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Old Dec 06, 2012, 12:03 PM
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Enrique K.'s Avatar
Montevideo, Uruguay
Joined Apr 2006
106 Posts
Question
Cogging of motor

After dozens of flights, my Boom fell into river water. When I picked it up the ESC -a Jeti Advance Pro 18A- gave up smoke -dead. I dried the RC gear and the engine -an AXI 2212/26- and waited a few days to make sure everything was dry.

Bought a cheap Chinese 30A ESC, tried radio gear, tried engine, everything worked -engine made no particular noise, so I believe no magnet is loose-, went flying.

Flew for 7 minutes, landed, noticed that engine had quite a bit of cogging. Did not think much of it, flew it again. After a couple of minutes, engine stops, model more or less glides to the ground. I notice an enormous amount of cogging. A few minutes pass, I test it again, cogging gone!

What is happening?

There is one thing that might be important. The motor mount is not as firm as it used to be. After the second engine failure I noticed there was a crack in the foam, so maybe the engine experiences a lot of vibration.

Thank you!
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Old Dec 06, 2012, 02:34 PM
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Ron H's Avatar
Bishopville S.C.
Joined May 2003
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Sounds to me like a magnet came loose. You might want to open it up and have a look.
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Old Dec 06, 2012, 03:03 PM
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modisc's Avatar
United States, MI, Ann Arbor
Joined Aug 2011
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Quote:
Originally Posted by Enrique K. View Post
After dozens of flights, my Boom fell into river water. When I picked it up the ESC -a Jeti Advance Pro 18A- gave up smoke -dead. I dried the RC gear and the engine -an AXI 2212/26- and waited a few days to make sure everything was dry.

Bought a cheap Chinese 30A ESC, tried radio gear, tried engine, everything worked -engine made no particular noise, so I believe no magnet is loose-, went flying.

Flew for 7 minutes, landed, noticed that engine had quite a bit of cogging. Did not think much of it, flew it again. After a couple of minutes, engine stops, model more or less glides to the ground. I notice an enormous amount of cogging. A few minutes pass, I test it again, cogging gone!

What is happening?

There is one thing that might be important. The motor mount is not as firm as it used to be. After the second engine failure I noticed there was a crack in the foam, so maybe the engine experiences a lot of vibration.

Thank you!
There are a few scenarios for cogging becoming stronger than before.
1, stator is not well centered due to crash or other type of impact on the shaft, resulting in the air-gap in certain region becoming smaller.
2, There is a short somewhere in the coils. If you connect 2 of the 3 wires together, the cogging will be stronger; if all 3 wires are connected, the cogging will be even stronger. but rathan than describing the cogging becoming stronger, it is more like a resistive feeling.

You can do a no-load test on the motor, and measure the no-load current at certain voltage, and compare it with the factory nominals.
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Old Dec 07, 2012, 11:27 AM
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gkamysz's Avatar
Chicagoland
Joined Feb 2000
7,387 Posts
Some ESC leave the brake on after powering down, until all the residual voltage bleeds off in the ESC. Disconnect the ESC from the motor next time you try.

Greg
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Old Dec 07, 2012, 03:41 PM
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Enrique K.'s Avatar
Montevideo, Uruguay
Joined Apr 2006
106 Posts
Thank you all!

Before opening up the engine -as suggested by Ron-, I tried the easier stuff suggested by Modisc. Results:

1. There was no appreciable increase or reduction of cogging neither with two nor three leads conencted. I used a screwdriver and connected first two leads and then three to make sure connection was good. I guess this is good.

2. With the prop off but still leaving the prop holder on, I got a current consumption of between 0.65 to 0.72 Amps at 12.36 to 12.41 volts. The motor specs say it sould be 0.45 at 10 volts, I don't know what this means: Should the current increase with voltage? Could the prop holder be the cause for the higher current? Is this increase significant?

Should I proceed to open up the engine?

Thanks!
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Old Dec 07, 2012, 10:05 PM
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Enrique K.'s Avatar
Montevideo, Uruguay
Joined Apr 2006
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I repaired the cracks near the firewall and installed some reinforcements around it. I then test ran the motor again to see what happened.

