Feasibility of an underwater DC motor

I have this idea for making a long-endurance underwater dc motor but I’d like for you guys to verify the feasibility of the design and give some suggestions for change if needed.

The design is attached

*Not to scale, it’s just a diagram
*And the white circles on the wheel’s front view are supposed to be magnets

Picture1.gif

I suspect it's a case of "suck it and see". It shouldn't be difficult or expensive to try.

It occurs to me that if you just had a series of electromagnets inside the container and used the Arduino to energize them in sequence (like a stepper motor) you wouldn't need any moving parts inside the container.

By the way regular DC motors work perfectly well when submerged in petrol/gasoline so, perhaps, they would also work submerged in water.

...R

Well that really made things a lot easier

-BTW I tried to use a dc motor underwater before and it short circuited miserably considering that the motor did not turn and the battery went dead shortly afterwards

In the diagram it says “inside”. Does inside mean inside a vessel open to the atmosphere or inside a closed submersible. If it means inside a submersible where does the power come from and how are commands sent to the arduino?

I tried to use a dc motor underwater before and it short circuited miserably considering that the motor did not turn and the battery went dead shortly afterwards

Salt or brackish water would do that, fresh water would generally take a lot longer.

I tried to use a dc motor underwater before and it short circuited miserably considering that the motor did not turn and the battery went dead shortly afterwards

The battery and the motor were not actually submerged in water were they?
The short circuit would kill the battery but why was there a short circuit?

Whirling strong magnets around in close proximity to an Arduino to it might give you inductive problems. If necessary, you could enclose the electronics in a conductive screen. You might also need to deal with inductive noise picked up by the wiring - the best way to find out is to try it, just bear in mind that the magnetic coupling is not the only part that you need to test.

Look at how a brushless motor is constructed. You could possibly build the coils in a waterproof enclosure, use water-lubed bearings, and put the rotor in water. Would be interesting, anyway.

The design you have is similar to the small chemistry mixers.

Robin2:
By the way regular DC motors work perfectly well when submerged in petrol/gasoline so, perhaps, they would also work submerged in water.

You sure of that?

Grumpy_Mike:

Robin2:
By the way regular DC motors work perfectly well when submerged in petrol/gasoline so, perhaps, they would also work submerged in water.

You sure of that?

Lots of people have built home made underwater ROVs with just regular DC motors. Most of them worked until corrosion jammed up the bearings. People have also experimented with escaping submerged cars, and found that the window motors still work with the entire car under water. The way its always been explained to me is that the high resistance of the water keeps a short from happening. As long as your controller is above the water, the "path of least resistance" is always along the wiring for the motor, keeping it from shorting out. Robin is probably referring to vehicle fuel pumps, which are entirely submerged in gasoline (even the wiring harness) with no special water proofing. The ground wire is usually just a crimp-on spade connector.

the “path of least resistance” is always along the wiring for the motor, keeping it from shorting out.

The path of least resistance argument is a bit of a myth. When you have a parallel circuit current flows down all paths not just the one with the lowest resistance.

the high resistance of the water keeps a short from happening

While pure water has a very high resistance, in fact it is an insulator, the slightest amount of dissolved salts cause a very rapid and marked reduction in resistance. So in practice the resistance of water is quite low as any health and safety officer will tell you.

Robin is probably referring to vehicle fuel pumps, which are entirely submerged in gasoline (even the wiring harness) with no special water proofing.

I would have thought that if the pump ever became only partially submerged the spark from the brushes would have a dramatic effect. But I am only guessing on this last one, but it seems a bit incredible.

So I'm sticking with the step-up motor idea, although I wonder if the water will provide enough lubrication for the moving parts ( assuming the parts are aluminum or steel and the water is freshwater ). Also, does anyone have a clue of about how long the parts will last before they corrode or wear out from friction.

If you want to make a proper design, you should buy a "stern tube". It lets you put a shaft through without water running in. They are quite inexpensive.
One could worry that if you made a motor through the container you would loose most of your power, remember that the magnetic force travels 1000 times easier in metal than in air.

