As a new electric excitation machine, EEDSVRM is widely used in popular fields such as aerospace and electric vehicles because of its simple structure and high robustness. However, due to its structural characteristics, VRM has problems such as excessive torque pulsation, which affects operating performance. Therefore, it is necessary to consider the machine structure and optimize the design of the machine to find the optimal combination of structural parameters to make the EEDSVRM achieve the best operating performance, which is of great benefit to the large-scale application of VRM in industry. The optimization design of EEDSVRM ontology was researched in [16,17,18,19,20,21]. There are many types of EEDSVRM ontology optimization designs, which can be roughly divided into five types: novel machine topology-based optimization, finite element simulation-based optimization, mathematical analysis-based optimization, intelligent algorithm-based optimization, and multiple fusion-based optimization. These five optimization design methods have their own characteristics. It is relatively time-consuming to propose new machine topology optimization and finite element optimization. Although mathematical formula analysis meets the requirements of real-time optimization, the optimization accuracy is still insufficient compared to finite element. Multi-objective optimization using intelligent algorithms can improve optimization efficiency. The optimization method of multiple fusion combines the advantages of multiple optimization methods and greatly improves the optimization efficiency as well as reduces the optimization cost.

2. Basic Topologies, Principles, and Performances

2.1. WFDSM

According to the machine excitation mode, the Doubly Salient Machine (DSM) can be divided into three categories, as follows: the Wound-field Doubly Salient Machine (WFDSM), which is shown in Figure 1a; the Doubly Salient Permanent Magnet Machine (DSPM); and the Hybrid Excitation Doubly Salient Machine (HEDSM), as shown in Figure 2 [22,23,24]. Compared with DSPM and HEDSM, WFDSM does not use permanent magnets for excitation. Instead, it only relies on the DC power supply to provide the excitation source. In this way, the magnetic flux of the machine can be controlled by adjusting the amplitude of the excitation current and the risk of irreversible demagnetization of the machine can be avoided. Therefore, the WFDSM can be used in harsh environments and has a strong magnetic flux adjustment capability.

Figure 2. (a) DSPM machine [22]. (b) HEDSM machine [23].

The working mode of WFDSM is different from DSPM and HEDSM, which adopt permanent magnet excitation because of the DC excitation mode. When current flows into the excitation winding, a magnetic field is established inside the machine. The magnetic flux generated by the main magnetic field passes through the yoke and magnetic poles of the stator and rotor and penetrates the entire magnetic circuit of the machine. When the machine rotor is driven to rotate by the prime mover, the flux linkage of the armature winding changes with the rotor position, resulting in a symmetrically induced potential. Unlike the permanent magnet excitation, the air gap magnetic field remains constant, which results in poor output voltage regulation and fault demagnetization capabilities. In WFDSM, the main magnetic flux of the machine can be controlled by adjusting the excitation current to affect the output voltage. The external uncontrollable rectifier circuit is shown in Figure 3 [25], where e_Af, e_Bf, e_Cf, and e_Df are the induced voltages of windings A, B, C, and D; W_A, W_B, W_C, and W_D are the windings A, B, C and D; R_A, R_B, R_C, and R_D are the self-resistance of the armature windings of A, B, C, and D; and R is the external load.

Figure 3. Full bridge rectifier circuit [25].

2.2. SRM

The stator and rotor of SRM are salient pole structures. As shown in Figure 1b, the salient pole of the stator is wound with armature winding, while there is no excitation winding. The magnetic flux moves along the path of minimum reluctance. When the stator and rotor poles are aligned, the air gap reluctance is the smallest. On the contrary, when the cogging is aligned, the air gap reluctance of the machine is the largest. Therefore, SRM follows the principle of minimum reluctance and the magnetic field is distorted to produce electromagnetic torque. Due to its simple structure, stable operation, and wide speed regulation range, it is widely used in aerospace, electric vehicle, and other fields. SRM have two-phase, three-phase, and multi-phase structures, and the stator and rotor also have different pole slot matching combinations. Generally speaking, if the multi-phase structure is adopted, the torque ripple of SRM is relatively small [26]. The relationship between the pole slot and phase number of SRM is as follows:

$$\{\begin{array}{c}{L}_{min}\left(N_S,N_r\right)=m{N}_r\\ L_{min}\left(N_S,N_r\right)>N_S>N_r\end{array}$$ (1)

where $L_{min}$ is the least common multiple of the number of poles of the stator and rotor, m is the phase number of SRM, and $N_S$ and $N_r$ represent the number of poles of the stator and rotor, respectively.

The pole slot between the stator and rotor of SRM is shown in Table 1. The most common one is the three-phase 12/8-pole machine, as shown in Figure 1b. Increasing the number of poles of SRM can reduce the torque ripple and vibration problems caused by SRM to a certain extent [27].

Table 1. Common pole slot fit combination of the stator and rotor of SRM.

2.3. EEFSM

Identical to the excitation method of DSM, in addition to EEFSM that uses only direct current for excitation, FSM also includes a flux-switching permanent magnet machine (FSPM) that uses permanent magnets for excitation and a hybrid flux-switching machine (HEFSM) in which permanent magnets and direct current are used for excitation at the same time. The topology of FSPM and HEFSM are shown in Figure 4. Although FSPM is considered to be the latest stator excited reluctance machine [28]，the permanent magnets used for excitation inevitably encounter demagnetization problems. It is because of the ability to achieve magnetic field adjustment and fault demagnetization under high temperature and harsh operating environments that EEFSM has attracted increasing attention and become one of the research highlights in the VRM field.Figure 1c shows the most widely used 12/10 pole for EEFSM.

Figure 4. ( a ) FSPM [ 12 ]。( b ) HEFSM [ 5 ]。

As shown in Figure 1c, it can be seen that there are 12 salient pole silicon steel cores on the stator. Slots are made on the stator teeth and the field windings are embedded in them. The excitation currents of the adjacent two sets of stator slots are opposite to each other.

Compared with conventional SRM, EEFSM can work in a full cycle and both its torque density and power density are higher, which makes it more suitable for applications with high performance requirements such as aerospace and offshore wind power.

With the periodic change of the overlapping area between the rotor pole and stator teeth, the air gap magnetic conductivity in the main magnetic circuit also changes, resulting in the magnetic linkage and back electromotive force in the phase winding, changing with the rotor position, which realizes the so-called “flux switching”, as shown in Figure 5. Reasonably designing the pole slot fit of FSPM and optimizing the structural size of the machine can make the waveform of the flux linkage and back EMF close to the sine wave.

Figure 5. ( a )Maximumforward position of flux linkage. ( b ) Maximumreverse position of flux linkage.