will a brushless dc motor work underwater if you just varnished/sealed/coated the windings?
First of all as per my knowledge there is no DC motor which can work without brush armature. Sealed type AC motors are available even immersion type which can work in the water. The varnish is not guaranteed against water protection, there could be leakage currents possible dangers
A brushless DC motor is actually an AC motor with an electronic switching network that serves the purpose of brushes, commutating the windings. Yes, if you could coat the windings as well as the electronics well enough, then yes, it would operate underwater. What is normally done for electric motors for ROV's is the motor housing is sealed and completely filled with oil which is not compressible and a diaphragm is used to allow the pressure inside and out to equalize. that way the shaft seals never see any difference in pressure. also the oil can be circulated through a heat exchanger/radiator to the surrounding water to keep the motor cool.
