Water proffing of thruster


#1

Its really a good work done by OPEN ROV team!

My query is on thruster. After making a electrical insulation on thruster wires..still copper wires(coils) are exposed to water..don't they really short when it is in water?


Rov Thrusters and motor questions
#2

the copper wires have a thin layer of coating, so they withstand the water.


#3

If you're in salt water, you'll want to be diligent about spraying (and coating) the motors with silicone mold release prior to your dive, and getting the motors a fresh water rinse afterwards.


#4

Does dipping on liquid electrical tape works?


#5

I treated my engines with liquid rubber. (plasti-dip)


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#6

So we need not worry about the coil even in saltwater?...Plastic dip has to be done only for the outer body not for the coil(copper)?


#7

I have dealt with only the metal parts.
I fear the copper wires will be damaged by the solvent.
The inside of the engine I have treated with silicone spray.


#8

I’m a bit confused, is this a spray silicone (like bike chain lubricant) or silicone mold release spray (kerosine,etc. for releasing mold from original positive)? I thought these were different substances (and I already have chain lube).


#9

Hi,

after having some trouble finding electric tape initially, I've tried to water-proof the motor solderings with the type of spray-on rubber (Plasti-DIP) which Holger mentions in his post.

It did not work too well and took some attempts until the soldering looked "covered". So to be on the safe side I've tried to measure insulation before actually assembling the ROV.

What I did was:

use a digital ohm-meter, put 1 end into a small bucket of water, put the motor into the water as well and connect the second end of the multimeter to one of the motor wires. I had expected to see inf ohms, however it showed some 300 KOhm ... looks BAD, doesn't it?

I finally bought some electric tape, removed the rubber as good as possible and applied the electric tape in 2-3 coatings (..at least it looks good now) - however I still can measure some 800K-1MOhm.

Am I overlooking something obvious or did the rubber-solvent maybe really ruin the copper insulation of the motor?

Has anyone performed a similar experiment in measuring insulation? What readings?

Thx for your insight!


#10

Hi,

Just a thought, has anyone tried stripping the motor to component parts and encapsulating the windings/stator in an epoxy? Using a lathe, the excess epoxy could be machined down, allowing the motor to be reassembled, thus protecting the windings from saltwater corrosion?


#11

Hey all,

This is a really good discussion! ...and I'm very interested in seeing the results of a bunch of methods tried out.

Here at OpenROV HQ we've been trying to get an idea of how bad salt water effects are on the brushless motors we use, so we started a little experiment which can be seen here:


...Alright, by showing this big fancy picture you probably think that our "experiment" is something elaborate involving quantitative measurements of performance tested against baselines, etc. Really it's just a cup with some seawater and a few motors in it.. but I'll get to all that later. Before I do, here's my experience and train of thought with the brushless motors so far:

We've flown OpenROVs with stock brushless motors in open ocean before, and they've generally done pretty well- especially if we take care of them before and after dives, but over time they do seem to deteriorate.

There are two main concerns with exposing the motors to water (especially salt water) that should be considered:

1. Electrical Shorting

2. Corrosion

... There is also sort of a #3 which relates to particles getting in-between the motor bell and staters causing abrasion on the system, but that seems to be a slower, less severe process so I'm going to table it for now.

1. Electrical shorting- since the magnet wire of the motor is completely insulated with enamel, the only exposed conductors that need to be insulated from water are the three solder joints between the magnet wire and the motor leads. We used to use several coats of liquid electrical tape to insulate this region with a layer of shrink tubing over that to keep it from getting scraped off over time, but it seems that even with multiple coats, some areas miss being covered and some small amount of leakage current can be detected. We now use a special marine shrink tubing that is lined with a sort of hot-melt adhesive. This stuff seems to work really well, and makes the process of attaching leads to the motor much easier.

2. Corrosion- This seems to be a bit more of a tricky problem. The brushless motors we use are not built to be around water, so the bearings, stater frame, and set screws are all made out of metal that corrodes (and of course, this process is hastened if there is any leaking electrical current from a bad insulation job on the motor). Also the neodymium magnets on the motor bell are very prone to corrosion wherever the metal plating that covers them is worn off.

In my mind, I break corrosion issues into two categories: things that directly effect motor performance right away, and things that are mostly aesthetic and do not have significant effect on motor performance until they become severe.

The only part of the motor that seems to be in the first section- things that directly effect motor performance right away- is the bearing set for each motor. There are two frustrating things I've noticed relating to bearing corrosion: (1) The ball bearings themselves corrode which makes them chatter and have a harder time rolling (which takes torque away from the motor), and (2) the inside racetrack oxidizes to the motor shaft which makes it very hard to remove the motor bell from the motor.

Recently, Colin Ho and I found some stainless steel bearings that would fit in our brushelss motors (frustratingly, the bearings on the top of the motor and the bottom of the motor are a slightly different size, so you have to buy two types), and we had a few shipped to us to try out.

