brushless motors and water

[hargard]

Hi again
I am a builder of RoV’s and think you might be able to sort me out.
We need to have drives to propel the RoV under water and if the drive is contained (waterproofed) then we need to seal the shafts, because of the pressure the seals often fail and screw things up.
I have been testing Standard brushed motors with NO waterproofing ie just placed in sea water. They breakdown after a few hours when the varnish softens and then the motor loses 90% of its power. But I am still ongoing with the brush motors by sealing the armature in epoxy then turning the excess off.
The question I have is: -
Will a brushless motor run under water and what are the predicted results will it remain controllable with a Esc. ??
I don’t want to get into discussions on shaft sealing please as I have been down that path.
Any help in this field would be most welcome and read with interest.

[Terry S]

I dont see why there is a problem with the varnish, I would have thought the problem would be with the brushes. The main problem I see in using brushless is that the bearings will fail after time.

Terry

[Trevor_G]

You seem to have very corrosive seawater:D . I don't understand why varnish should soften in seawater. But it sounds as though the seawater is getting to the bare metal of the windings in which case you will get electrolytic corrosion and the motor will need rewinding. The other problem is that most brushless motors use neodymium magnets which will rust unless they are protected.

If you really must run this way :eek: coat all exposed metal with a good waterproof varnish. I normally use Electrolube TRV its never let me down in saltwater but I have not taken things as far as you.

[MatC]

hmm... I'd try to avoid all the seal problems totally. I recently looked at a fishtank pump: it used a magnet on the output shaft, and electromagnets to rotate it. The beauty of this is that the electric stuff is all the other side of some plastic, and the propellor (in your case) is just attached to a magnet. You still have to arrange some supports and bearings, but at least that nasty seawater can't get into any contact with any motor parts.

The magnet was cylindrical, presumably with poles at right angles to the shaft.

Sadly this is probably less efficient (larger clearances) than a "proper" brushless motor. I'd think proper underwater seals are very difficult to make, but you might be able to track down an off the shelf solution from some engineering company.

[hargard]

[QUOTE=hargard]I don’t want to get into discussions on shaft sealing please as I have been down that path. QUOTE]
I am a engineer by trade.. like I said above :)
U might be right Terry S I never thought of that :(
I striped down the motor to find the Brushes very dirty
The armature is very badly clogged.
Cleaned and now is under test again.. Seams to be running at full capacity :)
But will check every hour.
Thanks for the post's
Regards Hargard RoV builder