Motor Direction

[Inq]

I thought I'd ask this here in the section where the real motor purist must hang out being that you actually build your own motors.

I read something years ago about motor spin direction. That some brushless motors are meant to spin in only one direction. I'm not talking about propeller CW/CCW or pusher/puller issues. That I understand. I'm assuming what I read was for extreme high-end motors.

1. Is that still the case that these motors exist?
2. Is it a mechanical issue - like where thrust bearings are?
3. Is it an electrical optimization - like windings are done a special way?
4. Do commodity type brushless motor have a preference, but we just ignore it?

Just curious.

VBR,
Inq

 

[quorneng]

A brushed motor can be mechanically adjusted to alter its 'timing' to extract maximum performance but this improvement will only work in one direction of rotation with a corresponding loss the other.
For brushless motors the ESC provides all the motor control including any performance timing. The ESC electronics are arranged so it performs equally in either motor direction. Of course there are different levels of quality with ESCs.

 

[Pieliker96]

Typically it's to do with the thread direction on the prop shaft - i.e. spinning the motor in the "intended" direction would tend to tighten the nut, whereas it risks backing off if reversed. This is why pusher props are usually the reverse pitch of their tractor counterparts: they spin the opposite direction to keep the nut tight even when the motor is pointed backwards.

Other reasons may have to do with asymmetric integrated cooling fans or fins that are optimized for a particular airflow direction.

 

[Merv]

...Is that still the case that these motors exist?

Yes, these motors still exist.

...Is it a mechanical issue - like where thrust bearings are?

The only mechanical issue is the thread direction, nothing else matters.

...Is it an electrical optimization - like windings are done a special way?

No, everything else about the motor is identical.

...Do commodity type brushless motor have a preference, but we just ignore it?

No brushless motors, that I am aware of, have any preference in rotation other than thread direction.


I agree with pieliker, its all about the thread direction. This only matters in quads, in airplanes, it doesn't matter.

In quads, the motor is starting and stopping many times per second, in order to maintain stability. It's the starting and stopping that can loosen the prop nut, if it is backwards. In airplanes we run the motor more or less at a constant speed, maybe changing speed at most once or twice a minute.

In a plane, just give the prop nut a firm tighten with a pliers and you will be fine to run a motor backwards. There will be absolutely no difference in performance. That said, don't run you prop backwards, that is in reverse rotation. if you do, there will be a significant loss of power, greater than 50% loss of thrust. Make the motor spin whatever direction your prop dictates.