Again with no prop, the amperage was 0.65 very stable (not as in the previous test).

I then put the watt meter away and ran the engine more or less as if I were actually flying the plane -throttle usually around the center and sometimes going WOT. After repeated stops to check, no cogging at all -even with the ESC still connected.

But then I noticed the engine became hot (probably some 50C or 122F). I could still keep my fingers on it , but I believe if I ran it one minute longer I would probably not have been able to. Approximately half of the 1300 mAh were consumed when I stopped the test.

I have never run the engine on the ground for so long until today, so I do not know if this is normal.

Ambient temperature was 24C - some 75F.

Can this help with the diagnosis?

Ron, as you can see, I am trying to delay opening up the engine at all costs -I have never done it and don't know what awaits me!
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Old Dec 08, 2012, 02:29 AM
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United States, MI, Ann Arbor
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Your no-load current looks normal. And the motor getting hot after no-load full throttle run (nominal voltage) is also normal. Is your motor an outrunner? 50 degree C is a little bit too hot for the rotor of an outrunner after 1-min no-load full throttle test run at nominal voltage.

A normal 3 phase brushless motor, if you short 2 leads or all 3 leads, you should be able to feel noticable increasing resistance when you turn the shaft by hand.
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Old Dec 08, 2012, 07:13 AM
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Enrique K.'s Avatar
Montevideo, Uruguay
Joined Apr 2006
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Quote:
Originally Posted by modisc View Post
Your no-load current looks normal. And the motor getting hot after no-load full throttle run (nominal voltage) is also normal. Is your motor an outrunner? 50 degree C is a little bit too hot for the rotor of an outrunner after 1-min no-load full throttle test run at nominal voltage.

A normal 3 phase brushless motor, if you short 2 leads or all 3 leads, you should be able to feel noticable increasing resistance when you turn the shaft by hand.
I am sorry for not having been clear. I was running the engine with an APC 10*3.8 slow fly prop. And I must have been running the motor for at least five minutes before I noticed the increased temperature.

Yes, the motor is an outrunner. See http://www.modelmotors.cz/index.php?...e=26&line=GOLD
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Last edited by Enrique K.; Dec 08, 2012 at 07:21 AM. Reason: Factual corrections
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Old Dec 08, 2012, 08:46 AM
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Dr Kiwi's Avatar
Chattanooga, Tennessee, United States
Joined May 2003
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A 10x3.8 APC SF on 3s takes an Axi 2212/26 beyond its rated maximum... it is rated for 10A-12A max and you are running it at ~14.75A/160W.
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Old Dec 08, 2012, 09:10 AM
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Enrique K.'s Avatar
Montevideo, Uruguay
Joined Apr 2006
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Yes, you are right! But in the link above, the manufacturer offers an example of a 3D use with a model weight of 450 grams or one pound -exactly what my Boom weighs- using 3S and a 10 x 3.8 prop.

I usually fly around mid-stick and only go WOT when climbing for loops or wingovers, which never takes more than 5 seconds. It has worked so far flawlessly, through summer and winter, for over two years.

Maybe you are right and I should prop down to 9 x 3.8 or 4.7...

Do yu believe ambient heat -which was around 28/30C -83/86F- could have had something to do with the recent motor failures?
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Old Dec 08, 2012, 09:49 AM
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lectroglide's Avatar
United States, WA, Puyallup
Joined Aug 2006
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yes , heat can do some bad things to anything rotating, if you dont have a wattmeter get one soon. also some maunfactures give faulty info on their stuff which is more reason to have a wattmeter
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Old Dec 08, 2012, 10:17 AM
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Enrique K.'s Avatar
Montevideo, Uruguay
Joined Apr 2006
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I do have a wattmeter and I just measured current... Dr. Kiwi is right, I am scratching 15 Amps with that prop! Stepping down to 9" diameter! What a dumbhead, I didn't use the wattmeter with this motor!

Thanks for your advice.
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