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

82 B rushless DC (BLDC) electric motors continue to be the most common choice for powering unmanned vehicles, particularly small-to-medium UAVs. Compared with brushed motors they have higher efficiency, reliability and ratios of thrust and torque to weight, while also producing less noise. Brushes can also cause sparking and wear out over time, making them less desirable for reliable, long-term propulsion. BLDC motors for unmanned vehicles can vary in a number of ways, such as their performance specifications, component types and counts, and configurations of internal parts, but they all share certain operating principles and facets. The most important parts of a motor are of course the rotor and the stator. Around the stator is a symmetrical arrangement of copper coils wound around the ‘teeth’ of the stator that behave as electromagnets when the coils are energised. The rotor, on the other hand, holds a symmetrical arrangement of permanent magnets. The most common motor design follows an ‘outrunner’ radial flux configuration, in which the stator is on the inner core of the motor, and the teeth and thus the coils run parallel along its cylindrical surface, while the rotor is enclosed around the stator with its magnets mounted radially inwards. The stator’s coils and rotor’s magnets are arranged in pairs of opposite (North and South) polarity. When a coil pair is energised, the magnets are attracted to their oppositely polarised electromagnets on the stator. As the permanent magnets approach them, the next coil along is energised, causing the rotor to turn towards the next pairing. This sequence continues until the rotor has turned 180°, at which point the original coil pair is again energised. In addition to energising the next pair of coils, the permanent magnets can also be repelled from behind by energising the previous pair in the sequence with a current of the same polarity as the next. This combination of pull and push from the stator means increased torque and power output from the motor, and reduces torque ripple (that is, keeping Rory Jackson explains how to match a UAV’s mission envelope to the most suitable electric power plant Power factors August/September 2018 | Unmanned Systems Technology Axial flux motors can be configured with a dual-stator, single-rotor design, enabling less permanent magnet material to be used and giving a large cooling area (Courtesy of Avid Technology)