Running an out runner motor under water?

[dshuen]

I am looking for a brushless motor for an under water ROV. A water
tight thruster housing is very difficult to construct. Pressure
compensation and shaft bearing seal are just a few of the problems. I
am wondering if the stator, the windings and the wires connectors are
sealed in waterproof paint/coating and we utilize a ceramic or
stainless steel bearing can we make the brushless motor usuable under
water without a water tight housing? I have looked into a LRK motor which I can wind it myself.

I am hoping a high torque low KV (200-250) motor will have the torque
to turn a 3" prop. A gear box will be one more thing to attract
corrosion. A 90mm ducted fan unit is also an option for the motor.
Maximun power output will be around 100-150W per thruster motor.

Regards,

Don

[GIFLYRC]

Hello Don,

I have no real knowledge and probably have no business replying to this thread, but my "gut feel" is that running BL underwater is doable

WHY??? Because brushed motors are run in to seat the brushes in water
they run and do not short out!

You mention packing boxes and bearing seals, personally I would not over engineer the prototype and would build the most simple unit, immerse it in water, run it and see what happens as proof of concept

If it doesn't short out and runs then I think that corrosion whoud be the biggest issue.

You can refine the application from there

Roger aka GIFLYRC

[latrans]

To the best of my knowledge brushless outrunners have no prblems with being run under water. I might give a little thought to the parts that would rust, bearings, shaft, flux ring and stator but other than that it should be no problem.

Latrans

[ImaBiggles]

To the best of my knowledge brushless outrunners have no prblems with being run under water. I might give a little thought to the parts that would rust, bearings, shaft, flux ring and stator but other than that it should be no problem.

Latrans

For grins, I ran a GBv in a glass of water before. Unloaded air amps = <0.5. "unloaded" submerged amps =2.4. Didnt give a hoot about RPMS so the were not measured. Eventually the quickey CA job I did to the shaft/rotor sheared (it was a loose fit when pressed so I CA'd the heck out of it)

To minimize the water issue, you can simply use a shaft and have the motor out of the water line if possible with your design - else I do not believe submerged running will be that much of an issue. I am an old weedeater boat guy and have a couple of electric boats as well. One of the things you want to watch out for is crap getting into the rotor - above the water line running not an issue. For the KV you are talking about, I think you would be better to run geared. However I dont think putting an entire gearbox in the water will give you results u are happy with. Water is obviously much denser than air and such a thing will creat LOTS of drag - as will a submerged outrunner. A gearbox will allow you to perhaps raise the motor out of the water if desired.

To water tight a shaft, all you really need is a stuffing box. For the scale we are talking above - a balsa/glassed box filled with cotton balls and axle grease will would well - be 99% water tight. Its really pretty easy to build a water tight thruster housing with this technique. If you desire, you can flood the motor chamber with clean water - go through the stuffing box to get the prop in the 'not so clean water' That way you do not have to add as much balast to the beastie.

The forces acting on the bell/rotor will also be MUCH higher in submerged operation- so its gonna have to be beefed up a bit. Additionally, although the bells look symetrical in air, things like cooling holes can (and did in my play test) cause cavitation. For rust protection, spray the thing down in CorrosionX (available at gun stores).

Good luck and let us know if/when the project progresses.

[flieslikeabeagle]

I don't see any reason to worry about shorting - pure water is a pretty good insulator, and even generic pond water doesn't conduct enough to matter to a motor (though it does conduct enough to seriously upset most modern electronic circuits).

I remember watching an episode of Junkyard Wars where one team built an underwater "torpedo" using a cooling fan motor from a car, fully immersed in water, and, IIRC, a 12 V car battery to power it.

Years ago, when I was in my teens, I built a model boat that was jet-propelled. Living in the third world at the time, I made the centrifugal pump myself, from scraps of whatever I could lay my hands on - I remember the impeller housing was a plastic measuring spoon from a tin of infant formula (powdered milk). The boat sucked water in from the bottom, and blew it out the back. It worked well, but had a tendency to suck in stuff if it got into shallow water. I took no special precautions to keep the water out of the motor, other than the fact that the motor was pointing downward and the impeller housing was the lowest item.

Is using a pump an option for you? That might simplify many of the issues Biggles raises. The motor and gearbox if any will be out of the slipstream; if necessary you could build a watertight enclosure around the motor, so if any water leaks out of the pump housing into the motor, it is still contained by the housing around the motor.

