That speed controller seems suspect. Actually the esc takes DC and converts it to three phase AC. With AC supplied to the coils of the motors the impedance is quite high, but if for some reason one applies DC, the excess current will really heat up the windings. You will probably fond the wires in the motor are black instead of a golden orange, Get another esc before connecting another motor. Just on an off chance another problem might be the control wire that goes to the FC. If that is shorted, then maybe something like that could also happen. Use your multimeter to check , you should have a fairly high resistance with no power. I use a Fluke 77 multimeter, the voltage used to test resistance is very low, 50mv I think which does not effect the delicate circuits.
