**5. Lightweight traction motor/generator using dual-rotor single stator split shaft axial-flux PMSM**

#### **5.1. Topology description**

In an effort to simplify the design of the two configurations (series and parallel), publications in the latest years released by the authors' team members, among which an international patent is present, treat a topic belonging to a complex scientific field of significant interest and certain actuality, being oriented toward identifying appropriate solutions regarding some proficient electromechanical converters and control structures of the drive systems based on axial machines excited by permanent magnets with a high number of poles, with applications in 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 corresponding stator winding operates as a generator most of the time (**Figure 2**) [29].

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

**5. Lightweight traction motor/generator using dual-rotor single** 

In an effort to simplify the design of the two configurations (series and parallel), publications in the latest years released by the authors' team members, among which an international patent is present, treat a topic belonging to a complex scientific field of significant interest and certain actuality, being oriented toward identifying appropriate solutions regarding some proficient electromechanical converters and control structures of the drive systems based on axial machines excited by permanent magnets with a high number of poles, with applications in

community.

**stator split shaft axial-flux PMSM**

**5.1. Topology description**

164 New Trends in Electrical Vehicle Powertrains

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].

regime). Fuel economy, which is obtained for the regime of urban movement, can reach 33%. For pure electric vehicles, two solutions have been found for the use of this electric drive system (**Figure 3**) where R is rotors, S is the stator, Inv. is the inverter, TM is the mechanical transmission, TD is the differential transmission, and RM are wheels.

An important advantage of using the synchronous axial air-gap single stator dual-rotor permanent magnet machine is representing the smaller length, this being able to be introduced in the clutch's place between the motor and the gearbox. A 3D drawing of the machine is shown in **Figure 4**.

#### **5.2. Control algorithm**

Traditionally, a three-phase three-leg bidirectional power converter is used in an EV. For this topology, the most appropriate power converter is the three-phase four-leg converter.

> The main feature of a three-phase inverter four-leg inverter, with an additional neutral leg, is its capability to handle load unbalance. In an automotive power system, the main goal of the three-phase four-leg inverter is to maintain the desired sinusoidal output voltage over all the ranges of loading conditions and transients [34]. Typically, the EV electric machine phases have a Y-connected winding. In order to maintain the phase voltage at the same level as in three-leg inverters, one may choose a Δ connection of the two windings. This means will result in the fact that the wire diameter will be smaller, the winding may be built easier and, for an existing machine, no rewinding is needed (**Figure 5**), and in **Figure 6** for

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**Figure 4.** A three-dimensional exploded view of the proposed machine [28].

Alternatively, a matrix converter solution can be used to operate the two rotating rotors (**Figure 7**). The matrix converter is an array of bidirectional switches that can directly connect any input phase to any output phase to create a variable voltage and frequency at the output.

serial HEV.

**Figure 5.** Three-phase four-leg inverter [28].

**Figure 3.** Using the synchronous machine with a stator and two axial rotors: (a) electric drive system for an electric vehicle with a four-wheel drive; (b) an electric drive system and a differential transmission at the same time.

Lightweight High-Efficiency Power Train Propulsion with Axial-Flux Machines for Electric or Hybrid Vehicles http://dx.doi.org/10.5772/intechopen.77199 167

**Figure 4.** A three-dimensional exploded view of the proposed machine [28].

regime). Fuel economy, which is obtained for the regime of urban movement, can reach 33%. For pure electric vehicles, two solutions have been found for the use of this electric drive system (**Figure 3**) where R is rotors, S is the stator, Inv. is the inverter, TM is the mechanical

An important advantage of using the synchronous axial air-gap single stator dual-rotor permanent magnet machine is representing the smaller length, this being able to be introduced in the clutch's place between the motor and the gearbox. A 3D drawing of the machine is shown

Traditionally, a three-phase three-leg bidirectional power converter is used in an EV. For this topology, the most appropriate power converter is the three-phase four-leg converter.

**Figure 3.** Using the synchronous machine with a stator and two axial rotors: (a) electric drive system for an electric

vehicle with a four-wheel drive; (b) an electric drive system and a differential transmission at the same time.

transmission, TD is the differential transmission, and RM are wheels.

in **Figure 4**.

**5.2. Control algorithm**

166 New Trends in Electrical Vehicle Powertrains

The main feature of a three-phase inverter four-leg inverter, with an additional neutral leg, is its capability to handle load unbalance. In an automotive power system, the main goal of the three-phase four-leg inverter is to maintain the desired sinusoidal output voltage over all the ranges of loading conditions and transients [34]. Typically, the EV electric machine phases have a Y-connected winding. In order to maintain the phase voltage at the same level as in three-leg inverters, one may choose a Δ connection of the two windings. This means will result in the fact that the wire diameter will be smaller, the winding may be built easier and, for an existing machine, no rewinding is needed (**Figure 5**), and in **Figure 6** for serial HEV.

