**2. State of the art**

A rotary switched reluctance machine consists of a stator, in which an even number of coils are placed in an laminated iron yoke, and a rotor, which comprises only a laminated iron yoke with no coils. Both parts have a polar configuration with different number of poles in each one. Only a restricted combination of pole number for rotor and stator is allowed to run the machine properly. The name of this type of machine comes from the fact that every time a coil is activated, the rotor moves trying to align one of its poles with the corresponding stator one, minimizing the overall reluctance of the magnetic circuit. The SRM is based on the concept of switching the stator coils sequentially. If the coil is switched on before the closest rotor pole is aligned with the active stator one, then the machine acts as a motor, while if it is switched after, then it acts as a generator.

Structural simplicity, high efficiency compared with the induction motor, low cost and control flexibility, rugged motor construction, large starting torque, wide speed range, inherent faulttolerance capability, and high operating efficiency are some of the characteristics that make the SRM a remarkable candidate for electric drive of electric vehicles and hybrid electric vehicles (HEVs) [7]. The authors of [2] also refer to the SRM suitability for high-speed appli‐ cations as well as for harsh environment such as EVs, considering high temperature and vibrations. Another important advantage of SRMs is their intrinsic fault tolerance, in the sense that they will continue to operate in a satisfactory manner, without the other phases being affected, after sustaining some types of faults [8]. Moreover, most SRMs can operate even if one of the phases is broken down (degraded mode or "m-1phases" mode), which means that they are intrinsically redundant [9–11]. Consequently, the SRM combines many desirable qualities of induction machines as well as PM brushless machines. Its performance and inherently low manufacturing cost make it a competitive option for this application.

Various motors have been considered for EV applications [2]: DC machines, induction machines (IMs), permanent magnet synchronous machines (PMSM) and switched reluctance machines (SRMs). DC machines are not used any more due to the high maintenance required and the lack of reliability associated to the commutator and the brushes. IMs, commonly used in industrial applications, are a robust option with more reliability and better efficiency than DC machines. However, they present a clear disadvantage which is the heat produced by the losses in the rotor, difficult to extract, requiring a special cooling system and reducing the overload capacity of the motor. PMSMs, and particularly Interior PMSMs (IPMSMs), are the most popular choice for EVs [3, 4]. These motors have higher efficiency, compactness, highpower density, fast dynamics and high torque-to-inertia ratio. However, they show some drawbacks due to the presence of rare earth permanent magnets. The price and the availability of rare earth material are considered a potential problem for EVs massive development [5]. Besides, PMSMs are more sensitive to high temperatures, and they suffer from risk of demag‐

Switched reluctance drives (SRDs) are getting increased attention recently, including in the EV industry [6]. SRMs have been well known for many years due to its simple operation principle. Relatively, advances in static switches and digital control devices have fostered its applications in many different fields, even replacing other types of electrical machines.

A rotary switched reluctance machine consists of a stator, in which an even number of coils are placed in an laminated iron yoke, and a rotor, which comprises only a laminated iron yoke with no coils. Both parts have a polar configuration with different number of poles in each one. Only a restricted combination of pole number for rotor and stator is allowed to run the machine properly. The name of this type of machine comes from the fact that every time a coil is activated, the rotor moves trying to align one of its poles with the corresponding stator one, minimizing the overall reluctance of the magnetic circuit. The SRM is based on the concept of switching the stator coils sequentially. If the coil is switched on before the closest rotor pole is aligned with the active stator one, then the machine acts as a motor, while if it is switched after,

Structural simplicity, high efficiency compared with the induction motor, low cost and control flexibility, rugged motor construction, large starting torque, wide speed range, inherent faulttolerance capability, and high operating efficiency are some of the characteristics that make the SRM a remarkable candidate for electric drive of electric vehicles and hybrid electric vehicles (HEVs) [7]. The authors of [2] also refer to the SRM suitability for high-speed appli‐ cations as well as for harsh environment such as EVs, considering high temperature and vibrations. Another important advantage of SRMs is their intrinsic fault tolerance, in the sense that they will continue to operate in a satisfactory manner, without the other phases being affected, after sustaining some types of faults [8]. Moreover, most SRMs can operate even if one of the phases is broken down (degraded mode or "m-1phases" mode), which means that

netization.

**2. State of the art**

98 Modeling and Simulation for Electric Vehicle Applications

then it acts as a generator.

On the other hand, SRMs have also some disadvantages, such as lower specific torque, hightorque ripple, low efficiency compared with IPMSM and high noise and vibrations [12]. Most authors claim that PMSMs can reach up to twice the power density of SRMs. However, this is only true when the comparison is done in unfair terms: PMSMs are not fault tolerant by default, but they must be for some applications. The good news is that they can be designed to be fault tolerant (although not for high-speed applications); the bad news is that doing so decreases their power density significantly [8]. Besides, PMSM lose torque capability with temperature due to permanent magnets flux reduction [13], much more than SRMs and IMs. As high temperatures could become a common requirement for electric motors in EVs, this fact will further reduce the difference between PMSMs and the other two alternatives. After consider‐ ing these two points, PMSMs could achieve "only" 25–50% more power density than SRMs [8].

