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

86 Focus | Electric motors Stators and coils In addition to the varying configurations, there are many options for the assembly and materials used to build BLDC motors. Stators are typically constructed by stacking multiple layers of thin, laminated sheets of iron-based or soft magnetic material. This is repeated until the shape for the desired orientation of flux, coils and teeth for windings has been achieved. Carbon steel is among the cheapest and most workable materials for stators, and an economical choice for motors with relatively short duty cycles, such as those for a small multi-copter or VTOL- transition fixed-rotor UAV. Silicon steel meanwhile is useful for motors designed to offer greater performance, such as for MALE-class or heavy-lift UAVs. Cobalt-iron alloys are more expensive, but offer the highest flux density before saturation occurs (when a material cannot be magnetised any further). The use of thinner layers and high- quality grades of steel in the stator can be desirable as it lowers the resistance of the motor, reducing the heat generated at a given amount of power (or increasing the power from a given level of heat). The coils can be wound in various ways depending on the desired drive and control system. Automated machine- winding can be cheaper, faster and potentially more consistent than hand- winding, but a human technician’s fine motor skills (biologically speaking) mean that hand-wound stators can squeeze 15-20% more copper wire than machine- wound systems. This can lead to a greater power-to weight ratio. Rotors and magnets Inrunner radial flux motors might have surface-mounted permanent magnets (SPMs) or internally mounted permanent magnets (IPMs). Those in SPM motors are mounted around the radius of the rotor core as individual curved bars, or they may be held in place using adhesive or composite tape. Alternatively, the magnets might be fabricated as a solid ring around the rotor’s surface. In an IPM motor, the separate bar magnets are inserted or moulded into symmetrical, radial slots inside the rotor. In this case, the rotor will typically have been manufactured in a similar way to the stator, by repeatedly stacking and bonding August/September 2018 | Unmanned Systems Technology The stators in electric motors are typically made from stacking multiple sheets of steel lamination (Courtesy of Sullivan) Silicon steel is useful for motors designed to offer greater performance, such as for MALE-class or heavy-lift UAVs