Balancing outrunner motor

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Balancing outrunner motor

Postby Joel Rosenzweig » Wed Jun 20, 2012 1:33 am

Does anyone have any experience balancing the can of an outrunner motor? What's a good technique for doing this? Are you adding material to the inside of the can so it doesn't fly off? What material are you using that won't impact motor operation?

I've been on a quest to minimize the vibration on my Schweizer 300C, which has had a small skid shake since the day it was born. I've carefully balanced the main rotor head and that has visibly improved the situation, but there's more work to do. I installed an SK-720 in order to do some vibration analysis, and the data shows that I have one large spike in the graph at precisely the motor RPM. By balancing the motor can, I hope to reduce the magnitude of the spike, and get rid of the skid shake by doing so.

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Re: Balancing outrunner motor

Postby cap231ex » Wed Jun 20, 2012 6:59 am

What kind of motor is it?

Does it have a fan mounted on the bottom ?

Does it have screw holes on the bottom?
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Re: Balancing outrunner motor

Postby Joel Rosenzweig » Wed Jun 20, 2012 7:46 am

This particular motor is an Actro 32-4. It has vents on the bottom, but I'm not sure it qualifies as a fan exactly. It has one screw at the bottom, that holds the can to the shaft.

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Re: Balancing outrunner motor

Postby toast » Wed Jun 20, 2012 8:04 am

Back in the bad old days when we used to build our own brushless motors, I used a magnetic prop ballancer and added small bits of solder secured with CA between the magnets. Same when doing EDF fans. I would try some plastercene first between the magnets to see if this helps. At least you can remove it easily and it wont ruin your motor if it comes loose.
You shouldnt need much weight at all as most motors should be pretty spot on. I will be interested to see how much difference it makes.

Oh yeah, there will be a difference where you put the weight both in circumference AND laterally (foward or back) very much like balancing a car tyre.
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Re: Balancing outrunner motor

Postby Héli Suisse » Wed Jun 20, 2012 8:14 am

You shouldnt need much weight at all as most motors should be pretty spot on. I will be interested to see how much difference it makes.

That's quite right! I'm interested as well :cool:
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Re: Balancing outrunner motor

Postby Joel Rosenzweig » Wed Jun 20, 2012 8:22 am

I'll share the before and after graphs of the vibration (these are FFT's). First I have to remove the motor can and see if it is actually out of balance though. Once I determine that, I'll see what I have to work with.

Butch emailed me and suggested I check the motor bearings and the trueness of the motor shaft itself. Those were both really good ideas, so I'll follow up on at least checking the trueness of the shaft. I'm equipped to do that, but not sure what's involved in pressing out the old bearings and pressing in new ones. Hopefully I'll learn something about the motor balance before I start pulling and replacing bearings. :-)

I think it is conceivable that the motor is properly balanced but still vibrating due to the magnets. The outrunner motors are all "notchy" when spinning the can by hand. By contrast, the inrunners are smooth. I might simply be getting vibration as an artifact of the magnet geometry. More to learn.

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Re: Balancing outrunner motor

Postby 1hander » Wed Jun 20, 2012 11:25 am

i would look to the swash follower as the possible source of that little shake...
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Re: Balancing outrunner motor

Postby Joel Rosenzweig » Wed Jun 20, 2012 11:43 am

The vibration log shows that the only peak of interest is the motor. The swash follower could be the source, but a rotor head imbalance does not show up on the log. Either way, I'll check into it.

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Re: Balancing outrunner motor

Postby toast » Wed Jun 20, 2012 11:53 am

Joel, I would be very supprised if it is anything to do with the magnets (inrunner vs outrunner) causing the vibration. The notchyness you feel is a function of the number of poles a motor has and that is usually why an inrunner feels so much smoother - they have a low number of poles. Once the motor is running the magnets are in a rotating magnetic field and so there is none of the notchyness you feel when the motor is not running.

I would be expecting an out of true motor shaft/bearings or possably the swash driver 'scizzors' creating an imballance. Have you run the mechs up with the swash installed but no head/blades?
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Re: Balancing outrunner motor

Postby Joel Rosenzweig » Wed Jun 20, 2012 12:37 pm

toast wrote:I would be expecting an out of true motor shaft/bearings or possably the swash driver 'scizzors' creating an imballance. Have you run the mechs up with the swash installed but no head/blades?


I haven't run it up without head and blades for a long time, so I don't know what it would do any longer. That's another good experiment.

