The design of electric motors leads to high specific powers, exceeding 11kW/kg by integrating the cooling system, with high efficiencies, greater than 97%. This design has been obtained thanks to high performance permanent magnet synchronous motor with Halbach structure. Two different design tools have been completely developed: a target setting tool for preliminary design and a second tool called "SM-PMSM" for Surface Mounted Permanent Magnet Synchronous Machine. The optimization under constraints of current densities, magnetic fields and the increase in rotation speeds are key drivers to which has been added the use of special windings (compact rectangular Litz wires) and high performance ultra-thin magnetic sheets to limit high frequency copper losses and iron losses: this is what makes it possible to obtain excellent efficiencies exceeding 97%. The concepts proposed by P’ Institute, an equally powerful cooling system, combine glycol water cooling for the stator and the rotor
to which it was necessary to couple an internal cooling, directly within stator slots, to achieve such specific powers. A detailed and accurate “Lumped Parameter Thermal Modeling” has been developed for the electric motor and its cooling system. Results show that the motor temperatures are maintained below the maximum allowed temperatures.
In order to efficiently convert the power between the electric distribution and the electric motors, solutions have been proposed to optimise the integration of power electronics. The joint use of a high voltage bus with its optimised mechanical structure is a first design solution. Then, the best electronic components (7th generation IGBTs) have been selected to limit conduction losses. These components are controlled from specific modulation strategies also optimised for various multilevel conversion structures (3 or 5 level NPC topologies). This set of design choices has proven to be particularly effective in terms of compactness and efficiency (yield of the order of 99%). As with the machine, optimising the power electronics is nothing without a very high performance cooling system. This is the case with the ultra-efficient two-phase capillary pumped (CPLIP) cooling systems. CPLIP concepts were optimized with analysis of the cooling system behavior during sudden and violent acceleration stages (realized during turbulence stages occurring during the mission of the aircraft): the device is capable of extracting 4.5kW of heat losses for 1 kg of cooling system, allowing the complete power conversion system to greatly exceed 30kW/kg, well beyond the targets set!
A specific WP was dedicated to partial discharge (PD) studies. For electric insulation of stator windings, PDs are unavoidable in non-pressurized zones. So the issue was to propose “PD tolerant” design solutions.
A preventive tool has been developed in order to prevent PD risks within power busbars. Simulation of PD risk in time-dependent conditions have been done.
Several tasks have been achieved related to stator winding EIS (electric Insulation system):
An accurate method to compute the electric field in a stator slot in order to apply the Paschen’s criterion; A second task dealing with the Paschen criterion. Achievements were focused on the definition and validation of a Partial Discharge (PD) criterion evaluated for a combined variation of temperature and pressure; An automatic tool to size the EIS was also built-up; Simple analytical models in order to perform parametric studies that have been integrated in overall design (WP6).
A study has also concerned the auxiliary electrical sources hybridized with thermal sources (gas turbine): simplified models have been developed for the most promising technologies of auxiliary sources. Specific power and specific energy are clearly assessed for current level and prospective level technologies.The conclusions show that, for this case of application to series hybrid regional aircraft, fuel cells with cryogenic hydrogen storage (stored at 20°K in liquid form) are almost twice as compact in specific energy (Wh/kg) than the best “energy batteries” by 2030 with energy densities beyond 500Wh/kg for the overall system (FC stack, balance of plant including major cooling devices and auxiliaries such as humidifier, air compressor, etc).
Regarding the optimized system, one of the major expectation was the definition of the voltage level of the electric distribution which constitutes a significant coupling factor on the mass of the main components by integrating environmental constraints (partial discharges, thermal aspects, flight mission profile, etc) specific to aeronautics. In the end, the best system compromise on the choice of this voltage is between 1300 and 2000V. This result could not be obtained without the overall optimisation of the propulsion chain: all the technological choices and scientific concepts were integrated through surrogate models making it possible to offer a global vision on the complete propulsion chain, from sources to propellers. A sensitivity analysis has been completed at the system level with a particular focus on the electric motor which involves the most important number of design variables. This step helps designers to define the convenient set of decision variables at the input of the optimization algorithm.
