What would cause this? Motor spark 3s

[Asuckaphish]

While doing a group trail run yesterday my normal 2S battery had died. I have never run 3S before and decided to throw one to get going on the trail again. As I plugged in the 3S I noticed a pretty good spark come from one of the taps on the motor where the wire is soldered on. Never seen sparks come from the motor before when plugging in on 2S. The motor is a new Yeah Racing Hackmoto V2 540 35T motor. I just bought this motor new and installed it last week and had two full packs (2s) through it when plugging in the 3S and getting the spark. Decided to keep running it down the trail and 10 minutes later could smell that telltale smell that something wasn't happy.

I have way more experience with brushless motors but as far as I know 3s shouldn't really bother a brushed motor. I'm at a bit of a loss as to what would cause such a large spark from the motor tab when nothing was physically shorting out. Note that the motor still runs but I stopped so I didn't damage anything further. Any ideas?

[Soldat Vaillant]

If the spark came from the solder joint then you probably have a weak/cold or partially broken solder joint. Reheat the joint making sure all the solder on the tab and wire melts and blends together, the joint should be fine after that.

[Olle P]

First: The switch from 2S to 3S is most likely irrelevant for the issue. It would have happened anyway.

Then my first question would be: Did the motor or ESC get hot in the 10 minutes you drove?
Running on 3S allows the motor to provide twice the power and heat compared to 2S, so if you used that opportunity it should run a warmer but also faster and stronger.
If there's just a bad solder point the motor would be running a bit more rough and neither get hot nor deliver all the extra power.
If there's a short (caused by bad soldering and/or debris) the ESC should get much warmer than usual while the motor would be running poorly in one way or the other.

[Asuckaphish]

Thanks for the input guys. Got a chance to check things out last night. Looked at the joints again and they appear to be shiny and smooth. Hooked it up to 3S and no sparks after plugging in several times. Ran it for about 5 minutes at full throttle and it appears to be running fine albiet with a slight hint of that burnt electronics smell that was there before. Motor was warm but not hot in any way and the motor leads were all cool. ESC and BEC were also running cool. Only thing i noticed was that in the coat of dust that was on the motor their appeared to be a hint of copper debris. Not sure if that has something to do with it. Anyway i'm going to run it tonight on the trail and see how it does. If it dies it dies.