I still wish that if the swash driver was responsible for the vibration though, that it would show up on the plot.

I'll take a screen shot of that and post it soon.

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Re: Balancing outrunner motor

Postby bwellmaker » Thu Jun 21, 2012 1:17 am

I have not messed with the devices which log vibrations. I may have to do this someday.

I was wondering if the data recorded shows if its vertical or lateral vibrations.

Does anyone know in what relation the velometer is mounted and how many there are?

That would certainly help to narrow down a source.

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Re: Balancing outrunner motor

Postby Joel Rosenzweig » Thu Jun 21, 2012 3:57 am

Here's a plot from the vibration analyzer. The large spike is at 255Hz, which shows my motor is running at 15,300RPM at this instant in time. There's an 11.6:1 gear reduction to the main rotor, which gives a rotor speed of 1318RPM / 21.96Hz. I've circled the spike in red due to the main rotor and all related harmonics. The tail rotor is approximately 4x the speed of the main rotor. I didn't check against the actual gears though, so I could be off by a small amount. Either way, that puts the tail rotor at about 88Hz, or 5275 RPM. I've circled the tail rotor and its harmonics in green. Because the tail is a 1:4 off the main rotor, the harmonics appear at every 4th main rotor harmonic, too. That's neat.

So when I forced myself to circle the harmonic vibrations, it makes it clear that the sum of the main and tail harmonics is not at all insignificant, even if their magnitude is low individually. There's a lot of energy there. That means that there's good evidence for the head being out of balance still, either due to the swash lock, or my balancing of the head with blades just wasn't accurate enough.

I also see that even though the tail rotor spikes are tiny, the fundamental frequency is still somewhat high, so I would benefit from balancing the tail properly.

Butch -- the view does let you observe the vibration from different axis. It plots each axis in a different color. I can take a look at that too. It was a bit of information overload (for me), so I filtered it out for now.

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Re: Balancing outrunner motor

Postby bwellmaker » Thu Jun 21, 2012 4:40 am

That's pretty cool.

Having all 3 axis is a good think .

With is the measurement base or how much is it moving?

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Re: Balancing outrunner motor

Postby vario » Thu Jun 21, 2012 6:35 am

I know they use a form of loctite to put the magnets on the inside of the case. Wondering if you lost one magnet what would happen?
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Re: Balancing outrunner motor

Postby 1hander » Thu Jun 21, 2012 10:08 am

definately the motor...easy enough to fix,

if its a magnet which i very seriously doubt just epoxy it back in place and rebalance...
bent shaft..replace.
could just be balance..
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Re: Balancing outrunner motor

Postby toast » Thu Jun 21, 2012 10:31 am

Joel, can you show a plot with all 3 axis on the same graph in different colours? Would be good to see the relationship and should narrow down the direction of vibration and point to particular components.

Very cool that it shows the harmonics for the tail! I would pull the motor and run it up in my hand to see how the vibration feel. Should be very noticable with a peak that high! Unless the airframe is acting like an amplifier...?
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Re: Balancing outrunner motor

Postby Multinut » Fri Jun 22, 2012 6:00 am

My experience with an actro 32-4 is if they are dropped or banged on the can the magnets do pop loose easy, I know this from experience, my fault :(. They are glued to the can with a PERMABOND 919-922 series super glue which is a high temp, 450 f once tempered correctly adhesive. My experience a magnet came loose (probaly from me banging on the can after changing shaft to get to run within .001" again I knocked a magnet loose and in running came free) and it just stuck itself to the magnet next to it (they are really strong) and centrifugal force spinning them inside the can keeps it in place but the motor did not run smooth in this conditon, maybe what you are seeing in your vib plots. I took the can off, be carful to keep magnet in correct orientaion and using PERMABOND 919 glued it back in place right in the glue slot it popped out of. Baked the can at the low temp in glue insructions to temper it so it can withstand high temps if encountered and now 20 running hours later all is still good with it and it has been thorugh 3 mishaps (in my electrified 222) so far and has never come apart again, never checked balance on it either as the 919 is a super thin wicking cyro and I'm sure added nothing to put the magnet back in the already "potted" place it came out of. Hardest part was keeping the magnet in place while gluing as it did not want to stay were it was supposed to stay, magnets all around it wanted to play kissy kissy with it. I clamped in place with very small c clamps and wicked 919 in, hit it with accelerator, let dry, baked it, put it back together and used a different method to get it to run true again (no banging on can). You can look into the motor and see if one has come loose with out taking it apart, maybe it is your problem since plots are showing the vib at motor speed. Could be the ESC "plugging" the motor and inducing a thumping, you have a O-scope to look at the ESC output? I have seen such on Industrial motors with such drives often, all think motor bearing bad until they put a new motor in and it acts the same, change drive and it goes away, some times it is even speed dependant were the firing of the IGBT's is not exact at certain frequencies but OK at others, have you tried differnt head speeds and see if vib changes or follows?
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Re: Balancing outrunner motor