Alternatively, a matrix converter solution can be used to operate the two rotating rotors (**Figure 7**). The matrix converter is an array of bidirectional switches that can directly connect any input phase to any output phase to create a variable voltage and frequency at the output.

**Figure 5.** Three-phase four-leg inverter [28].

line voltages to the center point of the DC link. However, by connecting a second two-phase inverter and motor to the same DC bus, as shown in **Figure 5**, it is possible to compensate for the single-phase current in the DC link capacitors. A specially designed vector control is required for this reason. In this way, the compensation current influence on the torque is limited in the case of surface PM machines (with no saliency) [34]. The neutral voltage regulation with different current compensation is based on a simple PI controller since the motor divides the current compensation effort to both motor sides, rotor 1 and rotor 2 (in a direct ratio with their rated current) and at the same time avoids power oscillations (power pulsation occurs when two independent controllers are used). In **Figure 8**, the proposed vector control strat-

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In order to evaluate the behavior of the combined solution dual-rotor single stator axialflux PMSM machine powered by a three-phase four-leg converter, a digital simulation was performed. Using the equivalent model of the axial-flux permanent magnet machine, a Matlab Simulink model was implemented. The actual test conditions were represented by a step in the speed reference consisting of a value of 150 (rad/s) which is given for first electric machine M1 at the starting moment (t = 0) and a second step in the speed reference consisting of the value 230 (rad/s) which is given for machine M2 at t = 0.8 s (**Figure 9a**). In this scenario, the machine M1 reaches the reference speed in 0.175 s at a rated torque as load. In an EV configuration after the thermal engine starts, at t = 0.5 s, the machine M1 switches to a generator mode with 20% load at the same speed (**Figure 9b**). The speed overshooting during starting process and torque perturbation is around 2 [rad/s] (under 1.5%) which represents a very good feature of the proposed vector control. The second machine M2 is started at t = 0.8 s (**Figure 9a**), with 10% of the rated torque as load (**Figure 9b**), and it reaches the reference speed in 1.3 s. A small perturbation on machine M1 speed occurs during the machine M2 starting (**Figure 9a**, **b**). The machine M1 is starting with a two-time rated torque, and it runs at a rated torque between 0.2 and 0.5 s and then at 25% of the rated torque as generator. Small torque perturbation could be

egy is presented.

**Figure 8.** The proposed vector control strategy [28].

**Figure 6.** The four-leg inverter for serial HEV.

**Figure 7.** Parallel HEV based on matrix converter topology.

However, despite the benefit of very compact construction (no DC capacitors), this type of converter is not very easy to control, and for this reason, we only mention it as an alternative topology to be used.

#### **5.3. Field-oriented control with four-leg inverter**

The proposed configuration of a dual mechanical rotor with a single stator requires a singlephase AC current flow through the capacitors. This is due to the connection of the machine line voltages to the center point of the DC link. However, by connecting a second two-phase inverter and motor to the same DC bus, as shown in **Figure 5**, it is possible to compensate for the single-phase current in the DC link capacitors. A specially designed vector control is required for this reason. In this way, the compensation current influence on the torque is limited in the case of surface PM machines (with no saliency) [34]. The neutral voltage regulation with different current compensation is based on a simple PI controller since the motor divides the current compensation effort to both motor sides, rotor 1 and rotor 2 (in a direct ratio with their rated current) and at the same time avoids power oscillations (power pulsation occurs when two independent controllers are used). In **Figure 8**, the proposed vector control strategy is presented.

In order to evaluate the behavior of the combined solution dual-rotor single stator axialflux PMSM machine powered by a three-phase four-leg converter, a digital simulation was performed. Using the equivalent model of the axial-flux permanent magnet machine, a Matlab Simulink model was implemented. The actual test conditions were represented by a step in the speed reference consisting of a value of 150 (rad/s) which is given for first electric machine M1 at the starting moment (t = 0) and a second step in the speed reference consisting of the value 230 (rad/s) which is given for machine M2 at t = 0.8 s (**Figure 9a**). In this scenario, the machine M1 reaches the reference speed in 0.175 s at a rated torque as load. In an EV configuration after the thermal engine starts, at t = 0.5 s, the machine M1 switches to a generator mode with 20% load at the same speed (**Figure 9b**). The speed overshooting during starting process and torque perturbation is around 2 [rad/s] (under 1.5%) which represents a very good feature of the proposed vector control. The second machine M2 is started at t = 0.8 s (**Figure 9a**), with 10% of the rated torque as load (**Figure 9b**), and it reaches the reference speed in 1.3 s. A small perturbation on machine M1 speed occurs during the machine M2 starting (**Figure 9a**, **b**). The machine M1 is starting with a two-time rated torque, and it runs at a rated torque between 0.2 and 0.5 s and then at 25% of the rated torque as generator. Small torque perturbation could be

**Figure 8.** The proposed vector control strategy [28].

However, despite the benefit of very compact construction (no DC capacitors), this type of converter is not very easy to control, and for this reason, we only mention it as an alternative

The proposed configuration of a dual mechanical rotor with a single stator requires a singlephase AC current flow through the capacitors. This is due to the connection of the machine

topology to be used.

