# Topology Comparison Study of Five-Phase Wound-Field Doubly Salient Fault Tolerant Generators
## 1 Introduction  
With the continuous development of science and technology, more and more devices are beginning to shift from full-size to micro-sized portable, and small-scale energy conversion devices are being gradually developed and applied [1; 2]. The integrated internal combustion engine and generator as a universal power constitute an independent power generation system. In order to meet the rapid development of portable electronic products and MEMS, it is of great theoretical and practical significance to develop a small internal combustion engine power generation system [3]. As the core component of the electric power system, the reliability of the generator directly affects the performance of the power system. Aircrafts, automobiles, and other transportation vehicles have higher and higher reliability requirements for the electric power systems [4; 5; 6; 7]. In these applications where reliability is critical, the generator is required to have fault tolerance for normal or low grade performance in the event of a fault [8; 9; 10]. The multiphase electric machines with more than three phases are very suitable to be used in high reliability situations because they can limp operate even with one or two open-circuited phases [11; 12]. Recently, most of the studies have concentrated on the multiphase driving motors and their fault-tolerant inverters and controllers [13; 14], which have shown the multiphase machine improve the fault tolerance performance of the traditional motors. For the generators at present, the aircrafts usually use three-stage synchronous machines, permanent magnet synchronous machines, and switched reluctance machines, etc. The rotor ofthe three-stage synchronous machines has the three-phase windings and rotating rectifiers of an AC exciter. And the stator also has three-phase windings of the permanent magnet auxiliary exciter and excitation windings of the AC exciter. The overall structure of the machine is complicated and the cost is high [15]. Permanent magnet synchronous generator also has shortcomings, such as difficulty in voltage regulation and flux-weaking in failure condition, especially at high-speed [16; 17]. When the switched reluctance machine is used as a generator, the voltage control is more complicated than the wound field machine in controlling the phase current, and a larger filter circuit is needed [18; 19]. Most of the generators used in automobiles are claw pole alternators [20]. The machine is generally structured with a carbon brush and slip ring, and the carbon brush on slip ring is easy to wear out, which is not conducive to the reliability of the automobile generator. Compared with the above generators, the wound-field doubly salient generator (WFDSG) has a simple and reliable rotor with salient-poles, as well as a field winding capable of regulating voltage [21]. Moreover, the position sensors are not needed for power generation operation, and the excitation current can be directly adjusted like the wound-field synchronous generator to keep the output voltage stable, and the voltage regulation performance is good, which is very suitable as a generator [22; 23; 24; 25]. For example, a three-phase WFDSG is used to construct a brushless DC extended-range power generation system for electric vehicles in [26], and the power generation control strategy is proposed. In addition to the abovementioned low failure rate, high-reliability generators should also have the ability to operate under fault conditions. Conventional generators with three-phase star windings do not operate properly with a single-phase fault. Electric generators structured with multi-phase or redundant channel can achieve certain output after single-phase or single-channel faults by using multi-phase or redundant channel windings, which have fault tolerance or additional capacity [27; 28; 29]. For the multi-phase generator, it reduces the voltage ripple rate after rectification because of the electric angle of the electromotive force (EMF) waveforms are larger than 120\({}^{\circ}\).