Here's where the little experiment in a blue cup comes into play:


We placed one motor with stainless steel bearings and one motor with normal bearings in a cup filled with sea water we got from San Francisco Bay for a week, and this is what they looked like afterward:



Clearly there is some corrosion on both parts of each motor, but after a week of salt water submersion, I have to say, things didn't seem as bad as I had suspected. What was even more surprising to me- both the stainless steel and regular steel bearings seemed to work just fine! They both seemed to spin easily, and I suspect that if I had run the motors, I would hardly be able to tell the difference in performance.

It occurred to me that there were two things missing from this experiment that would increase the speed of corrosion, and also simulate more accurately the conditions the motors would normally experience while mounted onto an ROV: I need to bring the motors in and out of the water to be exposed to air frequently, and I need to run current through them.

I haven't had the time to make an electrical test stand for the motors this week, but I have started taking them in and out of the water every day, so I'll be sure to report back about what happens after another week of torture!

As for the second type of corrosion issue- things that rapidly become bad aesthetically, but only start effecting performance when they get severe... things like corrosion on the motor bell magnets, and stator frame, I think you guys have exactly the right approach: There should be some sort of very thin but resilient coating that can be used to further protect the parts without interfering with how the motor works.

For some reason, what keeps coming to mind is wrapping the whole darn inner part of the motor with wax paper and potting it into one big cylinder of E-90, save for the holes that the bearings go in, then making a plug that fits into the motor bell and potting around it's magnets in the same way.

Whenever I think about what's entailed with doing this, the lazy part of me chimes up and points out that the motors are only $10 a piece, and that spraying with silicon spray or WD-40 before dives, and rinsing with fresh water after dives will do just as well.

More tinkering is certainly needed in this realm, so let's keep brain storming. In the mean time, things seem to work well enough as is in my experience, but "better" is always better!


#12

Very cool! How does this hold up after a lot of use??

E


#13

Excellent post Eric. It seems to me that the biggest obstacle (both cost and technology wise) for the typical DIY ROV seems to come down to its thrusters. Several years back the topic of using outrunner RC brush-less motors came up on the Yahoo RobotROV group. Outrunner motors have lots of torque and are cheap so are an attractive option. One method to protect them that was discussed was to use a "dielectric epoxy" to coat the motor's components. I never tried it myself, but some reported that it helped a lot.

A couple other thoughts:

As I'm sure you are already aware, non-galvanic corrosion requires the presence of oxygen, so I wouldn't be surprised if phase two of your experiment exhibits a lot more corrosion. The addition of an electrical current may also increase the likelyhood of galvanic corrosion (electrolysis) as well. I don't know if such a thing is available, but have you considered using non-metal bearings, ie. ceramics or plastic?

Also, you probably don't want to use "WD-40" as a lubricant, it's petrolium based and not particularly enviro-friendly. My vote would be one of the many dry, silicon-based lubricants.

Keep us posted, this is really interesting stuff! :)


#14

One thing that I do as a best practice is to also fill the void between the two bearings with white lithium grease. I also put some on the outside of both bearings in the hopes that it will get forced into the bearings before the saltwater gets in.

I suspect water will sit in this cavity between the two bearings and continue to erode the bearings when the ROV is out of water. with the lithium grease in there it should reduce this issue.


#15

EDIT: just noticed this post went in the wrong place- somehow I reread the inital post asking about the magnet wires seeming to exposed copper.

****

Nope! Although they look like exposed copper wires, they're actually covered with a sort of varnish like insulation that keeps each winding from shorting with the windings next to it. If you were to take one of these wires and sand it, you'd notice that beneath this layer, the actual copper is brighter.

E


#16

That's a great idea- I'm going to try that out when I get back to the lab. Thank's, David!


#17

On commercial ROVs they use AquaShield as lubricant and corrosion protection for exposed gears and bearings.

http://www.marinesan.com/Aqua_Shield_Grease_s/239.htm

i've used this on underwater applications at work with fairly low rpms(20rpm) and the grease has worked very good.

it has a rather "slimy" viscosity, but if one remove the sealings on the bearings one can fill the bearings and cavity by pushing the noozle of the tube onto the bearing on one side of the motor and squeeze the grease trough the bearings and cavity.

one could also cover both the rotor and stator of the motor in the grease.

one downside: particles stick to the grease!


#18

How about using a sacrificial anode?

A small piece of Zinc ( or something else higher up the galvanic series than the metal you are trying to protect) on the motor frame (or elsewhere on the ROV but will require some wire to provide a path for electrons) should provide some protection.


#19

Good point Owen,

3.1/3.2U?

This technique is used extensively in the marine construction industry to protect structures.

Using a multimeter and "half cell" or Buckleys CP meter ;) you could measure how effective it is, (i bet everyone has one of those tucked away in their workshop)

Assuming that there is an electrical path between the spindle/mount & stator, then a continuity strap/conductor could installed from the spindle/mount to an anode.... Good idea, easy to replace when depleted!


#20

3.2u

:slight_smile: