Comparison of Permanent Magnet Synchronous Machines for Hybrid Light Aircraft with traditional and additive winding technologies

Electrical motors are key components in the electrical transition required for greenhouse gas reduction programs. Future aircraft will certainly employ electrical motors for primary and secondary surface control, for landing gear and for other actuations such as high-lift systems, and they will also need electrical motors for propulsion. The motors pose the hardest challenge due to the high-power density required. Hybrid-Electric propulsion represents one of the most important topics in the aerospace industry and offers an opportunity to pursue the electrification change especially in light aircraft segments. The focus of this paper is the comparison among permanent magnet synchronous motors for Hybrid Light Aircraft propulsion with traditional and additive winding technologies.


Introduction
Electrification is the key activity in pursuing the goals of the Flightpath 2050 [1], by reducing pollutant emissions, fuel consumption, operating and maintenance costs, noise, and on improving aircraft performances and reliability.
Electrification represents the transition from traditional aircraft to More Electric Aircraft (MEA) and All Electric Aircraft (AEA).It concerns both the substitution of the traditional hydraulic and/or pneumatic actuation systems with electromechanical actuators (EMA) both the replacement of the aircraft internal combustion engines (ICE) with electric motors (EM).
The first task has no excessive limits as EMAs have been and will continue to be used in several applications on different aircraft segments such as primary and secondary flight surfaces control [2], landing gear and on secondary systems such as opening doors and food trolley elevators and others, often also with fault tolerance capabilities [3].About the second task, the full electric propulsion has more criticalities due to high power-density requirements and storage capabilities, not yet reachable due the actual developments in electric motor, energy storage and power electronics converters.Currently, they can only be used for commuter and small rotorcraft at a small range or flight duration [4], and could be adopted also in the realm of urban air mobility (UAM).
Also consider, the current technology in electrical motors, energy storage systems and power electronics converters ensure the development of the hybrid electric propulsion (HEP).This is where ICEs and EMs are combined in the propulsion system which ensures high conversion efficiencies as well as low emissions and noise pollution.HEP is considered an effective substitute for conventional aircraft up to regional segment [5].

Hybrid electric propulsion systems
Hybrid Electric Propulsion Systems (HEPS) could have different configurations according to the way the propeller the ICE and Electric motor are linked [6].Each one is characterized by an index called Hybridization Index (HI) that represents the ratio between the power of the electric motor and the sum of the electric motor's power and ICE's power.
Briefly, the HEPS architectures are distinguished in three categories: • Parallel: ICE and EM are both mechanically connected to the propeller.Both EM and ICE can provide propulsion and EM can operate as electric generator charging the batteries when only ICE drives the propeller.Different configurations could be possible according to the position of the motor/generator in the drivetrain (single-shaft or double-shaft)) • Series: the ICE drives an electric generator that supply the electric motor and charge the batteries.The EM drives the propeller • Series-Parallel: propeller, ICE, motor and generator are mechanically connected in a combination of the two previous architectures.

Application
The following study refers to the hybridization of the propulsion system of the Cessna 337 aircraft.The EM will be installed in the fuselage nose and fully integrated inside the housing of the ICE.The EM's rotor will be rigidly linked to the ICE on the same shaft, through an anti-rotation key in a parallel configuration as shown in figure 1.The propeller is connected to EM and ICE by a gearbox (GR).The EM will be electrically connected by a DC/AC converter and the battery management system (BMS) to the battery pack.The requirements for the electric motor design are summarized in table 1.

The Electric Motor Design
The electric motor design process can be summarized as follows: • Requirement analysis (Environmental conditions, Electronic Constraints, Geometric constraints, Working Points, Duty Cycles) We changed the design in terms of slots/poles combination, retaining magnet system and winding configuration.Starting from the design presented in [7], with unchanged active materials and kept the same motor typology (i.e.surface permanent magnet brushless machine), which is one of the most referenced for propulsion application.New thermal verifications and cooling circuit refinements were performed.The details of the electromagnetic and thermal designs are not described here to avoid losing the focus of the paper.
For this application two electric motor designs with different winding technology were considered to explore the Additive Manufacturing (AM) (or so-called 3D printing) technology in winding manufacturing.
Both designs have some points in common which satisfy the design-to-cost approach (e.g.stator sheet, housing, rotor, magnets).This approach required some iterations among the two designs, in fact the final shape of the slots bottom was chosen to enhance the process connected with the additive manufacturing winding.
The optimization of the AM winding motor mainly concerned the shape and the arrangement in slot of the conductors to reduce the AC losses as much as possible.

Winding classification
Windings can be divided in two categories basing on the shape of the conductors: • Random winding • Form winding The term random winding refers to traditional winding with cylindrical wires while the term form winding frequently refers to the hairpin winding, less frequently to the flat coil winding.In both cases the coils are obtained through a manufacturing process that shapes the copper.Hairpin winding is considered the most robust solution in traction/propulsion motors where a high level of specific power is required.
One of the major advantages of form winding as opposed to random winding is the greater slot fill factor, as shown in figure 2: random windings have a maximum value of slot fill factor around 0.45 while form windings have a maximum value of slot fill factor around 0.8.Of course, a greater slot fill factor means more copper cross section in the same slot which leads to a minor current density, lower DC resistance, thus minor DC Joule losses and higher efficiency.Another advantage of form winding is better control over the voltage difference between the turns.It should be noted that form winding requires considerable initial costs for tooling justified only for large production.The attention to components needed for winding connections is also costly.There are also more AC losses due to the skin effect especially at high frequencies.
Apart from the shape, we can divide the types of windings in two categories based on their arrangement in the stator slots: • Distributed winding • Concentrated winding The advantages of concentrated windings compared to distributed windings are: • shorter end windings • non overlapping coils • independent magnetic fluxes • low or null mutual inductance • minimal electric isolation between phases • reduced phase to phase fault probability Considering that random winding can be arranged both in distributed both in concentrated winding while hairpin form wound winding can be arranged in distributed winding only and considering that the reliability it's fundamental in aerospace applications, it seems that winding for electric motor for aerospace applications can be only of the random type and of the concentrated type but the flexibility of the additive manufacturing could be applicable and make possible to combine the benefits of form winding and concentrated winding.

