**4. State of the art in axial-flux electric machines for HEV and EV propulsion**

The initial selection of electrical machines for hybrid (HEV) or electric (EV) drives includes a variety of different topologies. According to outcomes of literature survey, induction machines alongside synchronous machines take the major place in HEV or EV power trains. Both of these different families of machines topologies, in sense of operational principle, may be laid out in an axial or a radial plane. Out of radial synchronous machines, surface-mounted, inset and embedded-permanent magnet topologies, and switched reluctance machines are considered as competitive for traction purposes. Among the synchronous permanent magnet, axial-flux machines are subject of ongoing research and therefore recently the need to be included in selection process due to their advantageous axial length. The search for more efficient, cost-effective, and fault-tolerant layouts drives the design of electrical machinery up to its limits. The adoption of new materials and topologies for lightweight vehicle and power trains is extremely investment intensive. This chapter proposes to initiate the development and adoption of a new class of propulsion systems incorporating advanced electrical drives. By advanced electrical drive, it means the use of axial-flux dual mechanical output machines (the electrical machine has two independent rotors) with a single stator winding capable to control the mechanical output independently (**Figure 1**). Moreover, the requirements of many applications both in the industry and in the field of renewable energy conversion are so tough that traditional layouts are abandoned in favor of new topologies or new light is shed over older ones. We keep in mind the fact that the presence of the permanent magnet in the structure of the propulsion machine is rather a limitation which can be avoided. In order to build highly reliable propulsion systems, the use of a highly permanent magnet-depending generator/motor will limit the lifetime of the electrical machine and drive it.

Due to their high torque density capabilities, favorable aspect ratio, and the possibility to implement a large number of poles [14], axial-flux PM machines (AFPMMs) are used in many up-to-date applications. In fact, AFPMMs are applicable to fans, diesel and wind generation units, elevators, ships, and vehicles [15]. The first-harmonic approximation of the MMF field commonly used for design purposes is sometimes inadequate, for example, with trapezoidal back EMFs or with fractional slot windings. In fact, these windings operate with a high level of MMF harmonics, although they produce sinusoidal EMF. The technical literature offers many machine models, each one addressing specific issues. Some models provide a precise description of the MMF content [16], some focus on skewing [17], some provide guidelines for the use of finite element method (FEM) [18], and others account for saturation, iron losses, and temperature effects [19, 20]. Most models, however, are based on the reduction of the 3D problem to a 2D problem by the use of a cylindrical cutting plane at the main flux region [21–25]. Moreover, 2D and 3D FEM analyses fail to account for iron losses, especially in fractional winding machines with soft magnetic composite (SMC) core [25, 26]. Some models rely on FEM simulations [27]. The influence of rotor eddy currents on the efficiency of fractional slot AFPMM is significant, as emphasized in [23]. As a result of the fundamental intricacy of the magnetic circuit, extensive papers have been devoted to the design and modeling of AFPMM. Actually, the usual first-harmonic approximation of the MMF wave is not suitable for designing windings with a fractional slot number per pole. These windings present short links, compact poles, and sinusoidal EMF, even if the MMF wave has higher- and lower-order spatial harmonics. The harmonic content does not alter the quasi-sinusoidal terminal EMF, but it alters core loss and machine inductance. The professional data available show specific models addressing certain issues: provides a detailed MMF portrayal even if the analytic model cannot be held responsible for machine curvature and presumes that the coil pitch is detailed

intermittent overload capability for overtaking, high reliability and robustness for vehicular environment, low acoustic noise, and reasonable cost). On the other hand, when the electric machine needs to work with the engine for various HEVs, there are some additional requirements (high-efficiency power generation over a wide speed range, good voltage regulation

The initial selection of electrical machines for hybrid (HEV) or electric (EV) drives includes a variety of different topologies. According to outcomes of literature survey, induction machines alongside synchronous machines take the major place in HEV or EV power trains. Both of these different families of machines topologies, in sense of operational principle, may be laid out in an axial or a radial plane. Out of radial synchronous machines, surface-mounted, inset and embedded-permanent magnet topologies, and switched reluctance machines are considered as competitive for traction purposes. Among the synchronous permanent magnet, axial-flux machines are subject of ongoing research and therefore recently the need to be included in selection process due to their advantageous axial length. The search for more efficient, cost-effective, and fault-tolerant layouts drives the design of electrical machinery up to its limits. The adoption of new materials and topologies for lightweight vehicle and power trains is extremely investment intensive. This chapter proposes to initiate the development and adoption of a new class of propulsion systems incorporating advanced electrical drives. By advanced electrical drive, it means the use of axial-flux dual mechanical output machines (the electrical machine has two independent rotors) with a single stator winding capable to control the mechanical output independently (**Figure 1**). Moreover, the requirements of many applications both in the industry and in the field of renewable energy conversion are so tough that traditional layouts are abandoned in favor of new topologies or new light is shed over older ones. We keep in mind the fact that the presence of the permanent magnet in the structure of the propulsion machine is rather a limitation which can be avoided. In order to build highly reliable propulsion systems, the use of a highly permanent magnet-depending genera-

over a wide speed generation, capable of being integrated with the engine).

tor/motor will limit the lifetime of the electrical machine and drive it.

