Unmanned Systems Technology 021 | Robot Aviation FX450 l Imaging Sensors focus l UAVs Insight l Liquid-Piston X-Mini l Riptide l Eurosatory 2018 show report l Zipline l Electric Motors focus l ASTS show report

84 Focus | Electric motors motor, and the environment, should all be considered when deciding between an inrunner and an outrunner. Outrunners have other disadvantages. Their rotors suffer from a high degree of inertia, as all the rotating mass is on the outer diameter, far from the central axis relative to inrunners. This reduces the motor’s transient efficiency (as in when accelerating or decelerating), but also reduces torque ripple. Also, full sealing of outrunner motors is challenging, again because of the rotor being external to the stator, meaning that two sets of clearances need to be maintained (and the alignment of the bearings and rotor are critical to maintaining a tight air gap in the motor). Without proper sealing, a vehicle using radial flux motors can suffer from ingress of water or dust, and icing-up. This can severely reduce the longevity of such motors in inhospitable environments such as rainforests or deserts. Sealing can be less of a problem for axial flux motors than the radial flux configuration. As their name suggests, these use a rotor and stator shaped as flat discs placed against each other, with the permanent magnets and electromagnets arranged such that the magnetic field is axial to the rotational axis of the shaft. The operative magnetic surfaces are therefore the flat, circular faces of these two discs, rather than the cylindrical circumferences in the case of radial set- ups. As a result of this shape, axial flux motors are sometimes called ‘pancake motors’. Axial motors have also been developed that use two stators (one on either side of a single rotor), or two rotors (one on either side of a single stator), effectively doubling the amount of magnetic flux. This increases the torque density and power-to-weight ratio, while reducing torque ripple. The single-stator, dual-rotor type, also referred to as a ‘torus’ axial flux motor, can offer more compact stator windings, which makes them less copper-hungry and easier to manufacture in bulk, resulting in a relatively less expensive product. However, they too can suffer from poor transient efficiency owing to their high inertia (which again stems from the increased rotational area of an outer-rotor configuration) as well as difficulty in thermal management. Such configurations may need a separate cooling system using water, oil or a combination of both. Also, the complexity of a dual-rotor set-up can cause issues such as oscillation of the rotors inside the housing if additional and potentially extensive control mechanisms have not been integrated. A dual-stator, single-rotor axial flux design benefits from having its single rotor kept stable and relatively compact. Given the increased magnetic area, this configuration can also use fewer permanent magnets, which can be expensive and difficult to source, particularly if they are made using rare earth elements. The broad surfaces of the stator rear ends also allow for a large cooling area and therefore less complexity and expense for thermal management systems, with some featuring air fans of roughly the same diameter integrated inside the casing. As mentioned, having these on the outer part of the motor makes this configuration easier to enclose fully. August/September 2018 | Unmanned Systems Technology Brushless DC electric motors are a vital technology for the propeller drives of electrically powered unmanned systems (Courtesy of KDE Direct) Without proper sealing, a vehicle using radial flux motors can suffer from ingress of water or dust, severely reducing their longevity