What are the differences between brushless motors?

The mainstream motor used in brushless motor multi rotor unmanned aerial vehicles is the external rotor three-phase AC brushless synchronous motor. The casing of the multi rotor outer rotor motor rotates together with the shaft, is short and thick, has low speed and high torque, and is suitable for carrying low-speed large propellers. The outer shell of the inner rotor motor does not rotate, but the shaft rotates. It is slender, with low torque and high speed, suitable for carrying high-speed small propellers. Most brushless motor ducted models are inner rotor motors.

The brushless motor removes the electric brush, and the direct change is the absence of the electric sparks generated during the operation of the brushed motor, which reduces the interference of electric sparks on wireless equipment; Without electric brushes, the friction is greatly reduced during operation, resulting in smooth operation and low noise; Without mechanical contact, the internal resistance is much smaller; And modern brushless motors commonly use neodymium iron boron magnets instead of ferrite magnets, which have much stronger magnetic forces.
Blade and motor matching: The matching of motors and propellers, as well as the matching of motors, propellers, and multi rotor aircraft, are all very complex issues, so it is recommended to use empirical configurations. Large propellers use low KV motors, while small propellers use high KV motors (as speed is needed to compensate for the lack of lift). If the high KV is equipped with a large propeller, the torque is not enough, and it cannot rotate or rotates quickly, the motor and the electric controller are easily burned out. If the low KV is equipped with a small propeller, there is no problem at all, only that the lift is not enough at the speed, and it cannot lift off the ground.
Brushless motors typically require the use of an oscilloscope to observe the electrical performance of the motor. Oscilloscopes can monitor the voltage or current values of circuits. The different induced voltage waveforms of brushless motors can affect various motor and control characteristics.