Having a SRM with a higher number of rotor poles than stator poles results in higher efficiency and torque to weight ratio [14, 15]. This is particularly convenient for outer-rotor machines, usually used in in-wheel designs. In such case, a better performance is achieved if a segmented rotor topology is used [16]. Segmented rotor SRMs have a stator with full pitch coils and a rotor with discrete segments embedded in nonmagnetic material.

Regarding torque ripple, several alternatives related to rotor geometry have been extensively studied [17], based on slanting, serration, chamfering or skewing, demonstrating a certain improvement in torque ripple. Other studies have been accomplished varying yoke thickness, stator pole width, internal stator diameter [17, 18] and pole shapes [19].

Various SRM have been designed and built during the last two decades for EV and HEV applications; examples are found in [11, 20–27]. The key points to improve a SRM efficiency are to increase the number of stator and rotor poles, reduce iron losses using 0.1-mm-thick high-silicon steel laminations and use continuous current mode (single pulse) operation above rated speed [20]. It seems that torque density and efficiency cannot be improved at the same time: for instance, [26] achieved an efficiency of 96% but torque density was 25 Nm/l, while [27] reported a torque density of 38 Nm/l, although the efficiency was 90% due to the lowgrade iron steel utilized to maximize torque density. As usually in electrical machines, getting both high-torque density and high-energy efficiency is difficult without rare earth permanent magnets.

In SRDs, the power electronics topology is usually chosen as a compromise between perform‐ ance and the number of power semiconductor devices. The higher the number of switches, the higher the cost and the failure rate, but the controllability of the system increases, improving performance. Different power topologies with specific advantages and disadvantages are usually found in the literature [1, 28, 29]. The most common one is the asymmetric bridge converter, comprising two switching devices and two power diodes per phase. **Figure 1** shows the circuit schematic of this converter for a 4-phase SRM. The main advantage of this topology is the capability of supplying each phase individually. This, in turn, allows efficient phase overlapping, increasing average torque and reducing torque ripple. For the same reason, this circuit is very convenient for achieving fault-tolerant SRDs. In terms of cost, the main drawback of this topology is the high number of switching devices in comparison with other alternatives.

**Figure 1.** Asymmetric bridge converter topology for 8/6-pole switched reluctance machine.

The above topology requires two more diodes per phase compared with the most conventional topology used for PMSMs and IMs [30]. As aforementioned, some alternative topologies have been proposed in order to reduce the number of devices, including one that uses the SRM phases as inductive filters to charge the EV battery [31], which adds an additional value to this type of machine over IMs and PMSMs. However, these alternatives reduce the controllability and efficiency of the drive and are generally avoided in applications with demanding torque ripple requirements.

SRMs require very precise position determination, since the activation of the phases must be synchronized with rotor position to achieve good performance. Conventionally, SRDs have a measuring device (either an encoder or a resolver) to determine the speed and position of the rotor. These sensors are accurate and relatively inexpensive and have very good perform‐ ance. However, it would be desirable to remove them (without deteriorating performance considerably) in order to make the drive cheaper, less voluminous, less noise-sensitive and more reliable [32, 33]. This can be done by means of a "position sensorless control", also known as "position self-sensing control" given that the external sensor is replaced by the electrical machine itself, which acts as its own position sensor [34].

As the cost and the volume of an external position sensor is not significant when compared to the whole traction system, the main advantage of position self-sensing in SRMs for EVs is the possibility to continue operation in case the external sensor is out of service. This redundancy increases the fault tolerance of the whole drive, which is critical in traction applications and one of the main approaches of this chapter.

During the last decades, many self-sensing techniques have been proposed for rotating electrical machines in general [32, 33] and SRMs in particular [35, 36]. Commonly, these methods perform well either at high speed (such as BEF-based, flux linkage-based or induc‐ tance-based techniques) or at low speed/standstill (such as injection-based or di/dt-based techniques). This implies that at least two of these methods are usually combined in the same drive, the transition between both being particularly delicate.

Besides, self-sensing normally causes a slight reduction in performance, since the position is estimated with some error. This in turn implies that phase activations and deactivations are not applied exactly when desired, leading to a reduction in average torque, an increase in torque ripple and its consequences, and a decrease in energy efficiency. Of course, all these disadvantages are minor when compared to having the possibility to operate the EV even after the external position sensor fails.

Electromagnetic interference (EMI) and electromagnetic compatibility (EMC) are also very important aspects of an EV. In this sense, SRMs could present worse EMC behavior due to higher di/dt and higher switching frequencies in general. Ample research is missing on this topic.
