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

83 the torque and speed at which the rotor revolves consistent over time), so long as the currents are applied in an accurate and timely fashion. To ensure this smoothness of rpm, a system of controlling which coils receive a current and when they receive it is necessary. While brushed DC motors would typically use a physical commutator, these are inefficient and inappropriate for BLDC motors, which instead use electronic speed controllers (ESCs) for switching. These can also take different forms, but are generally based on a combination of sensors, PCBs and processors, and must be matched carefully to the motor. Motor configurations While outrunner BLDC motors are generally the most common for UAVs, two other configurations are also popular. So-called ‘inrunner’ BLDC motors are the other type of radial flux motor. Unlike outrunner motors, the rotors (and hence the permanent magnets) of an inrunner configuration are placed in the inner circle of the motor, with the electromagnets encircling it. The revs per volt of inrunners are higher than for outrunners, but to match outrunners for torque they must integrate a gearbox. Also, the torque generated from inrunners can fall short of the requirements at the low revs needed for propeller drives, depending on the motor and application loading. Inrunners are therefore more common in helicopters and ducted fan-driven systems than multi-rotor and airplane- type craft. The main advantage of an inrunner motor is that having the coils on the outside of the air gap makes it far easier to cool the windings. For mission applications or propulsion set-ups that have a challenging thermal environment – and therefore a more critical need to dissipate heat from the motor – a powertrain based on an inrunner BLDC device might be preferable. An outrunner rotor will give a higher torque-to-weight ratio, because of the larger radius of the permanent magnets that stems from having the rotor outside the stator. However, that comes with a challenge in terms of countering the heat it generates. The duty cycle of the application, the amount of thermal mass in the Electric motors | Focus Unmanned Systems Technology | August/September 2018 Outrunner radial flux motors have the rotor placed outside the stator. This larger radius gives them a higher torque-to-weight ratio than inrunners (Courtesy of Xoar International) Inrunner radial flux motors are easier to cool than outrunners, and can have their permanent magnets surface mounted or internally mounted (Courtesy of Aveox)