Repair brushless motor?

My quad had a mild crash upside down (hitting the field with its motors). One of the motors stopped to work, but at the end of the day it started again.

Now when compared to others it's clearly visible that it spins slower. In flight the quad is still fully capable of stabilizing itself, but it will naturally move in the direction of the faulty motor (like if there was wind from the opposite direction).

Should I buy a new motor or is there something that maybe I could try?

I still never opened it nor have done any kind of analysis.

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  • If I were you I would try to fix it. But motors are cheap enough that having a replacement handy never hurts.

    Your motor will probably need new bearings. If the motor makes a rumbly type of sound when spun by hand your bearings are not that great anymore. 3x8 mm bearings can be purchased online and can be replaced at home fairly easily. Be sure to immerse in motor oil or triflow (bicycle linkage oil) for a few minutes to ensure they are nicely lubed. Bearings need to be pressed evenly by the outer race so they don't come in "sideways" and loose shape. Find online instructions if you are not mechanically inclined.

    The bell might have shifted down on the shaft so that it is clamping the bearings way too hard. Re-seat the bell at a reasonable level of tightness (just after no slack remains) after loosening up any setscrews, then re-tighten.

    Also, the plugs might need to be re-soldered. Most outrunners have magnet wire leading right up to the plugs and each strand of wire needs to be individually attached to the plug, not just a few of them. Otherwise some windings will not receive power. If a wire broke or came unsoldered the motor will spin slower. My quad flew just fine like this for weeks before it caught my attention, except that rapid yaw would cause one corner to dip. Assuming your magnetometer is tuned right and GPS is working, corner dips on yaw indicate a weak motor.

    When re-soldering the plug, simply set your iron to about 800 degrees and hold a drop of solder up against all magnet wires until you see the enamel burning off. This will probably take several seconds. Continue until you have a nice solid covering of solder over the tips of all strands, then solder the plug. Do this for all three wires to be certain you've done it right.

    -Aurelio

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