## 2 General Structure of Fully Pitched SRMs
### Fully Pitched AFSRMs  
AFSRMs, which have flux flowing in the axial direction, feature a larger and more effective air gap compared to radial machines. The flat structure of AFSRMs allows for the easy adjustment of their air gap, making them well-suited for various applications, including electric powertrains, wind turbines, microelectromechanical systems (MEMS), simple devices such as fans, and in-wheel motors (IWMs) in electric vehicles [30]. AFSRMs are generally categorized into three main types: (1) the \"single-stator and single-rotor AFSRM\", which is the most basic structure with one stator and one rotor; (2) the \"double-stator and single-rotor AFSRM\", which includes two stators and one rotor; and (3) the \"single-stator and double-rotor AFSRM\", featuring one stator and two rotors [8].  
The \"single-stator and single-rotor AFSRMs\", also known as \"single-sided (SS) rotor and stator AFSRMs\", have only one air gap. One major drawback of this design is the non-uniform distribution of the forces between the stator and rotor. This leads to axial force issues, which can cause high noise levels, the deformation of windings, collisions between rotor and stator pole heads, and bearing damage. Additionally, because the forces disrupt the symmetry of the air gaps, they can increase motor vibrations and torque ripples. For these reasons, it is crucial to ensure that the rotor is properly aligned [8; 30].  
The \"double-stator and single-rotor AFSRMs\", also known as \"double-sided (DS) rotor(s) and stator(s)\", consist of a single rotor placed between two stators. This model, which features two easily cooled air gaps, has the significant advantage of balanced axial forces. The double-stator design aims to reduce the effects of electromagnetic forces without increasing the length of their air gap, because this model cancels the electromagnetic force in the axial direction due to its double stator. The axial force of each stator on the rotor is compensated by the force of the other stator. However, the twin air gap of this model reduces the maximum inductance and causes the output torque to decrease. This negativity  
Figure 1: Fully pitched SRM.  
causes a significant disadvantage for this model. In addition, although the torque ripple can be reduced by angularly shifting the stators, the angular shift can also cause unbalanced axial forces [8,31].  
Another model of the \"double-sided (DS) rotor(s) and stator(s)\" structure is the \"single-stator and double-rotor AFSRMs\". This model consists of a single stator in the middle and an external rotor on each side. The cooling situation is complicated; the windings are difficult to place and as a result, it is a high-cost model [8].  
In addition to these three models, multi-layer/multi-stack AFSRM structures can be created using all three model structures. Depending on the model structures used in the multi-layer structure of AFSRM, the stator and rotor numbers can be equal, less or more [30]. Figure 2 shows (a) short-pitched AFSRM and (b) fully pitched AFSRM (FP-AFSRM). The stator and rotor geometric structures of both motors are the same, but the winding structure and, therefore, the control algorithms are different.