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Someone that understands brushless motors ?

cpper

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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 ?
 
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@cpper the motor is an induction motor and it works by passing a current through the windings which produces a alternating magnetic current to pulse between the stators and this induced magnetic force makes the outer shell of the motor spin because of the magnets that are attached to it ,this is controlled by the ESC which is wired up so that the motor it powers always turns in the correct rotation ie CW or CCW it also alters the amount of current that the windings receive so the speed of the motor can be varied ,the props on a drone are fixed pitch so its only motor speed that controls them to provide lift ,from what you describe its possible that some residual current was being fed to that particular motor somehow ,because of an open circuit in the ESC when the drone was powered down
 
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 ?
I would definitely make double sure that you do not have any kind of debris in the motor like the center shaft like lint or a small strand of hair or something like that and also I would remove the propellers just on the motor that has the issue and if you have any kind of spray cleaner just for these kind of brushless motors because you can not just spray them with any kind of cleaner you might have just got moisture in it especially flying early in the morning or maybe you could have gotten something in the motor housing when you put it in your bag but I would start with a good cleaning and go from there let us know how it goes with it and God Bless Brother ✝️ Sincerely Clay B.
 
Cut a sliver, about 5mm wide, of thin paper and slip it between the bore (inside) of the rotor ( the silver bit that the props screw into) and the outer end of one or two poles ( the things the copper is wound around). This is a bit tricky and might be easier with the props off. See photo.

Once you can ONLY JUST see the paper between the rotor and the plastic of the arm, move the rotor through one or more full revolutions, the paper should not stick anywhere. If the paper protrudes too much into the arm I would suspect it will foul stuff inside the arm so I do mean ONLY JUST being visible between the rotor and the plastic.
Also make sure you can see into the motor, through the gap between the rotor and the arm, all the way around that gap.

With regards to OMM's residual current thing, just to ensure that that is unlikely, remove the battery before you try turning the motor concerned. That is if this happens again and you are checking the motor again.

If vacuuming again just put the vacuum to the openings in the top of the rotor (the surface where the propellor blades attach) and turn the drone upside down. Separately run the vacuum around the gap between the rotor and the arm.
 

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@cpper the motor is an induction motor and it works by passing a current through the windings which produces a alternating magnetic current to pulse between the stators and this induced magnetic force makes the outer shell of the motor spin because of the magnets that are attached to it ,this is controlled by the ESC which is wired up so that the motor it powers always turns in the correct rotation ie CW or CCW it also alters the amount of current that the windings receive so the speed of the motor can be varied ,the props on a drone are fixed pitch so its only motor speed that controls them to provide lift ,from what you describe its possible that some residual current was being fed to that particular motor somehow ,because of an open circuit in the ESC when the drone was powered down
I'd be prone to agree with your prognosis, maybe a small current leakage through the ESC, I have no clue how though. If it happens again, detach and replace the battery to see if the problem goes away
 
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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.
 
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Just suggesting that you removed the battery rather than just power it off, clutching at straws I have to admit and its hard to imagine that would resolve it. but it does sound like an ESC issue to my (fog filled) brain. Good luck and I hope it does not happen again.
 
@cpper the motor is an induction motor and it works by passing a current through the windings which produces a alternating magnetic current to pulse between the stators and this induced magnetic force makes the outer shell of the motor spin because of the magnets that are attached to it ,this is controlled by the ESC which is wired up so that the motor it powers always turns in the correct rotation ie CW or CCW it also alters the amount of current that the windings receive so the speed of the motor can be varied ,the props on a drone are fixed pitch so its only motor speed that controls them to provide lift ,from what you describe its possible that some residual current was being fed to that particular motor somehow ,because of an open circuit in the ESC when the drone was powered down
EXCELLENT answer!... so much good information packed into such a small space. Someone who didn't know all that would have to read it twice (3 times?) to 'get it all'. Good job! ThumbswayupThumbswayupThumbswayup
 
for me if it had been debris in the motor ,then his attempts at trying to discover any restriction in the motor should have been a success ,and then later when he tried again the issue had gone away,it was just a theory that could have explained what was happening ,
 
Something that might help 'clean out the motor area would be some canned air. Use sparingly, not full blast. It would an alternative to vacuuming.
After reading this thread, it does seem like something else is going on, but having the air handy would clear up the 'debris' thought.
 
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A can of compressed air with a spray nozzle can be used to blast air through the gap between the motor shell and the coils inside. All the motors should feel the same.
 
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 ?
I would definitely listen to the others posts. Every once in a while try spraying canned air in the motor, spraying a SMALL amount of wd-40 in, and them more canned air. If you have ever bought a drone new, there will be a small amount of oil on the motor, including the outside, to help with this problem. If the drone is turned off and the motors don’t have the “clicking” feel, then maybe try replacing the motor. I’m sure this is not your problem, but a crash may have slightly bent the motor. Again, yours sounds more of an “electronic” issue.
 
spraying a SMALL amount of wd-40 in, and them more canned air.
I wouldn't use WD-40 inside a drone motor.

I used to think WD-40 was the 'magic bullet' for lubing anything and everything... over time, I observed that the residual left after the aerosol evaporates will eventually get gummy and sticky. Sure, it'll penetrate and slick-up anything 'right now'- from door hinges to rusty bolts, and it's very good for cleaning rust off things I refurb, but still...