Due to their high torque density capabilities, favorable aspect ratio, and the possibility to implement a large number of poles [14], axial-flux PM machines (AFPMMs) are used in many up-to-date applications. In fact, AFPMMs are applicable to fans, diesel and wind generation units, elevators, ships, and vehicles [15]. The first-harmonic approximation of the MMF field commonly used for design purposes is sometimes inadequate, for example, with trapezoidal back EMFs or with fractional slot windings. In fact, these windings operate with a high level of MMF harmonics, although they produce sinusoidal EMF. The technical literature offers many machine models, each one addressing specific issues. Some models provide a precise description of the MMF content [16], some focus on skewing [17], some provide guidelines for the use of finite element method (FEM) [18], and others account for saturation, iron losses, and temperature effects [19, 20]. Most models, however, are based on the reduction of the 3D problem to a 2D problem by the use of a cylindrical cutting plane at the main flux region

**propulsion**

162 New Trends in Electrical Vehicle Powertrains

**4. State of the art in axial-flux electric machines for HEV and EV** 

**Figure 1.** Lightweight power train solution based on axial-flux dual rotor. (a) Hybrid vehicle and (b) electric vehicle.

at the mean core radius, skewing-focused, aids in using the finite element method (FEM), and others are saturation, iron losses, and thermal effects accountant. However, a fair amount of models cut down the 3D issue to a 2D portrayal of a cylindrical cross section. Thermal models require a detailed analysis of rotor iron losses. Mention is based upon FEM simulations and emphasizes the impact of rotor eddy currents. The thermal transfer in axial-flux machine geometries was explained. Thermal simulations done on a machine with a massive iron rotor core showed that the rotor temperatures are above those measured over the windings surface. Actually, a considerable contribution to losses comes from the rotor and is basically due to the sub-harmonic and high-frequency parts of the stator MMF, including slotting. Recent investigations on the subject demonstrated that the sub-harmonics are, however, the main causes of rotor losses. Axial-flux induction machines are also possible. The Australian company Evans Electric has developed induction axial-flux machines for electric car as in wheel motors. These motors deliver 625 Nm and a peak power of 75 kW in an AFIM (axial-flux induction machines) configuration. This machine has the particularity that the stators cover only a part of the machine and not the complete disc. This enables a better air cooling of the machine but reduces the performance. In-wheel direct-drive motors represent the simplest and lightest method for propelling wheeled vehicles, but due to the reduced suspension performance of vehicles with increased wheel mass, the mass of in-wheel motors is a major concern. The axial-flux switchedreluctance motor (AFSRM) topology for in-wheel drive vehicle applications is presented. For a high-speed automotive generator, the axial-flux reversal machine was proposed. The highspeed AF FRM electrical machines offer great advantages of reduction in size per unit output and improved efficiency [25–27]. Small-scale generating sets of high power densities, previously used predominately in military and aircraft applications, are attracting growing attention for a wide variety of automotive applications: onboard charger, compact range extender, and turbine-based HEV. One salient feature of these sets is the use of a high-speed generator directly coupled to a small gas turbine, resulting in a significant reduction of weight and size. The path of the magnetic flux distribution in the air gap is in the axial direction. The rotor of the machine has no permanent magnet and field winding. The permanent magnets and armature windings are on the stator cores being very easy to be cooled. For this reason in the future, we start our investigation including the induction machine, the SRM machine, and the flux reversal machine. The building experience will be valuable for industry as well to the scientific community.

electric traction, operating in different modes: engine, generator, the brake, or combinations in between [28–35]. It proposes **an international original solution** [32] in which the two electrical machines (generator and motor) and static converters related are replaced by a single synchronous permanent magnet machine having an axial air gap, a central stator with slots on both sides, and two different windings supplied from a single PWM inverter having two output frequencies, and two independent rotors. This machine efficiency is high, and the torque density developed by the two rotors is also high. The inverter output voltage ripple contains a combination of voltages each having different frequency and determining a different rotational speed Ω1, Ω2, for the two rotors. The first rotor, coupled with the combustion engine, together with the

Lightweight High-Efficiency Power Train Propulsion with Axial-Flux Machines for Electric or Hybrid Vehicles

http://dx.doi.org/10.5772/intechopen.77199

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corresponding stator winding operates as a generator most of the time (**Figure 2**) [29].

It can be used and as a starter for the combustion engine startup. The second rotor, coupled with the differential mechanism and the drive wheels together with the associated stator winding, operates as a motor (in traction) as well as a generator (regenerative braking

**Figure 2.** Principle of construction of the proposed solution: (a) parallel planetary mechanism; (b) series version [28].
