The Schaeffler eDrive Platform
The development of electric drives goes way beyond the electric motor. Optimal efficiency, range, and system costs can only be achieved through the interaction of the motor, power electronics, sensors, mechanical integration, and control strategy, which is why a system-oriented approach is needed.
Schaeffler already proved back in 2011 that it possesses system expertise with the “Active e-drive” concept vehicle. The combustion engine in a Škoda Octavia was replaced with two electric axle drives, each with 105 kW of nominal power. Even then, these motors were units that had been developed within the company. While the gear ratio of the motor and axle speed was fixed initially, a third generation rear axle drive with a two-speed transmission and nominal power reduced to 65 kW was employed in the same concept vehicle beginning in 2014. The drive now included new power electronics developed within the company. Both generations of the concept vehicle had torque vectoring in each wheel independently. While the power electronics for the tractive drive was still a separate system at this time, it was already installed on the axle for torque vectoring.
• Hybrid modules for integration in the powertrain of the combustion engine
• Dedicated hybrid transmissions for specifically realizing hybrid and electric driving modes
• Electric axle drives for dedicated hybrid powertrains and purely electric vehicles
• Wheel-hub drives for new mobility concepts.
Their complexity is increased by the fact that electric drives can be operated at various voltage levels, ranging from 48 V for initial hybridization to 400 V in purely battery-driven vehicles. The first manufacturers have already launched projects with an 800-volt on-board electric system in order to achieve short battery charging times at acceptable current levels. Accordingly, the power spectrum of electric drives is very broad – from 20 kW to more than 400 kW.
Figure 1 Powertrain concepts with an integrated electric drive
The large number of requirements listed necessitates a modular approach for various installation spaces and power classes. This is even more true since – depending on the design – the drive’s electric and electronic components make up to 80 percent of the total added value and are therefore key elements in the total cost.
Figure 2 Modular technology platform for electric drives
The electric motors form the base of the platform. Due to the different requirements described at the beginning, multiple series need to be defined, each with a scalable output. According to the current estimate, the complete spectrum of future applications can be covered completely with six series. Depending on the necessary power density and other requirements, both permanently-excited motors as well as asynchronous machines will be used for this.
The middle level, to begin with, includes the power electronics with all key components, such as the power switches, capacitor, bus bars, driver stages, and sensors. Another part of the platform concept is a carrier frame that serves to channel the coolant and as a heat sink for all components of the power electronics. Also pertaining to the second level is the hardware for the drive control, the specifications of which not only depend on the electric motor and power electronics, but also on the functions carried out at the vehicle level, thereby requiring it to be implemented specific to the application. A key example of this is the communication network, which may be executed as a FlexRay, CAN, or CAN FD network.
For this reason, it cannot be separated from the third level where the software platform is located, which has likewise been developed according to a function-oriented approach. The software includes a functional library based on AUTOSAR, which also defines the requirements for the associated hardware.
When regarding highly-integrated powertrains such as hybrid modules, electric axles, or wheel-hub drives, it is difficult to identify the electric motor as an independently functioning unit. The complete motor function often cannot be tested until the powertrain has been assembled, since multifunctional modules are also determining factors for the functioning of other subsystems. Examples of this are bearing arrangements that also provide support for an integrated drive, or rotor carriers that are also plate carriers for a clutch. The upshot of this is the high requirements for the test concept du-ring production. It is necessary to utilize the measurement of parameters such as winding resistance, inductance, or magnetic field distribution of the components to assess the quality of the magnetic circuit assembly (stator and rotor) in order to immediately sort out faulty parts and not fail to identify them until during the end-of-line test.
In addition to the main function, that of representing defined speed-torque behavior, other demands need to be handled by this magnetic circuit as well:
• Ideal cooling-down capacity and a high copper filling ratio to ensure long-lasting continuous performance
• Minimal use of materials in order to optimize costs
• Minimized harmonically occurring radial and tangential force effects in the stator for NVH optimization
• Minimized harmonics, which for their part likewise generate force effects, but also induce eddy-current-driving voltages and ultimately contribute towards losses and stator/rotor heating
• Minimal cogging and ripple torques.
And the list goes on and on, as any detailed analysis of the individual phenomena will show. Thus there is an optimization problem, the solution for which must be oriented towards the requirements of the application and the expected load cycle.
Distributed winding has proven to be advantageous for a high torque density along with low harmonics and good heat flow from the current-carrying winding into the stator’s laminated core. Since the coils extend over slots at different angles, the coil ends are large compared to concentrated winding. Round wire distributed win-ding, known from industrial electrical engineering, has proven to be poorly suited for use in automobiles. More and more solutions are shown that use what is referred to as hairpin or I pin technology, which involves inserting copper bars and welding them to the end faces of the laminated core. The requirements for this production technology are high, since there is a high number of weld points for each stator. Compared to coiled wires, the bars have a much larger cross-section, leading to eddy current and skin effect related losses during operation that greatly increase the more frequent changes in polarity become.
Figure 3 Application areas of concentrated and distributed winding, depending on the active length of the electric motor
Figure 4 Application areas of concentrated and distributed winding, depending on the active length of the electric motor
The extent to which the advantages and disadvantages come into force depends on the specific application case. In order to quantify the differences, the comparison between the hairpin and wave winding was depicted using a specific example, namely their use in an electric axle of a purely electrical powertrain in the C segment. In coordination with the downstream gear stage, this resulted in the following specification values:
• Pmax = 147 kW
• Mmax = 265 Nm
• nmax = 18,000/min.
Figure 5Figure 6.