Postby seattle_helo » Sat Jun 23, 2012 11:21 am

Joel, the MR guys have a post somewhere on RCG showing how they mount an iPhone to the booms and use a vibration app to isolate and them eliminate motor vibes by applying vinyl tape to the can. The dude was just doing trial-and-error applications of the tape- if it was worse he moved it. :laugh: But damn if it didn't smooth right up when he found the right spot. I'd probably substitute aluminum tape for vinyl. Once you burnish that stuff down it ain't coming off and a little bit of tape weighs a lot, relatively, so you don't need much on there.
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Re: Balancing outrunner motor

Postby 1hander » Sat Jun 23, 2012 2:19 pm

you could do that with a zip tie to foind the heavy side, quite easy, just by sound you can get it way better..doublesided 3m tape will stay in place outside the can, if want to balance internally usesome ca...ive used all three methods succesfuly on my outrunners
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Re: Balancing outrunner motor

Postby Joel Rosenzweig » Sat Jun 23, 2012 2:56 pm

I ran up the heli without main blades, and the air frame is pretty smooth. There's a hum throughout the mechanics and fuselage, and that feels just enough to give you the sense that something is spinning. It would be spooky if it was so smooth while running that you couldn't feel it.

I did a series of experiments, taking vibration measurements for a baseline with no changes, then after adding metal tape to the motor can. I found a spot by trial and error that appeared to lower the magnitude of the motor spike by approximately 25%. This was using 0.22g of metal tape. Going with the theory that "0.22g was good so 0.33 might be better", I added more weight, measured again, and this made it much worse. Hah.

It's getting late here, and running up the helicopter in the basement workshop is still relatively loud. Not wanting to wake up the kids, I put an end to the experiments for the night. Tomorrow, I'm just going to fly it as it was and enjoy. Then, I'll get back to seeing what I can do to further balance the motor. And to see if that makes any difference. The heli is otherwise really smooth except for the skid shake.

The interesting piece of this, was that of course there was no visible skid shake when running up the machine without main blades. That points back to the head still being out of balance, even if it's still much better than it ever has been. I may have more work to do there. And of course, as mentioned, it could be the swash plate follower. I'll investigate more as time permits.

Thanks for everyones suggestions and feedback so far. I appreciate what you have to say about it.

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Re: Balancing outrunner motor

Postby toast » Sat Jun 23, 2012 3:50 pm

I hate trying to find vibes on helis! I have had a consistent nodding on my Trex 700 at low head speads, but it goes away when I flick to 3D mode. Finally found it last night. Is a bent head block! Ran a brand new main shaft in the head block on a prop balancer and it has around .5mm runnout in it. Obviously enough to cause a problem. Been scratching my head as I have replaced all the shafts and it still did it. Maybe you have the same issue? This was a "new" second hand head....
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Re: Balancing outrunner motor

Postby Joel Rosenzweig » Sat Jun 23, 2012 3:55 pm

I checked the runout on the main shaft, and it's under 0.001". I was pleased with that. I'll see what I can find. It's not my plan to drive myself too crazy over it. I was hoping that I'd find something obvious early on, but that hasn't panned out. I'll do some more work with it to see what I can learn.

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Re: Balancing outrunner motor

Postby 1hander » Sat Jun 23, 2012 4:19 pm

it waas more than .ikely not balanced properly, i had two scorpions i had to rebalance...bran spanking new too..
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Re: Balancing outrunner motor

Postby Multinut » Sat Jun 23, 2012 11:30 pm

Did you check the run out of the can top, middle, and bottom? Mine took allot of fiddling to get it to run true again after I took it apart.
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Re: Balancing outrunner motor

Postby wingtip » Sun Jun 24, 2012 1:53 am

its not entirely impossible to have an inproper machined rotor head hub where the shaft bolts at the top... I hate chasing vibes as well too... Dont remember if you said you ran it without the headblock on or just blades off ...
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