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Old Jan 17, 2013, 10:11 AM
Walkera fan-boi
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USA, FL, Sunny Isles Beach
Joined Jun 2009
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Question
Brushless Inrunner 8750kv torque issues

I have several Losi Micro Brushless 1/24 scale cars that I enjoy upgrading and tinkering with. I recently made a dive into upgrading the power system as much as possible and started looking at improving the quality of the brushless motors the vehicles came with. I ordered replacement ceramic bearings for the motor from Boca Bearings and once they arrived, I replaced the stock ones. I guess I had bad luck, as after having everything all back together and running the car the first day with the upgraded bearings, one of the bearings failed. I took everything apart again and put back in a stock bearing, and now the motor cogs when first trying to take off from a standstill. If I give it a tiny push, or sometimes by itself given enough attempts at accelerating, it will go.

I have no issue with just buying a replacement motor, but I am too inquisitive to just do that without giving it a college-try at repair.

I have several of these motors and have tried interchanging parts from them (spindle, spacers, bearings, shims, cover) to see if I can fix the problem, but I have come up with several questions along the way which I would appreciate feedback from those with more motor experience on:

1) What are "factors" which would impact the torque a motor is able to generate with all other factors remaining constant (voltage, connectors, ESC, etc.)?
2) Is the position of the magnet along the length of the main shaft of the motor important? Is having it in the dead center of the coils critical? Does the magnet guide itself to that spot?
3) How important is the air gap between the magnet and the coils? Are there any readily available methods to measure it and more importantly with this motor type, is there any way to correct it if issues are discovered?
4) The magnets used in these motors are rare-earth magnets, not neo. As a result the magnet is porous. I soaked the magnet at some point in Tri-Flow and am wondering now if this could have affected its strength?

Any and all feedback would be appreciated!

MiniChopper4Me
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Old Jan 17, 2013, 12:24 PM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
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Quote:
Originally Posted by MiniChopper4Me View Post
I replaced the stock ones. I guess I had bad luck, as after having everything all back together and running the car the first day with the upgraded bearings, one of the bearings failed.
"If it ain't broke, don't fix it!"

Quote:
I have several of these motors and have tried interchanging parts from them (spindle, spacers, bearings, shims, cover) to see if I can fix the problem,
Did you manage to isolate the faulty part?

1) What are "factors" which would impact the torque a motor is able to generate with all other factors remaining constant (voltage, connectors, ESC, etc.)?

Torque is produced by interaction between the rotor magnet and magnetic field generated by current flowing through the windings. Low torque might be caused by a broken wire, bad connection, shorted winding, or weak magnet. Mechanical issues such as bad bearings, swarf in the airgap, or a slipping shaft may soak up some of the torque before it gets out of the motor.

2) Is the position of the magnet along the length of the main shaft of the motor important? Is having it in the dead center of the coils critical? Does the magnet guide itself to that spot?

Ideally the magnet should be positioned where it 'wants' to be, but slight misalignment won't make much difference.

3) How important is the air gap between the magnet and the coils? Are there any readily available methods to measure it and more importantly with this motor type, is there any way to correct it if issues are discovered?

The air gap is very important, but in an inrunner it's almost impossible for it to go wrong.

4) The magnets used in these motors are rare-earth magnets, not neo. As a result the magnet is porous.

Neodymium is a Rare Earth element. The porosity may be due to it being sintered.

Neodymium magnet

I soaked the magnet at some point in Tri-Flow and am wondering now if this could have affected its strength?

It shouldn't have any effect - unless the magnet was already loose on the shaft, in which case lubrication could allow it to spin more freely!
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Old Jan 17, 2013, 12:38 PM
Scratch builder
USA, PA, Telford
Joined Apr 2004
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1) factors. Air gap (ID, OD of motor components), strength of permanent magnets, and magnet strength of the wound field. Wound field strength has to do with lam material, (iron losses), wire resistance, and therefore copper volume variations from motor to motor. Lam design itself is a factor. (thickness and shape)

2) Yes the position does matter, BUT, usually the magnets are designed to be longer than the lam stack to account for some axial misalignment due to tolerances of the parts lengths. So, as long as the endbells don't push the alignment enough to cause a "magnet spring", the alignment is good.

3) Airgap: small = good, large = bad. Some motor manufacturers spend the time to actually hone parts to keep this as small as possible. Measure by OD. and ID. of parts and subtract the difference. The only way to enhance it is to mix and match motor parts the get the smallest difference components in one motor.

4) NEO is rare earth too. I know you mean SmCo. These are extremely hard to de magnetize using large magnetic fields let alone any cleaner, solvent, or oil products. The only liquid that could harm them is a potent magnet eating acid

Hope this helps!, I'm sure others will add to and correct as usual
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Old Jan 17, 2013, 02:02 PM
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You may have damaged the bearings during installation. It is important to press only on the outer race or you risk denting the races or seals, or end up with a flat spots on the balls. Any of which may feel like cogging.
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Old Jan 17, 2013, 08:47 PM
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USA, FL, Sunny Isles Beach
Joined Jun 2009
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I have a total of 3 motors components. In an effort to make the "best motor possible" I probably ended up mixing and matching pieces from the 3 starting motors. I realize now that the most likely culprit in my case is that the 2 main components of the 3 sets were probably all "paired" and if I take them all apart again and measure the ID and OD, I'll find that what ended up happening was that I made a bad combination of windings to spindle.

Incidentally, I only them all apart because 1 of the motors had the magnet detach from the shaft and needed to be re-glued, and another had a missing spacer, causing the shaft to have lateral movement during operation of the motor. It was eating up pinions and spurs to no end. The third motor was rubbing while spinning, and it was evident that it was because the magnet was touching the coils in one point during its rotation. It was so minor that the motor worked, but made noise and did not have nearly as much power as it should have (similar cogging issue, which is why I got the third motor in the first place).

