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

90 Focus | Electric motors popularised for BLDC motors. While most of the elements used in samarium-cobalt magnetic alloys are still mined in China, sources also exist in the USA and Europe, which can lower their cost relative to neodymium. Sintered SmCo 5 has a flux density of 0.8-1.1 T and an energy product of 120- 200 kJ/m ³ . While these may be lower than for sintered neodymium, the greater corrosion resistance, the comparable coercivity levels (600-2000 kA/m) and far higher Curie temperature (720 C) can make the material preferable for extended use in motor configurations that are prone to generating heat. Another samarium alloy – Sm(Co, Fe, Cu, Zr) 7 – has lower coercivity than SmCo 5 but increases the flux density and energy product to 0.9-1.15 T and 150-240 kJ/m ³ respectively. That makes it more useful for motors with reduced duty cycles but higher performance requirements. Bearings The most common cause of electric motor failure is damage to the bearings. Most bearing failures occur from incorrect lubrication, which can usually be avoided by following manufacturer guidelines on the frequency, type and quantity of lubrication to apply. That can vary according to the application, duty cycle, environment and size of the bearings. Ball bearings tend to carry an ABEC (Annular Bearing Engineers’ Committee) rating, which indicates the tolerance and accuracy of the bearings, allowing them to function efficiently at high rpm. The established ABEC ratings are 1, 3, 5, 7 and 9, with 7 being a typical class for high-quality, cost-effective BLDC motors. However, over-reliance on the ABEC rating can be risky. They do not specify ball precision, load-handling capabilities, vibration, material or other factors that might be more relevant to an unmanned vehicle application. The bearings themselves tend to be made from chrome steel or stainless steel. The former will support higher loads, while for vehicles facing exposure to salt, moisture or debris, stainless steel is better at resisting corrosion. That makes them suitable for maritime systems, for example. Also, the contact angle of the ball to the raceway inside the bearing can merit investigation. This is inversely proportional to the radial play, which is the maximum distance by which one bearing ring can be displaced with respect to another, perpendicular to the bearing axis. A looser radial play gives a greater contact angle, and therefore greater bearing longevity as the balls are kept less ‘forced’ against the raceway. Proper construction of motor housings must also be conducted to absorb all the forces on the bearings and motor. Steel is the strongest housing material, but aluminium will save weight for aerial applications. Depending on the motor and application, the vibrations of the rotor and bearings can actually provide enough force to loosen the fastener holding down the enclosure (if only one is used). A secondary method of holding down the fastener, such as a Heli-Coil or adhesive patch, is therefore recommended, although using additional fasteners will also add weight. Importance of torque A range of motor quality metrics can be investigated to gauge its suitability to the propeller. The Kv rating of a motor, measured in rpm per volt, is the ratio of unloaded rotation speed to the peak voltage of the wires connected to the coils, and tends to be the typical specification used to define unmanned vehicle motors. The wattage is often also looked to for the power of the motor. Relying on these is a common mistake, however. It is essential to remember that torque is the most important product an unmanned system can derive from a motor, as this is what actually drives the propeller. Rating torque against speed, efficiency August/September 2018 | Unmanned Systems Technology Use of samarium-cobalt permanent magnets can be preferable to neodymium, which can have corrosion issues (Courtesy of MTC Industries) Bearings tend to be made from chrome steel or stainless steel. Chrome supports higher loads, stainless better resists corrosion