**5.3. Field-oriented control with four-leg inverter**

**Figure 7.** Parallel HEV based on matrix converter topology.

**Figure 6.** The four-leg inverter for serial HEV.

168 New Trends in Electrical Vehicle Powertrains

**Author details**

Sorin Ioan Deaconu<sup>1</sup>

**References**

Springer; 2017

Lucian Nicolae Tutelea1

\*, Vasile Horga2

\*Address all correspondence to: sorin.deaconu@fih.upt.ro

1 Politehnica University Timisoara, Romania

3 University of Cassino and Southern Lazio, Italy

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Optimization. Berlin: Springer; 2007

Oxford: Oxford University Press; 2001

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Congress, San Francisco, USA. 1996. pp. 7732-7737

Singapore: Wiley; 2015

2 Technical University, Iasi, Romania

and Ilie Nuca4

4 Technical University of Moldova, Chisinau, Republic of Moldova

, Marcel Topor1

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

[1] Ehsani M, Gao Y, Gay S, Emandi A. Modern Electric, Hybrid Electric, and Fuel Cell

[2] Egede P. Environmental Assessment of Lightweight Electric Vehicles. Switzerland:

[3] ISO (14040:2006). Environmental management—life cycle assessment—principles and

[4] Guzzella L, Sciarretta A. Vehicle Propulsion Systems. Introduction to Modeling and

[5] Chau KT. Electric Vehicle Machines and Drives: Design, Analysis and Application.

[6] Chan CC, Chau KT. Modern Electric Vehicle Technology. In: Oxford University Press.

[7] Njuguna J. Lightweight Composite Structures in Transport: Design, Manufacturing,

[8] Koffler C, Rohde-Brandemburger K. On the calculation of fuel savings through lightweight designe in automotive life cycle assessments. International Journal of Life Cycle

[9] Redelbach M, Klotzke M, Friedrich H. Impact of lightweight design on energy consumption and cost effectiveness of alternative powertrain concepts. European Electric Vehicle

[10] Fuhs A. Hybrid Vehicles and the Future of the Personal Transportation. CRC Press; 2009 [11] Isermann R. Mechatronics systems – with applications for cars. In: 13th Triennial World

, Fabrizio Marignetti<sup>3</sup>

,

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**Figure 9.** (a) Machine M1 and M2 reference and actual mechanical speeds; (b) achieved torque by machine M1; (c) achieved torque by machine M2 [28].

observed in machine M2 while starting. The machine M2 is starting with electromagnetically rated torque while the load torque is 10% (**Figure 9c**).

In terms of reliability, the presented topology combined with an adequate power converter allows to have the highest reliability in the single inverter configurations for EV.

#### **6. Conclusions**

The use of a very compact electrical drive will have the benefit of considerably reducing the weight and complexity of the power train. As it is known, the mechanical complexity of the power train is responsible for as much of 20% of the weight of the vehicle. The mechanical system of internal combustion engine (ICE) vehicle required a rather complex system of adaptation of speed and torque to the travel conditions. Our chapter is developed around the concept of a reduced number of mechanical elements included in the power train. This is possible by the close integration of electrical drive with the ICE and the use of the electrical differential concept. Special consideration is given to the power electronics required for the drive. Using a new approach, the number of converters is limited to one for each axle, each converter being capable to independently control the motion of the side wheels. Instead of a complex sophisticated gearbox, we propose to use a simplified gearbox or no gearbox in case of the electric differential, much of the function being fulfilled by the dual mechanical output electric machine controlled by a single power converter. A special control based on the dual vector control with operating on dual frequency will be investigated. In order to increase the ruggedness of the system, we investigate special power converters with a high degree of reliability (the four-leg converter and the matrix converter that makes no use of DC capacitors in the DC link), the multilevel inverter concept applied to EV which brings the benefit of a very reliable topology, a reduced harmonic pollution, and easy battery cell balancing. Although this seems to be an unnecessary complication to a rather proven technology, our chapter considers the fact that the existing power train solutions are not considering the problem of extra weight/complexity given by the electrification.