-Flieslikeabeagle

[birdofplay]

Sounds like you want to go to DEEP water work in high pressures.
A "relatively" contained motor housing filled with something
"less dense" than water would lower the fluid "Drag" issues
around the rotor but still not require pressure compensation fittings et al.
Any leakage could be remedied by fluid replacment between uses.

I have heard that the Brushed motor guys "Break In" their motor in water !
So theres another option.

So what are you planning ?
Sneeking up on Killer Whales or someting like that ? :-)

[epilot]

Salt water will be a problem so you need to make sure your wires are insulated completely. The ESC might not like the short that will occour in salt water. Hardly enough to fry the ESC but it might "sense" something and get the timing screwed up. As soon as the motor is out of the salt water it must be washed in fresh water and have the bearings oiled otherwise it will not live very long. Even stainless steel can be tarnished by a saline solution.

[grouchy1]

If I remember right, I saw some kind of "stuff" at a trade show that waterproofed anything. The display had electric trains and servos and things all running under water. Just spray it on. Don't remember what it's called. :censored:

[grouchy1]

I found it :cool: It's called "aeroplate" and you can get it at www.aerotrend.com :D

[flieslikeabeagle]

If I remember right, I saw some kind of "stuff" at a trade show that waterproofed anything. The display had electric trains and servos and things all running under water.
What you didn't see was the employees frantically replacing all the trains and servos on their lunch breaks and after show hours. :D :D

Salt water? I did not see any mention of that in the original post. If we're talking about salt water this is a whole other ballgame - the stuff is incredibly corrosive, and the salt makes it conduct electricity well enough to seriously upset most electronic circuits. Not for long, though, as the corrossion usually destroys copper traces and soldered joints on circuit boards within hours. :eek:

-Flieslikeabeagle

[radfordc]

It's dead easy to insulate all the wiring in a brushless motor.

I wouldn't worry about pressure compensation, etc. Just build a low Kv motor and run it direct drive. Lube the bearings with WD-40 each time you run it. Should work great.

[dshuen]

Thank you for all your suggestions. I am building a deep water ROV in open seawater. Operation depth hopefully will be 200-300 feet deep; maximum hull pressure will be approximately 140psi. Traditional method of water proofing the thruster housing in commercial ROV is to fill the housing with oil and an oil reservoir. Shaft seal bearings need to be checked and serviced frequently.

I am trying to bypass all that to drive the prop with a brushless motor. A sealed housing does have the advantage of permitting a gearbox to increase the efficiency. However, I like the simplicity of a direct drive with an off the shelve motor.

The motor shaft will have to be changed to stainless steel. Shaft bearings replaced with ceramic bearings. What is the best way to seal the stator and wire connectors? I want the RPM to be 2500-3000 under 12 volt. All ROVs require low speed and high thrust propulsion.

Don

[olmod]

Titanium shafts are available (forgotten who} epoxy will seal winds and stator done in a vac' tank ;) whilst rotated.

[ImaBiggles]

It's dead easy to insulate all the wiring in a brushless motor.

I wouldn't worry about pressure compensation, etc. Just build a low Kv motor and run it direct drive. Lube the bearings with WD-40 each time you run it. Should work great.

WD 40 is the last thing one should use on bearings (matter of fact, I bet ya it would be the last thing).

WD 40 turns to sludge at high RPMS. It is not made to be a lubricant of fast moving stuff.

[ScubaSteve]

The stator, if exposed to water or even extreme humidity, will rust. Even with insulated stators, the most susceptible areas to rust are the stator arm faces. Other than that I as well don't see any issues with running it underwater. The saltwater will just accelerate the oxidation. Make sure all your electrical connectors are sound.

Your best bet would be what Biggles suggested - keeping the motor out of the saltwater. I'm ignorant to the electric boat aspects of the hobby, but i remember "air mover" posting several months ago about a ~flexible motor shaft that apparently RC boaters use regularly. Wish I had more details.

steve

[PaulVi]

Im thinking some of us may have it wrong as to the application hence we keep suggesting that he keep the motor above water.

"under water ROV" hence I think this is more along the lines of a sub.

and he mentions thrusters. so i assume it would have a pair or more that can be moved or rotated to control pitch and directin

So we need to rethink the water proofing.. If this is not to be kept under water long then maybe it is just as simple as you would treat your outboard motor when you were done running salt water.. Lots of freash water to rinse with..

With that said you could also control corosion by providing a sacrifitial metal. one type of metal that corodes faster and is mor conductive than the surounding metals.. On my bouat it is a chunk of magnisuim and that prevents the suronding aluminum from pitting or coroding.

