Cooling Technologies for High Power Density Electrical Machines for Aviation Applications

Abstract

This paper is aimed at giving an overview of possible cooling technologies for electrical machines and their assessment for aviation applications, e.g., fan or propeller drives. The most important demand for aircraft is the minimization of the drive system weight comprising electrical machine, power electronics, and the cooling system. The potential of aluminum winding an overview about several cooling technologies with the Rankine or Brayton cycle or utilizing the phase change of the cooling fluid is given. As an alternative approach, the cooling structure inside the machine is studied. A very interesting potential was discovered with direct slot cooling (DSC) removing the heat where it is produced and, thus, simplifying the cooling system effort and its weight. Since it is one of the most promising approaches, this cooling method is studied in depth. Furthermore, it can also be combined with one of the cooling technologies discussed above.In the overview paper [6], focus is set on the most recent progress in thermal management of electric motors for aviation. Besides air cooling, oil bath cooling, water jacket cooling, heat pipe cooling, intra winding cooling channels with a thermal conductive polymer [7], and direct winding cooling with tube-like conductors are cited. In this case, the problem of large pressure drops in the cooling ducts of six bars or more occurs. As experimental results, current densities of 24.7 A/mm 2 and transient current density of more than 40 A/mm 2 with class F insulation were achieved. With class H insulation even higher, values up to 58 A/mm 2 could be possible. By using conductors with integrated cooling channels, current densities possibly reaching 130 A/mm 2 were anticipated. However, the losses increase with the square of the current density and are linear with the volume of the winding even though the temperature is within the limits of the insulation system. The extremely high copper losses will increase weight and size of the cooling system and the reduced efficiency also requires an increase of the on-board power supply (generators and prime movers, fuel tank or energy storages, wiring, switchgear).The weight advantages of such machines can be quickly overcompensated by the weight of the cooling system and the increased power supply. Therefore, an accurately balanced design of the drive system is necessary. A proposal for the improvement of this approach is given in Section 5.A reduction of the winding temperature level by lowering the coolant temperature below standard ambient temperature promises a reduction of the copper losses in the electrical machine and a longer lifetime of the insulation. The higher effort for such cooling systems may be compensated by the higher efficiency of the drive. Due to the usually high temperature drop between winding and cooling fluid, this requires low ambient temperatures-as given in high cruising altitudes-or the use of refrigerant cooling systems, which allow coolant temperatures below ambient temperatures. In a literature study, no pap...

Impact of Current Density and Cooling on the Weight Balance of Electrical Propulsion Drives for Aviation

“…In Reference [8], a heat pump is proposed to increase the temperature of an oil cooling system to achieve smaller and lighter coolers. The authors also realize the problems with the additional weight of the heat pump components, as also mentioned in Reference [1]. Nevertheless, this may be an option in some special cases.…”

“…4 mm would be a good choice for the sizing of the cooling channels. Compared to reference [1], the number of the cooling channels in the slot was increased from 2 to 6, while one cooling channel also works as an insulating phase separator in the center of the slot (Figure 6). Due to the net slot width of 10.5 mm, a channel height of 2-3 mm is suitable.…”

“…The main data of the machine are given in Table 2. Further features to reduce the weight were the choice of a Fe-Co lamination (saturation flux density 2.4 T) and a direct slot cooling as proposed in reference [1].…”

“…100 passengers per airplane PAX) in a standard tube and wing configuration with two drives in nacelles and an electric energy supply (battery, turbine-generator set, or fuel cell), a comprehensive study of the thermal dependencies in the drive and cooling system was conducted. In this study, an improved direct slot cooling of a permanent magnet excited electrical machine, as proposed in Reference [1], in combination with an electrically 2 of 22 isolating heat transfer fluid on polysiloxan basis [2] is assumed for the cooling system, allowing very effective heat removal from the electrical machine. Regarding the power electronics, weight data and efficiencies for a two-level inverter are used.…”

Perspectives and Development of Electrical Systems in More Electric Aircraft

“…Additionally, the operational conditions of machines have to be considered because aircraft machines operate at a very wide range of temperatures and air pressures. The winding temperature cannot exceed 180°C for standard H-class insulation and 220°C for R-class insulation [123]. Sufficient cooling has to be secured.…”