MrBear:
If you want to make a proper design, you should buy a "stern tube". It lets you put a shaft through without water running in. They are quite inexpensive.
One could worry that if you made a motor through the container you would loose most of your power, remember that the magnetic force travels 1000 times easier in metal than in air.

Or just buy a bilge pump and pull the motor from that.

Grumpy_Mike:

the "path of least resistance" is always along the wiring for the motor, keeping it from shorting out.

The path of least resistance argument is a bit of a myth. When you have a parallel circuit current flows down all paths not just the one with the lowest resistance.

And 'all paths', in this case, include the whole body of water, not just a staight line. Millions of paths, all in parallel, through the water will almost always be the path of least resistance.

Part of London was flooded this morning by a burst water main. The fire brigade had to call in the electricity supplier to disconnect the electricity before they could go in to help, as the water could have been live.

I have this idea for making a long-endurance underwater dc motor but I'd like for you guys to verify the feasibility of the design and give some suggestions for change if needed.

So what is the design function of the motor and what type of environment will it need to operate? Most efficient would probably be to have the motor operate in some type of inert gas environment and use a low friction shaft seal arrangement between the motor area and the outside environment. How are electric trolling motors made?

A similar design of a motor like OP’s:

http://www.homebuiltrovs.com/magneticcoupler.html

(actually, that entire site is a great reference for ROV design ideas)

Grumpy_Mike:
I would have thought that if the pump ever became only partially submerged the spark from the brushes would have a dramatic effect. But I am only guessing on this last one, but it seems a bit incredible.

It actually wouldn’t; in a sealed gas tank, there isn’t enough air in the space for the correct fuel/air ratio to ignite. In the case of an in-tank fuel pump, the gasoline actually lubricates the motor (which is why you don’t want to run your tank dry on any vehicle with such a pump, because you could ruin the pump and be looking at a fairly expensive repair bill). In fact, running the tank dry is the extreme of what you are describing, GM - a completely empty tank, with just fumes. The motor will run (and eventually seize) - but the tank won’t suddenly explode (again, wrong fuel/air ratio). Remember, for an engine to run, you need three things: spark, fuel, and air (and those last two have to be in the correct proportion for the engine to run right, if at all).

Henry_Best:

Grumpy_Mike:

the "path of least resistance" is always along the wiring for the motor, keeping it from shorting out.

The path of least resistance argument is a bit of a myth. When you have a parallel circuit current flows down all paths not just the one with the lowest resistance.

And 'all paths', in this case, include the whole body of water, not just a staight line. Millions of paths, all in parallel, through the water will almost always be the path of least resistance.

Part of London was flooded this morning by a burst water main. The fire brigade had to call in the electricity supplier to disconnect the electricity before they could go in to help, as the water could have been live.

Theres a bit of a difference between 12v DC and 100,000v AC. Mains voltage can arc through the air if the conductors get close enough, so of course water could be a problem. Also, i'm fairly certain copper wiring has a lower resistance than water. The resistivity of copper is 1.68x10^-8, while drinking water is 2x10^1 and sea water is 2x10^-1.

It's quite normal to run small DC brushed model motors under water, but the drag of the water saps the power and the water also causes accelerated brush wear. I don't know what percentage of the current leaks through the water but with ordinary tap water, ponds, puddles etc I've never noticed any significant outgassing which makes me suspect that it isn't a large amount. I've never seen it done with salt water, but I'd suspect that drag and corrosion would be more of an issue than electrical leakage. I don't know whether a shaft seal is practical in this application but it would seem like the best option if it is.

I am confused here. If you look at the diagram in the original post there is an "inside" and an "outside". The DC motor is "inside" and is magnetically coupled to the "outside". The Arduino is also "inside". It seems clear to me that while the whole device may be submerged "inside" is dry. The purpose of the magnetic coupling is to avoid any breach between the wet "outside" and the dry "inside" - or have I missed something?