Look into that to help things out..

As to fluid dynamics if you use a Low KV motor and a fan unit you may do real well with speed and thrust. i would us a good marine RTV sealer where the wiring has to go from the motor to the seed controler which i assume will be mounted in the body of the craft. you will still need to define what is going to keep the water out at the depth that you intend to go.. (Pressure) so it may be a combonation of running the wires through a water compressin fitting and then additinal sealing..

You will also have this same water tight problem where you rotation shafts exit the body of the craft as i assume you will be faving a sing shaft that is connected to the thruster housing providing a rotation point and the other end connected to a servo of some sorts to rotate the housing..

You would have the same problem there in keeping the water out of the body at pressure..

On last thought you may want to use bonse bushings in the motor not only will that soak oil up but they will be less susseptible to corosion in a by metal enviroment.

Thanks for listening..

[flieslikeabeagle]

I think the original intent of the thread was to use an off-the-shelf brushless motor, but I just had a crazy thought or two:

What if the ROV hull had a tube designed to pass through it, fore and aft, open at the front and rear (i.e. a jet duct open to the seawater). Conventional inrunner type windings would go around the tube, therefore outside the water entirely, as they would be in the hull of the vessel. Inside the tube (and therefore immersed in the water) would be conventional inrunner type rotor - a permanent magenet with a suitable number of poles. If this were made of, say, ferrite, it would be quite impervious to sea water.

With this design, the only things immersed in the salt water would be the rotor, the bearings and supports, and the actual propeller. The windings, the steel stampings susceptible to corrosion, and the electronics, would all be dry and comfy in the hull.

An inrunner can be designed with a large number of poles like an outrunner (Astro has a new motor of this design, I believe Neumotors has a few), so direct drive is still a possibility.

My second idea is to use a bellows pump for propulsion - the bellows sucks in water from the front via an intake valve, and blows it out the back via an exit valve. Mounting the bellows inside the hull ensures the salt water is contained entirely inside the bellows itself, which should be made of plastic or other material unaffected by water. The motor setup inside the hull would simply elongate and compress the bellows - one easy way to do that would be to use a piston/crankshaft setup as used in steam and internal combustion engines - the motor goes round and round, the piston (here, the top of the bellows) goes up and down. Suitable gear reduction, would, of course, be necessary, but it is all inside the hull and does not need to be waterproofed.

What do you folks think?

-Flieslikeabeagle

[dshuen]

Thank you again for all your interests. Sorry I did not make it clear regarding underwater ROV. Here are a couple of links:
A Seabotics thruster and ROV.
http://www.seabotix.com/products/hpdc.htm
A Spyfish ROV
http://www.spyfish.com/
I think the Spyfish is using a direct drive brushless motor.
There is a water tight compartment for the electronics. The whole thing is 200-300 feet underwater. Unless the pump, drive, prop etc. has a magnetic coupler., there is always a shaft seal problem. I am not familiar with outrunner motors but I think if I can seal the whole stator, wiring and connectors with some epoxy paint I can water proof the core. The aluminum housing and magnets can be coated with epoxy paint too. Is there enough space between the stator and the magnets for the epoxy paint?

Don

[rkopka]

What if the ROV hull had a tube designed to pass through it, fore and aft, open at the front and rear (i.e. a jet duct open to the seawater). Conventional inrunner type windings would go around the tube, therefore outside the water entirely, as they would be in the hull of the vessel. Inside the tube (and therefore immersed in the water) would be conventional inrunner type rotor - a permanent magenet with a suitable number of poles.
I think it would be easier to stay with the outrunner scheme. Similar to an impeller. The coil fixed and sealed in the inner part and outside the bell integrated into the prop. Remember: you need very low kv values because of the medium.

RK

[westfw]

So you want to put the stator and all the wiring inside a water and pressure proof
compartment (which ought to be relatively easy if it doesn't have stuff going in
and out of it)? Then put the can, magnet, shaft, and bearings (none of which have
electrical connections of any sort, so can presumably be coated with corrosion
preventing stuff) out in the wet.

I was thinking of doing something like that, though I had in mind a toy to run
around my pool rather than a "real" thing.

Doesn't this boil down to how wide a gap can be allowed between the stator and
magnets? There are LOTS of candidates for housing the powered side...

So, how badly does the power output go down as the gap increases?

