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Thread: Diagnosing Post-Crash Problem (RCX 1804)

  1. #1
    Navigator Sbatty's Avatar
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    Diagnosing Post-Crash Problem (RCX 1804)

    So despite the durability problems I have seen posted online I took a gamble on the 1804's and have been pleasantly surprised. Multiple severe crashes (some onto concrete) have not budged the magnets and everything has been going fine until yesterday.

    I managed to drive my quad onto a cement surface at about 25 mph. Both of the front motors came out with severe deep scuffs on the cans and the the props from all four motors literally shattered into tiny pieces. Pretty impressive crash and condition of the quad speaks highly of the designer of the 3D printed frame I am flying. I pulled all the motors off and inspected the magnets and to my surprise everything was still in place. I replaced the props and launched again about an hour later.



    The initial flight went okay, but soon I noticed some surging and weakness form one of the damaged motors. I set the quad down and found that one of the crashed motors was considerably hotter than the other three. The ESC was still cool to the touch. I quickly swapped out the damaged motor and set it aside for testing.

    After pulling the motor apart (and shooting the eclip across the room) I can find no obvious signs of damage. The windings are not discolored, the magnets are still firmly attached an aligned to the housing and the bearings are turning freely. Even thought I don't see any damage at the end of the arms of the stator, I suspect this magnets and stator may have come into contact during the crash. There was a slight deflection in the housing and maybe a 20% reduction in the gap between the magnets and the stator in one spot. Using a fairly light force I was able to tweak the can very slightly and restored the gap. The motor now feels like it turns a little smoother, but I could be imagining things.

    I am going to turn the motor in drill to determine if the excessive heat is friction or electrical. I was going to check the voltages with a meter. I am also going to take a thermal camera (office toys are fun) to the problem and see if I can isolate where the heat is being generated.

    Assuming that the bearings are still good and not the source of the heat, does anyone have any suggestions about what other problems I should be looking for?

  2. #2
    Navigator STI-REX's Avatar
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    I use a large plastic see through bag to take motors apart then if the Clip pings off it doesn't go far
    4 wheels moving body 2 wheels moving soul

  3. #3
    Navigator Sbatty's Avatar
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    Last time I worked on anything RC related was about 20 years ago. I forgot how far those things fly.

  4. #4
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    Not sure if it would be extremely useful but you may want to measure the resistance between each winding, in case some insulation was damaged and you have some arcing or shorting going on inside the motor. I measure about 0.6 ohm on each winding of my RCX motors.

  5. #5
    Navigator Sbatty's Avatar
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    So IR checked out fine. Bearings appeared to be in good shape. The only physical damage I found was a slight flattening of the housing that may have caused the magnets to be closer to the stator face in one area, but no evidence of a strike.






  6. #6
    Navigator Sbatty's Avatar
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    Just got done playing with the thermal.

    ESC on power-up:

    I was actually surprised to see that on power-up it jumped to 96 degrees almost instantly.

    Underside of motor after 1st run with no prop:


    After confirming that I wasn't producing a short that was generating extreme heat in either the motor or the ESC I moved on to testing at speed with a prop.

    Motor at shutdown after 50% throttle run with prop:


    ESC after 50% throttle run


    Stator after 50% throttle run dissembled


    As best I can tell the motor is functioning acceptably now. Operating temps are consistent with the undamaged motors.

  7. #7
    Navigator Sbatty's Avatar
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    Alright, now I am stumped.

    I ran the motor under load yesterday. After about a 90 seconds the motor slowed dramatically, I cut throttle and thermaled the ESC and motor and temps were no bad. I then setup a good motor and ran it in a simlar test. I was shocked to see that the operating tempts of the "good" motor were significantly higher around the stator than damaged motor. The ESC got a little hotter on the good motor, but there was no overheating and the motor never lagged like the damaged motor. IR was pretty much the same across all the leads of both motors.

    Since I have a damaged motor to play with does anyone have any suggestions or tests that they want done? Otherwise I am going to cannibalize the bearings for another issue I am working out.

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