Additive manufacturing technology for copper conductors
The copper conductors are manufactured by selective laser melting (SLM).In that process, pure copper powder is added layer by layer, while a laser system melts specific locations.The trajectory of the laser beam shapes the final part's geometry and is directly deduced from CAD data.In that way, also complex designs are feasible, since common restrictions like bending radii are not present.After completion of the copper building process, the part is de-powdered and cleaned.SLM processes commonly produce part surfaces that are quite rough directly after building, showing Ra-values (the arithmetic average of surface heights measured across a surface) up to 30μm.To improve the surface smoothness, the copper parts are post-treated by different procedures.The post-treatment is also important to activate the copper's surface in preparation for the electric insulation.Focusing on the copper SLM process can achieve near maximum conductivity, which is referred to as 58MS/m at 20°C or 100% IACS.
In this regard, it is crucial to use copper powder with a purity >99.95%.Minute contamination or powder oxidation will massively decrease the conductivity.Apart from that, the laser melting process must be finely controlled to avoid porosity.A functional conductor for electric applications involves two components: the conductor material itself, and a primary insulation that prevents the conductor from short circuits.The primary insulation layer is applied after the AM and surface smoothening process.It is crucial that the insulation material adheres perfectly to the copper surface.Otherwise, a delamination of the insulation layer would cause dielectric problems in the application, like partial discharge.For that application an epoxy resin was chosen as insulation material.

The process of stator assembly with additive copper-coils
Additively manufactured conductors require insulation before they can be assembled in the stator core.
Before the insulation is applied, the conductor surface needs to be treated to reduce roughness.For an optimal slot fill factor, a dedicated insulation material with an appropriate layer thickness is needed.Once insulated, the coil-conductors are inserted into the stator and fixed with wedges as shown in figure 3. The completed assembly is illustrated in figure 4, showing a very compact end winding structure.The single coils now needed to be connected according to the winding scheme.Therefore, customized copper busbars were printed and directly integrated into the winding end turn.For an easy connection between inverter and stator, insulated power cables were crimped and soldered to the busbars.The finished stator with AM winding is shown in figure 5 while the wound stator is shown in figure 6.

Comparison of electric motor design
In table 3 the characteristics of the two motors are summarized.After examining the data, the motor with winding in additive manufacturing has a higher cross section, lower end winding height, higher efficiency and a better thermal behaviour.A new technology for winding manufacturing was explored recognizing its strengths: • no design restrictions • integration of busbars into the winding, so no further welding is needed • less stress on copper conductors and insulation because no bending is needed • eligibility for busbar system manufacturing Another advantage of the AM is the cost containment in the development of prototype, in fact, on contrary of the traditional process, it doesn't need tools and set up of the production machine.
At the same time the weaknesses of 3D printing process were recognized: • AM machine size defines the maximum part size • proprietary manufacturing routine • dedicated insulation process is needed The challenges faced in this project were: • for both technologies, the connection of power cables to phase connection points at busbars, due to size (very big) of cables • for AM: geometry of end turn side (connection side) needed to be customized for printing process • for AM: fixation of coils inside stator slots via wedges needed to be very precise, due to no allowance of impregnation process • for traditional technology: to be compliant with end winding height requirements two star points were needed.
The next steps consist of: • test campaign on dedicated test bench (figure 7) • post processing of results to validate the performed analysis • integration on aircraft for on ground tests • improvement of the electric motor performances with AM winding using the space in slots for direct stator winding cooling, improvement of the cooling jacket with housing in additive manufacturing [8], and possibly utilizing more performant material for magnet and magnetic steel in order to reach a greater power density.

Figure 3 .
Figure 3. Stator stack with lower part of insulated single coils (view from other side).

Figure 4 .
Figure 4. Completed assembly of single coils inside stator stack.

Figure 5 .
Figure 5. Finished stator with AM winding.Figure6.Finished stator with traditional winding.7.Comparison of electric motor designIn table 3 the characteristics of the two motors are summarized.After examining the data, the motor with winding in additive manufacturing has a higher cross section, lower end winding height, higher efficiency and a better thermal behaviour.

Figure 6 .
Figure 5. Finished stator with AM winding.Figure6.Finished stator with traditional winding.7.Comparison of electric motor designIn table 3 the characteristics of the two motors are summarized.After examining the data, the motor with winding in additive manufacturing has a higher cross section, lower end winding height, higher efficiency and a better thermal behaviour.

Table 1 .
Electric motor requirements.The duty cycle in table 2 refers to a typical training mission. 3

Table 3 .
Electric motors' comparison.Two designs of electric motors for Hybrid Light Aircraft propulsion were performed.