I wouldn't use WD-40 inside a drone motor, or any other electronics.

Deoxit D5 contact cleaner is what I use to clean up old electronics. It dissolves the oxidation and leaves behind a thin non-corrosive lube that won't gum up over time. Deoxit (the best brand) makes other formulas too, some that don't have lube.

I'm not saying spray D5 in your motors.... but it's gotta be better that WD-40.
 
I wouldn't use WD-40 inside a drone motor.

I used to think WD-40 was the 'magic bullet' for lubing anything and everything... over time, I observed that the residual left after the aerosol evaporates will eventually get gummy and sticky. Sure, it'll penetrate and slick-up anything 'right now'- from door hinges to rusty bolts, and it's very good for cleaning rust off things I refurb, but still...

I wouldn't use WD-40 inside a drone motor, or any other electronics.

Deoxit D5 contact cleaner is what I use to clean up old electronics. It dissolves the oxidation and leaves behind a thin non-corrosive lube that won't gum up over time. Deoxit (the best brand) makes other formulas too, some that don't have lube.

I'm not saying spray D5 in your motors.... but it's gotta be better that WD-40.
Thanks, yes I’m sure there is better stuff out there.
 
Thanks, yes I’m sure there is better stuff out there.
Make use of a specific electrical cleanser that doesn't leave anything behind but evaporates completely and then cleaning it out with the air as suggested by others. When you said you tapped the sides of the motor and vacuumed it and the problem was solved it sounds to me like you had a very small particle you could not see but was stuck between the rotor and the starter that's why you needed the extra bit of force when you used your hand to turn the motor. Like old man mavic explained the working to you is correct, it could also mean that perhaps a magnet is slightly loose and can cause the problem too. As for the electrical leak it's only possible if there is a problem within the connections on the circuit board soldering and the components or a diode went open circuit causing the problem. Never use WD40 on anything electrical. I am a qualified sparky by trade hence the proper electrical cleanser suggestion, no shorts or build up afterwards
 
Make use of a specific electrical cleanser that doesn't leave anything behind but evaporates completely and then cleaning it out with the air as suggested by others. When you said you tapped the sides of the motor and vacuumed it and the problem was solved it sounds to me like you had a very small particle you could not see but was stuck between the rotor and the starter that's why you needed the extra bit of force when you used your hand to turn the motor. Like old man mavic explained the working to you is correct, it could also mean that perhaps a magnet is slightly loose and can cause the problem too. As for the electrical leak it's only possible if there is a problem within the connections on the circuit board soldering and the components or a diode went open circuit causing the problem. Never use WD40 on anything electrical. I am a qualified sparky by trade hence the proper electrical cleanser suggestion, no shorts or build up afterwards
Thanks, good to know
 
Thanks everyone for the numerous answers and explanations :)

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. Without going online to check the advice you gave me, I tried some things.

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 the attached pics. 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.

I'm not sure what the conclusion is. Maybe the bearing was overgreased? Maybe some very fine dust go into it, and I solved the issue by cleaning the area and applying IPA? Regardless, thanks again to all of you for your time and help :)

IMG_20210820_154312.jpg


IMG_20210820_160503.jpg
 
  • 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 the attached pics. 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.

I'm not sure what the conclusion is. Maybe the bearing was overgreased? Maybe some very fine dust go into it, and I solved the issue by cleaning the area and applying IPA? Regardless, thanks again to all of you for your time and help :)
GOOD JOB!
Now that's some good thinking right there! Glad you it solved.
 
Great answer
EXCELLENT answer!... so much good information packed into such a small space. Someone who didn't know all that would have to read it twice (3 times?) to 'get it all'. Good job! ThumbswayupThumbswayupThumbswayup
But I suggest you take the "this is an induction motor" with a grain of salt until someone else investigates it... Why?

I count 8 pole stops as my motors rotate: the ONLY way a motor can have detent like that is if it is a PM synchronous, not induction, motor. 8 'clicks' means it is a 4 pole non induction PM motor. A PM (permanent magnet) motor has the coils in the stator (non moving part) and the rotor is made of high power magnets, not coils as an induction motor is. An induction motor has NO magnetic field - until powered up, so it cannot have detent torque as we feel rotating our motors here. I have not investigated these so will not guarantee this answer but it matches the physics. I have spent my 46 year ele engineering career designing & applying Induction and PM motors & controls like these (pico thru 750 HP), but I did NOT look into the specs on our drone motors, just going by 43 years experience.

If this is true, there is no residual magnetism potential issue to cause this failure. If PM motors and power was turned off at the time of feeling rotation differences, then it seems something adverse was going on mechanically inside the motor like something stuck in the airgap ID as others suggested, or miniscule particles stuck in the bearings. Since the issue happened hours after the last flight, it cannot be a heat issue causing magnets to momentarily demagnetize (would make the detent torque rotating get less or disappear). The type magnets used today do not typically stay demaged after an issue. I have no suggestions other than already proposed.
 
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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.

FYI, Hot magnets LOOSE their magnetism so the mag click would be LESS not more; therefore the heating did not temp change their strength.
 
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