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100 in CFD analysis and digital twinning (for simulating the magnetics, thermal behaviour and fluid flow), has informed the internal design and construction of the rotors and stators, enabling airflows to be directed at the windings for highly targeted and efficient cooling. Such designs have been validated through testing and are now commercially available. And as indicated, the iron cores of conventional electric motors pose further thermal issues in and of themselves. Their inherent resistance to rapid changes in the magnetic field during operation induces eddy currents, which creates heat in the laminated layers. Moreover, the lack of open surface area in stators relative to their mass makes it difficult for them to conduct their heat elsewhere. If these heat issues cannot be resolved then electric motors cannot realistically be made more efficient. Fortunately, e-motor manufacturers have come up with some creative solutions to these issues in the past few years. For one, switching to inrunner configurations means the stators have a much larger exposed surface area that can be conductively cooled. Standard outrunners can only achieve comparable cooling with open configurations, which as discussed leaves them vulnerable to particles or moisture that can disrupt their operation. And of course, iron-less core motors circumvent many of the typical thermal issues, so passive radiation of the heat generated by their copper coils tends to be sufficient for their thermal management without needing to go to active cooling. In circumferential flux motors, this is further assisted by the larger air gap, inherently providing a larger flow of air through which the windings can dissipate heat. Testing With widespread innovation in small electric motor architectures, it is critical that new motor designs are validated (and indeed certified) through extensive testing to confirm that they work reliably. That can be achieved through a combination of focusing on reliability during the design stages, strict quality control throughout the production processes, and extensive testing of individual sub-components at each level up to the whole propulsion system. Procedures and guidelines for quality control and testing are widely available, and are detailed in various standards, such as 810G for shock and environmental ingress or 461E for EMI/EMC. Going beyond these is advisable, however, given the untried nature of some of the latest motor designs. For instance, critical sub-assemblies such as the stator and rotor should be thermally cycled or shaken over long durations, tantamount to their expected lifetime (as in HALT or HASS practices) to simulate flying in harsh conditions for certain numbers of hours. They can then be examined under a microscope, thermal camera or other inspection equipment to quantify effects such as microfractures or damage to laminations or winding coats. In addition to these systems for testing entire sub-assemblies, motors and e-powertrains, some manufacturers are engaging with research organisations to run e-motor materials through magnetising equipment, to gather real-world data on their magnetic performance and losses. Such detailed knowledge will be vital in many ways for future innovations in electric motors. As an example, newer motor control technologies are being geared towards higher switching February/March 2022 | Unmanned Systems Technology Focus | Electric motors A wide range of tests over many thousands of hours is ideal for validating a motor’s performance, housing, bearings and more (Courtesy of Alva Industries) Sub-assemblies such as the stator and rotor should be thermally cycled or shaken over long durations to simulate flying in harsh conditions