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I have noticed that drones with brushless motors fly longer than those with brushed motors. Why are drones with brushless motors able to fly for longer amounts of time?

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Brushless motors (aka BLDC) are typically[*] more efficient than brushed DC motors. Higher efficiency equates to longer flight times for a same battery pack.

Amongst other reasons:

  • The inrush current at the moment of commutation can involve sparks (which is an inefficient waste of energy), and in any case is a free-running operation without the possibility for optimization.
  • The carbon brushes have a somewhat higher resistance, which is an electrical energy loss
  • The carbon brushes involve a certain amount of drag, which is a mechanical energy loss
  • The EM field is not easily controllable to reduce torque ripples, resulting in mechanical energy losses. Anecdotally, you can hear these ripples, especially when comparing power tools using universal motors (i.e. brushed AC wound in series) vs BLDC. Sounds represents energy, all other things being equal a noisier drive is a less efficient drive.

Brushless motors also are easily built into outrunner applications, which is appropriate for high-torque direct drive applications, such as a propeller. An inrunner needs an inefficient gearbox to develop the torque required to spin a blade (which might also be complex, costly, and weighty).

An oftentimes overlooked consequence of brushless efficiency is heat generation. Fundamentally, what limits a motor's power is heat, and producing less of it means that more energy can be supplied from a smaller package. The smaller package means lighter weight, which equates to longer-runtime.

Each of these on its own is small, but summed together they result in a far superior drone operation from brushless than brushed.

[*] There are some extremely efficient (>95%) brushed motors out there. However, the engineering required to get that high efficiency is costly. Cheap mass-produced brushed motors will never have those kinds of tolerances, but brushless motors are much less sensitive to manufacturing imprecision.

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There are multiple reasons.

Firstly, here is what I have been able to source: https://www.popularmechanics.com/home/tools/a8109/whats-so-great-about-brushless-motor-power-tools/

In this article, you can see that the addition of brushes causes friction and an extra voltage drop. This means more power is used for the same amount of work, so the battery will not last as long.

Now here are a couple of extra points from my experience:

Often, to improve torque, brushed motors use gears. These gears introduce more mass and friction, decreasing efficiency.

Additionally, brushed motors are generally used to spin smaller props. The smaller the prop, the less efficient in general most people in the hobby have found them to be (hence we use 7 inch for long range), therefore further decreasing efficiency.

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On a brushed motor, the permanent magnets are on the outside (stator) and the electromagnets are inside (on the rotor.) The coils are connected to the supply voltage by the eponymous brushes, which slide against the rotor to contact different ends of the windings as they go past, changing the direction of the magnetic field. This generates friction and sparks, which in turn produce heat - which is wasted energy. To effectively manage the heat, the motor may need to be made with more robust materials, adding to the weight.

On a brushless motor the coils are permanently connected to the speed controller, which is needed to change which coils are energised but this is done in a solid-state manner which does not spark. The only friction surfaces are the bearings holding the rotor in place, and these produce minimal friction.

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