Figure 5 Calculated level of efficiency in an electric motor with wave winding for various operating points
Figure 6 Comparison of the stator and rotor losses for the electric motor in an electric axle application with wave and hairpin winding
Figure 7 Comparison of the stator and rotor temperatures at two operating points in an electric motor with wave and hairpin winding
Finally, the efficiency was compared in the cycle (WLTP). For wave winding, there was an average level of efficiency of 94% for the motor, while the value for the motor in hairpin design added up to an average of 89%. One general advantage of the distributed winding is that the stator can be used for permanently-excited synchronous motors, asynchronous motors, or even separately excited synchronous motors, thereby making it suitable as the basis for a modular system.
Due to the use of power electronics in automobiles, its integration in powertrain components such as electric axles, hybrid modules, and dedicated hybrid transmissions, and its high quantities, it is also subject to specific requirements that must be carefully borne in mind:
• High level of robustness, since the variance of the user profiles increases at high quantities and there are greater environment-related requirements in highly-integrated powertrains (vibrations, ambient temperature)
• High power density in narrow installation spaces, resulting in special requirements for lea-ding away power losses
• Flexibility of the design, as the installation spaces vary in the different applications
• Optimal current formation to minimize harmonic losses
• High torque accuracy under all operating conditions
• Operational reliability in a wide range of operating voltages in order to respond to specific battery configurations
• Functional safety.
As with electric motors, a detailed analysis would be able to add to this list indefinitely. In addition, power electronics need to be adapted to the application-specific power requirements. To scale power electronics in various applications in light of the specified requirements, Schaeffler has developed a modular concept, which is explained as follows on the basis of a sample implementation in an 85-kW class hybrid module – Figure 8.
Figure 8 Modular concept for power electronics, taking a hybrid module as an example
Figure 9 Power scaling through the size and number of IGTB semiconductors and the number of phases in the electric motor
Figure 10 Power electronics setup for an electric axle drive
It was able to be demonstrated that the basic concept selected for the power electronics has the necessary flexibility for meeting the different requirements of P2 and P4 high-voltage drives with regard to the installation space and electric output.
In a further evolution step, it was checked whether the selected approach can also be used for integration in a coaxial P2 hybrid module with a voltage level of 48 V. Used as power switches in this case are MOSFETs (metal-oxide semiconductor field-effect transistors). The components are attached directly to a ceramic substrate (called “bare dies”). The substrate is connected with the coolant-carrying carrier across its entire surface. This makes for very good heat dissipation and thus very high power density, which is important for the relatively high current levels that can occur in powerful 48-volt drives. In the specific application with 15 kW of nominal power (20-second value), the maximum current that occurs is 650 Arms. The capacitors are attached directly via the MOSFETs in order to keep impedance low.
Since it was only possible to have an active length of 45 mm due to the available installation space, the electric motor was also designed with concentrated single-tooth winding. Despite the differences compared to a high-voltage application, key technology platform components were able to be used in this case as well, thereby allowing for a compact hybrid module structure – Figure 11.
Figure 11 Integration of the power electronics in a P2 hybrid module at the 48-volt level
The approach pursued by Schaeffler involves the development of an extensive function-oriented software library, which also includes the specifications for the required hardware. The following are key elements of the library:
• Analysis of sensor signals, such as for determining the rotor position, the phase currents, or temperatures at defined points
• Functions for motor control as a function of the motor type used (PSM, ASM)
• Functions for current control, such as a field-oriented control system that factors in all relevant influencing variables (e.g. field weakening)
• Superordinate controllers for functional integration in the powertrain, which can also be integrated as customer modules upon special request
• Monitoring functions, such as for controlling power derating for thermal reasons and for providing functional safety.
In addition to software modules, the library also contains any necessary hardware circuits, along with their definitions and preferred components. Special rules define the implementation in the final layout to guarantee optimum heat dissipation or electromagnetic compatibility. This layout is prepared specific to the application in order to react to special customer requirements. The various communication network options (CAN, FlexRay) have already been mentioned as examples.
With regard to the architecture, the software strictly follows the AUTOSAR paradigms in order to guarantee a high level of reutilization – Figure 12.
Figure 12 Software architecture
The system development for a specific drive unit based on the technology platform always depends on the boundary parameters required by the overall powertrain and – in part – the vehicle concept as well. This can be illustrated by a wheel-hub drive design in which disturbing acoustic phenomena occurred during operation. A systematic analysis of all of the components, the software, and the transfer paths that was conducted together with research partners KIT, FAST, and ETI revealed the cause: Magnetic field fluctuations generated longitudinal and transverse forces in the electric motor’s stator, which were transferred to the bodywork via the chassis.
Figure 13 Vibration reduction in a wheel-hub drive through targeted torque variation in the e-machine
Figure 14 Propagation of torque generation inaccuracies in a typical e-machine
Due to a growing variety of electrified powertrains in ever greater quantities, new solutions are needed for the electric drive. Schaeffler has found an answer to the balancing act between variety and standardization with its scalable technology platform for electric drives.
This platform includes both the electric motor as well as the power electronics and the hardware/software for drive control. On the basis of different applications (P2 hybrid modules with high-voltage and low-voltage technology, P4 axle drives), it has been shown that a targeted approach is able to cover a very broad application spectrum.
The technologies used in the platform correspond to the very highest demands for efficiency, power density, and scalability. It has been developed in a modular fashion so that hardware and software from third-party providers can be seamlessly integrated.
Figure 15 Comparison of electric axle drives from 2011 and 2017
 Reitz, D.: One Idea, Many Applications – Further Development of the Schaeffler Hybrid Module. 10. Schaeffler Kolloquium, Baden-Baden, 2014
Meanwhile, it is now foreseeable that electrified vehicle quantities will reach large proportions during the current decade and that the degree of mechatronic integration will increase considerably. At the same time, very different topologies are being implemented for the powertrain, which can be classified based on its installation position . Four classes of aggregates are needed for the design implementation of these powertrains: