Unmanned Systems Technology 007 | UMEX 2016 report | Navya ARMA | Launch & recovery systems | AIE 225CS | AUVs | Electric motors | Lethal autonomous weapons

70 Focus | Electric motors faster than outrunners, often up to 11,000 rpm per Volt, far too fast for most aircraft propellers. As a result, most inrunners are used in conjunction with a gearbox on both surface and airborne systems to reduce speed and increase torque. When a motor is supplied with a voltage, it spins. As the voltage increases, logically so does the rate of the spin. This rate of spin is quantified using rpm and the Kv rating, the rpm constant of a motor – the number of revolutions per minute that the motor will turn when 1 V is applied with no load attached to the motor. The ideal rpm of a motor depends on the type of device it is being used in. For example, in a high-performance, acrobatic ’copter, a high-revving motor is best, meaning a motor with a high Kv rating. This is intuitive because these ’copters are generally small, meaning a smaller motor and leading to smaller propellers. With smaller propellers though, the motors need to run at higher rpm in order to produce the necessary thrust, which means they consume more power than ones running at lower revs, and so are less efficient. On the other hand, if the system needs to use as little power as possible, and so have a greater flying endurance, then motors with lower rpm are best. An example here is a system for taking aerial photos: the type of ’copters used for such tasks are bigger in order to carry the necessary equipment, meaning larger propellers, which will create more thrust when rotated. They therefore require fewer rpm. More than a motor Although there is research on topics such as improving the efficiency and reducing the weight of electric motors, most of the development is in ancillary areas such as power electronics and power sources. The motors are controlled with power electronics – transistors that can switch tens, hundreds or thousands of Amps. These are becoming faster and more sophisticated, allowing motors to be controlled more effectively as well as making them spin faster. Research into the electric control system components in electric drives is focused mainly on silicon carbide and gallium nitride power semiconductors, lithium-ion capacitors and photovoltaics of high efficiency such as crystalline silicon and gallium arsenide. The efficiency of an electric motor is a function of power output against power input. Intuitively, the power output of the motor is the power input minus the energy that is necessarily lost, and there are two main ways that a motor can lose efficiency – current loss and flux loss. Current loss is the loss of energy through heat. To calculate it, we need to incorporate the electrical resistance of the motor, which is referred to as the winding resistance and denoted by R W , and measured in Ohms, and once we have that value, we can calculate the copper loss with the formula: Copper loss = I 2 x R W , measured in Watts. Flux loss is a little less intuitive but nonetheless important. This quantity explains the power lost due to the fluctuating magnetic fields in the motor. To calculate it, we need to know the amount of current the motor needs to run when it is unloaded, denoted by I 0 . With this term, we calculate the iron loss with the formula: Iron loss = V x I 0 . Combining these two losses together, we obtain the total power lost, so we can now calculate the efficiency of a motor. This is given by: High efficiency is of course desirable in any motor as the more efficient a motor’s performance the longer it can remain airborne with the power from its battery, which is the limiting factor on endurance with electric motor-powered UAVs. In addition, higher efficiency results in less heat produced, meaning the motors can be pushed harder at any given moment, or over a longer period of time. In control In essence, a brushless motor contains electromagnets (coils) that are connected together in specific pairs. The motor controller, commonly known as the electronic speed controller or ESC, activates and deactivates specific sections of the coils at specific times to cause the rotor of the motor to spin due to the magnetic force produced by the April/May 2016 | Unmanned Systems Technology Electric propulsion system with variable pitch propeller for greater inflight control (Courtesy of NWUAV)