[ScubaSteve]

As your air gap increases your efficiency and torque go down exponentially. Can you dip the whole stator assembly into some sort of resin/sealant? Sure. I'd use as little as possible to cover as much as possible, and i would try to remove all but the smallest amount that covers the face of the stator arms. As for the rotor, you don't need to seal over the face of the magnets (they're nickel plated already), but you may want to seal the rest of the rotor to that the steel components don't corrode.

When you're done swimming, just rinse it well with fresh water and dry it completely before storing it for the next dive.

[birdofplay]

Here is a depiction of my idea. see pic please

Fill the interior of the "Shell" ( shown in blue) with some low density fluid ie oil.
That fluid will not compress or be displaced at depths
because it's a FLUID. No need for fancy pressure compensation.
No need to add corrosion prevention coverings to the motor ( shown in green).

Said fluid if/when polluted can be changed out via fill and drain ports.
shown in red.

my philosophy is to ...
try not to FIGHT nature - work with it.

[dshuen]

Steve, thanks for the air gap information. I suspected the torque will go down drastically as the air gap is increased. Will a coat of epoxy paint make a huge difference? I guess I have to get a motor to test it out. My next task is to locate a high torque low Kv motor. Any suggestion?

Birdofplay, this is how commercial thruster works. The spinning shaft will open up the shaft seal. Some oil will always leak out. An oil bladder is usually connected to the chamber to compensate the oil leak.

Don

[birdofplay]

Oh well ... Seems like I always invent stuff TOO LATE :-(

Good luck chasing Orcas in the Johnson straights :-)

[flieslikeabeagle]

Unless the pump, drive, prop etc. has a magnetic coupler., there is always a shaft seal problem.
Don
A bellows pump has no drive shaft - and no need for magnetic coupling. The bellows is a completely sealed unit, with seawater inside it, entering through one pipe wia an intake valve, and exiting through another, via a suitable exit valve to prevent backflow. The motor is in the hull and simply pulls and pushes on the outside of the bellows, making it alternately shorter and longer.

Thrust would be intermittent, of course, and you'd have to find a bellows material that will take the repeated flexing long enough to give you the service life you need, but if it works, there will be no electrical parts exposed to salt water at all.

-Flieslikeabeagle

[birdofplay]

if you want big torque motors check this thread out ...

monster stators at
http://www.rcgroups.com/forums/showthread.php?t=458898

[rpage53]

Steve, thanks for the air gap information. I suspected the torque will go down drastically as the air gap is increased. Will a coat of epoxy paint make a huge difference? I guess I have to get a motor to test it out. My next task is to locate a high torque low Kv motor. Any suggestion?
As long as your bearings can handle it, the rest should work. Just run it in fresh water at the end of each saltwater run and immediately dry it out. You can re-wind most outrunner motors to get a lower kV (more winds = lower kV). Disassemble the motor frequently to see if the epoxy has been damaged and corrossion is starting anywhere (it will).

As someone else suggested, build one and try it out and solve the problems sequentially. The ocean has lots of junk that will get in and damage the motor. I think that would be the main advantage of keeping it sealed. I assume you are trying to do this on the cheap instead of buying a commercial saltwater motor.

Good luck,
Rick.

[z-matrix]

Maeby you should think about using some N50 Neo magnets to non - contact transfer the rotating force, this will be 100% water proof, no bearing problems, put some on the motor, and a few mms away, isolated, a few at same positions,
and put the motor, and the fan in 2 isolated containers, so now, motor and other things can be in 1 hermetically sealed container.
Now you only have to worry about the bearings' corrosion.

[ImaBiggles]

Here is a depiction of my idea. see pic please

Fill the interior of the "Shell" ( shown in blue) with some low density fluid ie oil.


Its not the density of the fluid that will eat your power up through loss, it is the viscosity of the fluid.

[nstaller]

I too am interested in building brushless thrusers for my new ROV. My current one is equipped with 3 trolling motors and while they work well they are power hungry and heavy.


Maeby you should think about using some N50 Neo magnets to non - contact transfer the rotating force, this will be 100% water proof, no bearing problems, put some on the motor, and a few mms away, isolated, a few at same positions,
and put the motor, and the fan in 2 isolated containers, so now, motor and other things can be in 1 hermetically sealed container.
Now you only have to worry about the bearings' corrosion.


Its sounds like you are referring to Magneticaly coupled thrusters, they are simple in theory but difficult to build. Thats why brushless is so appealing. :D

[birdofplay]

Bananna Banahnah

Tomato Tomahtoe

Lets call the whole thing off !

