter, through which noise is introduced by their switching frequency when there is a PWM technique employed and, often, by design failures. Therefore, they should be designed with the aim of reducing interference and noise in the machine. Several control strategies have been proposed and implemented to mitigate this issue, however there is usually a trade-off between minimizing noise and maintaining high efficiency along with average torque [87]. For instance, in [87], the authors present several NVH mitigation techniques based on the power electronic control Random Hysteresis Band (RHB) and RHB with Spectrum Shaping (RHB-SS). All methods demonstrated a reduction in acoustic noise, with RHB and RHB-SS proving the most effective, achieving a 13-15% decrease relative to the baseline. However, due to the inherent characteristics of high-rotor pole SRM, notably their pronounced saliency, these machines are particularly prone to noise and vibration generation. As a result, the observed reductions may not represent the near-optimal achievable mitigation in this SRM configuration.  
Efforts to mitigate Noise, Vibration, and Harshness (NVH) at the component level can often result in negative effects on other parameters, such as efficiency, or fail to achieve sufficient noise reduction, since NVH is influenced by both the machine and the controller. The use of SLDOM could address this issue by optimizing the entire system while setting NVH-related parameters as optimization targets, like mitigating radial forces or optimizing phase commutation timing through the control strategy. This holistic approach allows for the simultaneous optimization of all potential noise sources, both mechanical and electrical, leading to a more comprehensive and effective noise and vibration reduction strategy, especially for transportation applications.