A Review of Electric Aircraft Drivetrain Motor Technology

Abstract

In order to develop the next generation of fixed wing and vertical lift aircraft, the aircraft efficiency, emissions, reliability, and noise needs to be improved [1]. An electric aircrafts drivetrain needs to operate at a very high mass torque density whilst also being exceptionally reliable, efficient, robust and environmentally benign. Mechanically geared drivetrains are often utilized as they enable the aircraft drivetrain to be more compact. However, the use of mechanical gears within an aircraft introduces a number of system level consequences such as the need for a lubrication system to reduce tooth friction and remove heat [2]. A back up lubrication system is then also required to meet certification requirements in the advent of loss of lubrication performance. The mechanical gearing system also introduces significant cabin noise due to the gear-tooth vibrations [2]. To mitigate tooth wear and failure, the aircrafts need to be grounded periodically for inspection and maintenance.One of the main alternatives to using mechanical gearing is the use of a direct-drive permanent magnet (PM) motors [3]. The use of a direct-drive PM motor removes the reliability concerns with respect to the mechanical gearbox. However, the torque density of a direct-drive PM motor is thermally limited (by current) and therefore a PM motor does not normally achieve torque densities much greater than 50Nm/L [4-10]. For this reason, direct-drive PM generators become very large when scaled-up in size. Superconducting motors are being considered for use in electric aircraft as the torque density can be much higher [11], however the required cooling system is highly complex. Recently the use of magnetic gearing (MG) within an aircraft drivetrain has been proposed as a means of achieving a higher mass torque density and power density that is competitive to mechanically geared systems [1, 2]. Magnetic gears (MGs) utilize magnetic field space modulation to create speed amplification without any physical contact [12]. As a MG does not rely on current excitation, the MG can sustain a very high magnetic air-gap sheer stress value. MGs can operate with low noise and vibration and its noncontact operation means that no gear lubrication is required. In addition, MGs have the unique ability to pole slip when overloaded rather than catastrophically failing, thereby providing built-in overload protection [2]. Recently, MGs have been shown to be able to operate at 99% efficiency [13]. The MG can be nested internally with a much smaller motor or connected in series with a motor thereby enabling a compact electric drivetrain typology. This presentation will review the performance potential of MGs relative to alternative direct-drive aircraft drivetrains. The paper will provide an up-to-date review of electric aircraft drivetrain motor research conducted at government agencies, such as NASA [1, 2], as well as at universities and companies (such as Magnix and Siemens [14]) The performance potential of utilizing a magnetically geared aircraft drivetrain relative to the alternative direct drivetrain technology will be discussed. **