Thanks for all of the feedback! I'll report back my findings once I've taken everything apart again and done my due diligence in measuring stuff

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Old Jan 18, 2013, 02:46 PM
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I finished taking everything apart and measuring all of the components last night. I arranged the 3 sets of windings and spindles in order from smallest to largest and assembled them. Turns out that the middle outside piece (windings) was the "bad" component, all of the other pieces appear to work fine. Now I have 2 working motors again. I'm going to use the spindle from the middle motor on the motor with the smallest ID since as you guys mentioned it may be beneficial and then finish sealing the motors back up and reassembling the cars.

My plan was to use 2 dots of green loctite on opposite sides of the top and then clamping the motor until it dries. Is green loctite sufficient?

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Old Jan 18, 2013, 04:31 PM
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United States, CA, San Diego
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Quote:
Originally Posted by Bruce Abbott View Post
"
4) The magnets used in these motors are rare-earth magnets, not neo. As a result the magnet is porous.

Neodymium is a Rare Earth element. The porosity may be due to it being sintered.
If it is porous it is more than likely bonded material rather than sintered. Alot of the car stuff uses bonded neo due to cost considerations.

Steve Neu
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Old Jan 18, 2013, 04:37 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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Sounds like you've work this out well.

My experience with the various Loctite compounds is that spreading a very light surface film all the way around the ends (put it on the part that is easiest to work with) before putting them together will work well. That will give you a better grip over a larger area and also minimize the risk of that being little or no excess Loctite might get into a bearing.

Getting any Loctite into a bearing can be a real problem. Especially if you don't notice it immediately. If you do get any into a bearing washing it out with Acetone and starting over on the assembly is a good way to deal with it.

Jack
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Old Jan 18, 2013, 08:56 PM
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Originally Posted by sneu View Post
If it is porous it is more than likely bonded material rather than sintered. Alot of the car stuff uses bonded neo due to cost considerations.

Steve Neu
Sorry for skipping past all of the magnet talk, I'm a bit ignorant in this area. Up until now, I've only seen the insides of maybe 5 different motor types, and saw an inrunner that had the spindle that the magnet part appeared to be completely silver colored, smooth, and clearly non-porous. It was a Traxxas Velineon 380 motor. I assumed (clearly incorrectly) that this was "Neodymium" as opposed to all of the other magnets I've seen that have been ash-colored and feel porous, like refridgerator magnets, which is what I was referring to as "rare-earth". I'm guessing that the Velineon magnet was simply coated or something, and whether its Neodymium or not had nothing to do with its appearance. Thanks for the pointers!

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Old Jan 19, 2013, 06:31 PM
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Quote:
Originally Posted by MiniChopper4Me View Post
Sorry for skipping past all of the magnet talk, I'm a bit ignorant in this area. Up until now, I've only seen the insides of maybe 5 different motor types, and saw an inrunner that had the spindle that the magnet part appeared to be completely silver colored, smooth, and clearly non-porous. It was a Traxxas Velineon 380 motor. I assumed (clearly incorrectly) that this was "Neodymium" as opposed to all of the other magnets I've seen that have been ash-colored and feel porous, like refridgerator magnets, which is what I was referring to as "rare-earth". I'm guessing that the Velineon magnet was simply coated or something, and whether its Neodymium or not had nothing to do with its appearance. Thanks for the pointers!

MiniChopper4Me
A lot of the magnets are plated with nickel.

Steve
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Old Jan 19, 2013, 08:24 PM
Wake up, feel pulse, be happy!
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United States, AK, Fairbanks
Joined Aug 2009
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Looking inside a Mega motor (apologies to Steve ), you can clearly see the dull gray neodymium magnets inside. Neodymium is just one flavor of "rare-earth" magnet; there are several others as well. A common fridge magnet is unlikely to be of the rare-earth category, though the sale of them for general use has definitely increased in recent years and I know there are a few small neodymium discs in my house which we use for hanging things on the fridge or chandelier.

Many/most of the common brushed motors, for contrast, use ferrite magnets. This is one of the reasons brushless motors appear so much more efficient than brushed; they simple use much stronger magnets and have done so since their introduction into the mass hobby market. If a brushed motor was built around the very powerful neodymium magnets found in most brushless motors (and if the can was built to contain much more flux), a brushed motor's efficiency could easily outstrip the numbers seen in many brushless ones.
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Old Jan 20, 2013, 03:41 AM
Aka: Tom Jenkins
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Is the battery charged?
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Old Jan 22, 2013, 01:02 AM
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Australia, VIC, Melbourne
Joined Feb 2010
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Quote:
Originally Posted by MiniChopper4Me View Post
Velineon magnet was simply coated or something, and whether its Neodymium or not had nothing to do with its appearance.
Nickel plated, makes them a lot more durable. The Neodymium-Iron-Boron http://en.wikipedia.org/wiki/Neodymium_magnet mix is fairly fragile (almost ceramic like, it is quite brittle) so the Ni coating provides a fair amount of protection.

They are not really made from Neodymium, most of the structure is Iron (Nd2Fe14B).

Do motors use the sintered magnets, or the composite type? seems the latter would have benifts as there are much less eddy current losses but they are a bit weaker.
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Old Jan 22, 2013, 09:23 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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Here is a place that has a lot of info on magnets. I know they sell magnets but they also seem to be honest and helpful as far as sharing info honestly and objectively:

http://www.kjmagnetics.com/FAQ.asp

They have a page for info on Neodymium magnets:

http://www.kjmagnetics.com/neomaginfo.asp

and a very nice glossary of terms associated with magnets:

http://www.kjmagnetics.com/glossary.asp

Jack
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