Recently I had a strange issue with my Mini 2, I hope that there is someone here that understands brushless motors and could explain what went wrong. So, this is what happened:

In a beautiful morning, some days ago, I took my Mini 2 out to record some nice holiday footage. I landed the drone in my hand in order to avoid stuff getting into the motors when landing on the ground. All good and no issues. I folded the drone and carefully put it back in the case. Then, in the evening, I wanted to take some more shots, so I took the drone out again and put in on a clean picnic table in order to take off. When starting the motors with the CSC, the left rear motor wouldn't rotate, and the other three would stop after rotating for two seconds. In the DJI Fly app I got an error saying "Motor unable to rotate. Check motor". I tried for a few times, powered the drone off and on again, made sure the props don't touch anything, but with the same result. Then, with the drone powered off, I manually rotated the motor, and noticed it had a different feeling than the other three. The three motors that worked fine all had a similar feeling: they would spin very easily and had a regular magnetic clicking/snapping feel. The bad motor however didn't have this snapping feel, and was more difficult to rotate. It still was rotating very smoothly, but it just required more force to spin it. There was absolutely no grinding feel/sound, confirming the issues was not something getting into the motor.

I was 100% sure nothing got into it, also because of how careful I handle the drone. Regardless, I still removed the props, carefully inspected it using a magnifier, lightly tapped it with some plastic thing, and tried to vacuum out any potential stuff that got into it. All without results, because this was not the issue. The error was still there, and the motor wouldn't start. Disappointed, I folded the drone and put it back in the case.

The next day, I wanted to show the issue to my brother. So I took out and unfolded the drone, and I asked him to manfully rotate the motors to see what I was talking about. He couldn't feel any difference between the four motors, and neither did I. The issues was magically gone, the troubled motor would easily spin and have the snappy feel like the other three. I powered the drone and all motor worked fine, and the drone flew flawlessly.

So, what went wrong here? Is it possible for a bug in the software to cause this issue, and the different "feel" of the motor, when manually rotating it ?

Was the drone powered up when you manually rotated the motors? If it was it might have been some glitch in the software that partially fed power to the failing motor.

If on the other hand the power was off when turning the motors my guess is the same. The drone has always power from the batteri and might have fed power to the failing motor even when the power was off. If it happens again, take out the battery before turning the motors.
In any case, check the log files.

Johnny_J Posted at 8-19 05:18
Was the drone powered up when you manually rotated the motors? If it was it might have been some glitch in the software that partially fed power to the failing motor.

If on the other hand the power was off when turning the motors my guess is the same. The drone has always power from the batteri and might have fed power to the failing motor even when the power was off. If it happens again, take out the battery before turning the motors.

Thanks Johnny, the drone was powered off when manually spinning the motor. Is there some official DJI software to check the logs ?

cpper Posted at 8-19 05:33
Is there some official DJI software to check the logs ?

I'm sure someone here will answer how to check log files. I'm curios too.

I had a similar problem but it was the cause of the tangled hair making it difficult to spin the engine.

It is possible for the ferrous material inside the motor to disintegrate after a physical shock or perhaps water intrusion. The magnets and cores in the motor can release tiny metal particles into the very tight spaces between the motor armature and the magnets resulting in jamming. That happened to me with my 2nd Mini. I replaced both front arms/motors after I decided I could not trust that it would not continue.

The drone didn't suffer any physical shock at all.
Yesterday the issue occurred again. I unfolded the drone and could feel the motor is again more difficult to turn by hand. As expected, when I tried to start the motors I got the "Motor unable to rotate" error message.
Firstly, I've noticed that if I turn the motor by hand for like 20 seconds (in any direction, or just repeatedly left-right half a turn), it would somehow 'loosen up' and spin more freely, almost like the other three motors. Then if I left the drone still for some minutes the motor would stiffen up again. I did this experiment for about 6 times, and every time manually rotating the motor would loosen it up. This happened both with the battery inserted or not.
The next thing that came to my mind is using a hair dryer. I left the drone still so that the motor was stiff again, and then lightly heated it up without touching(rotating) it. Surprisingly, this action would also loosen up the motor. At this point I had two theories:

• The hot air interacts with the magnets, temporarily changing their power. When the motors was loosened up and spun easily, I could clearly feel the magnetic clicks, so something else was going on. The temperature was also too low for this, I believe.
• Maybe something related to the grease/oil in the bearing. Heating the motor would maybe make the lubricant more fluid, resulting in an easier rotation. Repeatedly rotating the motor by hand maybe had the same effect.
  

Based on the second theory, I decided to try to slightly degrease the motor bearing. I removed the rubber/foam pad that covers the 3 screws of the motor. Unscrewed those and lifted the motor so that the bottom part was visible. A good amount of dark brown oil was visible there(not sure how that part is called), so I wiped the area with a Q-tip, being careful not to leave any cotton hairs. You can see this operation in two pics here. In the second pic is the Q-tip, after cleaning the area. Finally, I soaked the tip of the propeller screwdriver in some isopropyl alcohol in order to leave 3 tiny drops of IPA around the shaft. Maybe a degreaser would have been more adequate, not sure. I then rotated the motor for a minute or so, then cleaned the area once again using a new Q-tip.
After putting everything back together, the issue seems to be gone. Every few hours I checked if the motor would stiffen up again, but it would not. I did some test flights, going through 3 batteries and everything seemed fine.