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Brushless unpowered torque

jhitesma

Some guy in the desert
Mentor
#1
Usually I'm pretty good at finding answers to my questions, but this one I just can't seem to get a search to return anything. I'm new to RC Aircraft - but not new to RC or aircraft individually ;) But this is also my first time using brushless motors.

When unpowered how hard should it be to turn a brushless motor by hand? I've only completed one plane so far, a nutball built from the swappable plans. I used the 24g HK motor and was worried about it from day one. The soft foam box it came in was crushed and wet. But the motor inside seemed ok - other than being much harder than I expected to turn by hand. I figured it was just strong magnets and when I mounted it up wired it up and tested it the prop sure seemed to move a LOT of air. And it flies. At first I thought it seemed overpowered - but once I learned where I needed to be on the throttle for various things I thought it was pretty good, maybe actually a touch under powered in some ways...but maybe that's just the many hard hits the motor has taken introducing me to flying :)

But the motor is still pretty hard to turn and I'm not sure it's just the power of the magnets. I wonder if some bearings were bad before I even ran it. I've had no other motors to compare it to though so I haven't really questioned it.

Then yesterday I got two new motors. A D2826-6 2200kv (to go on an Axon I'm working on finishing) and a L2210-1400 which to be honest I'm not 100% sure what I was planning on building that I wanted it for..but I have some ideas.

Since my little 24g motor was so hard to turn by hand I expected the new motors to be similar. But when I picked them up I found they were way looser than I would have expected. Now, only 3 motors isn't really enough experience to draw any conclusions...but I'm really starting to suspect my original 24g motor has a bit of a problem.
 

earthsciteach

Moderator
Moderator
#2
I would agree that your 24g has an issue.

You should feel some resistance when turning a brushless motor by hand, but not much. There are a couple of things to look for. First, as you said, it could be the bearings. Second, the magnets or front of the bell housing could be rubbing on the stators. Or, there could be a short between to legs of the motor (which should prohibit it from running at all).

Brushless motors are very easy to take apart. First, remove the clip ring at the rear end of the motor. After doing this, you may be able to simply pull the shaft and motor housing forward, exposing the stator, windings and bearings. If the end of the motor shaft is slightly larger, it won't pull through. Just loosen the set screws that affix the bell housing to the shaft, support the motor with something like a deep socket and gently tap the shaft "down" through the back of the motor with a hammer.

The bearings will be easily removable (probably just drop out) of the center of the stator assembly. If it is a bearing, you will probably have tiny bearings stuck to the magnets. Make sure you hunt down all of those little buggers or they will end up ruining your windings.

If the issue is due to something rubbing against the stator, you should be able to see areas of wear (shiny spots) on the stator. You can simply file the offending stator area down until it clears. Be careful not to file the winding. If you break the insulation on the wire, you could be out of luck!

When turning the motor by hand, make sure none of the motor leads are touching. It won't hurt anything if they do so, but it imposes a load on the motor (actually a generator at this point) and makes turning harder.

I'm actually quite surprised that the motor has not burned up, yet. If it is as hard to turn as you describe, I'd think it would pull too much power and produce that little puff of white smoke - POOF!
 

jhitesma

Some guy in the desert
Mentor
#3
Thanks, I'll give it a teardown and see how it looks. Probably pretty ugly...even if it wasn't before it most likely is after today. Went through 3 props (but over 3 batteries which is pretty good for me so far.) and probably another half dozen other poor quality landings that resulted in the motor buried in sand and gravel.

I knew it was too windy to for me but I couldn't resist trying...and there were a few calm moments that were a lot of fun. Need to finish the axon I'm building...I'm getting tired of going through propellers :)
 

jhitesma

Some guy in the desert
Mentor
#4
Well, I tore it down yesterday...the bearings seem fine. I put the shaft back in backwards so the magnets wouldn't grab the stator and it spins just as loose and free as I would expect a bearing supported shaft to spin.

The bell is pretty nasty though. I cleaned out as much sand and grit as I could...but there's a LOT of really fine metal particles that are sticking to the magnets that I can't seem to clean out. Any suggestions on how to get them out of there?

After cleaning it up it spins a bit better...but still just feels like the magnets have a lot more "grab" than the two other motors I just got. Looking at them I see that on the 24g motor the magnets are shoulder to shoulder and packed in there...but on the other two motors that spin more freely the magnets are spaced apart a bit...maybe that's what's causing the difference I feel? It's just odd that this 24g motor feels like it can only spin in distinct little steps while the other two can spin fairly freely (steps still noticeable but light enough you can spin the bell and it will keep spinning unlike the 24g which only spins when actively spun.)

As for clearance issues, it's hard to say. There was so much sand packed into this motor from my crashes that I can't tell if the wear on the stators and magnets is from that or from something rubbing. When cleaned though it doesn't feel like anything is rubbing...just like the magnets are really grabby.

I'm about to order some more parts so I may grab another 24g motor just to compare....would be nice to have a spare anyway and I could always build up another power pod and pick up a cheap TX since my wife wants to learn to fly....