A Brushless Direct Current (BLDC) machine and converter are applied for the electric propulsion system of the solar aircraft. BLDC motors are electronically commutated instead of mechanically, as Direct Current (DC) motors do, so the position of the rotor must be known to do the commutation at the right moment. The control of BLDC motors can be done in sensor or sensorless mode. being the use of Hall-effect sensors the most widespread use inside the sensored control. The use of the hall sensors divide a complete revolution of the motor in 6 sectors, where in each sector only two phases are conducting (block commutation, trapezoidal control). However, considering the complexity of the high altitude, the electrical propulsion system cannot work properly if the hardware of the Hall sensors is broken. Therefore a sensorless control is necessary. The BLDC motor provides an attractive candidate for sensorless operation because the nature of its excitation inherently offers a low-cost way to extract rotor position information from motor-terminal voltages. The implemented sensorless technique consists of sampling the non-fed phase voltage and comparing it to half of the DC-link voltage to detect the zero crossing point. The most important part of the sensorless control is the sample of the non-fed phase, so the software must synchronise the sample with the PWM signal and apply the necessary delays in order to avoid commutation spikes and execute a correct BEMF measure. The software must be able to control the motor in both sensored or sensorless mode and to change from one to the other if an error occurs, so the code has to be compatible between the two modes.

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