[dshuen]

nstaller, you are correct magnetic coupler drive is not easy to fabricate. Trolling motor are not designed to go very deep. I am not sure if the seal is safe beyond 100ft.

I have located a Torcman motor as a kit. I am trying to figure out what guage wire to use, and how many wind etc. to get a kv of approximately 250. any ideas?

Don

[nstaller]

what sux is my motors probably wont last more than 100 feet yet my flotation is rated for 300 meters.

As far as the torcman I am still trying to learn the ins and outs of winding a motor.

[ScubaSteve]

Not to ruin the day or anything, but have you thought about how you're to control this thing thru 10 ft of seawater, let alone 100?

man i hate playing devil's advocate. :/

[Ron van Sommeren]

... I have located a Torcman motor as a kit. I am trying to figure out what guage wire to use, and how many wind etc. to get a kv of approximately 250. any ideas?Wire as thick as possible, always. Torcman has a wind calculator in their site:
http://www.torcman.de/index_e.htm
-> TM Configurator

Vriendelijke groeten ;) Ron

[Prof. Maneuver]

Interesting Design Project! Somewhere above I note that 3,000 RPM on a 3" dia prop is desired with a 250 Kv? Does that mean your target is 12VDC supply? Most empirical propellor power absorbsion equations are based on air vs water. I can't even fathom how many watts is required, thus be able to size the motor off the top of my head.

Believe you must get everything wet; nothing you can put into the airgap will withstand 300 ft water pressure without increasing the airgap to an intolerable level.. Also as pointed out above, water in the airgap (windage), not to mention drag from the rotor exterior increases the no load current but also provides infinite cooling! This should allow any motor design to operate at higher power than in air as conductive heat transfer from water can dissipate a lot more heat than air. I think you'll have more fun trying to come up with a water lubricated bearing that will fit in the same package as the stock motor design. Also don't feel constrained to an outrunner. You can get a low Kv from a large OD inrunner (i.e. Neumotors 19XX series).

For sealing the wires, use a solid conductor "bulkhead fitting" between the (3) phase motor wires and speed control (inside the vessel). To gimble the motors, you'll need something flexible somewhere. If these are flexible wires, you might have trouble with saltwater getting between the insulation and the strands.

All in all an interesting project. If you are serious and have real $$, contact Neumotors and Aveox both develop custom motors for industrial applications.

Good Luck to You,

Steve Manganelli

[nstaller]

Not to ruin the day or anything, but have you thought about how you're to control this thing thru 10 ft of seawater, let alone 100?

man i hate playing devil's advocate. :/

With a looonnngg tether :D

I am running 24v for the motors and 12v for the Cameras,Lights and Manipulator.

[nstaller]

What about a disc-style brushless?

[VWtechie]

Why not make the thruster housing the motor? Put magnets on the tips of the prop and the coils in the housing.

[RobinBennett]

> I saw some kind of "stuff" at a trade show that waterproofed anything

The hydrofoam threads had several people trying this stuff, apparently with success.
http://www.corrosionx.com/

[floss]

Interesting thread, I work for the worlds largest submersible pump manufacturer so have enjoyed the theories. The motor must of course be in a dry cylinder with a seal arrangement where the shaft exits the motor cavity. Double lipped oil seals come awfully small these days and some pumps rely entirely on 2 or 3 of these equipped with stainless springs. If your motor shaft is too small then use a sleeve to make it suit the seal size you wish to use. Salt water will eat through the wire coating in no time, even a whiff of salt air can cause a green growth on winding wire that will strip the insulation off and cause turn to turn shorts. O-rings will need to be used on any removable end-shields etc. Getting an electric motor to run underwater is very simple unless you want to redesign the wheel and make it like going to the moon from your backyard. You guys have a great lip-seal company in the USA called Chicago Rawhide or CR seals, and they are cheap. Get some aluminium, borrow a lathe and go for gold. :D

[dshuen]

I have looked into CR seals. Their smallest dual lip seal is 0.125" rated at 90psi static and 50psi at 1000fpm; so a 0.125" shaft running at approximately 2500rpm is only rated at 50psi. 50psi is only approximately 100' deep.

Even with a epoxy coating as suggested before, I am worry about the effect of sea water on the stator and copper winding too.

Don

[pem]

Hello dshuen

I do not really understand why you do not go for an inrunner with
large diameter that has the fan entirely within the water and NO
opening between the electrical partes and the inrunning rotor ?

there are still bearings within water unless you make them
magnetic but the seal to the dry cell is nowhere broken at all.

PM