DC Motors in Oil 1

[mgopalan]

A project I am working on requires that we use a DC Motor in an Oil Fileld Environment. This is due to hydrostatic Pressure compensation issues. The motor that I have selected and designed into the system was tested over this last weekend and started to show some problems.

The motor was filled (flooded) with a 50 Visc Synthetic Silicone Oil and left to run with no load over the weekend. When I came in Monday, the motor had slowed down considerably AND was pulling less current (obviously not following the Torque-Current Curve). After several hours of (rather oily) labor, I figured out that the problem was not related to bearings, loads, or any other mechanical factor. Cleaning the motor and Graphite Brushes completely (with Alcohol and Contact Cleaner) solved this problem. But Im still stuck. I need this thing to work IN oil...

So heres the Hypothesis : OIL makes the motor's commutation Process erratic and unreliable. Maybe it is introducing the oil as a dielectric between the Commutator and Brushes?

Question? Is this true? The manufacturer isnt much help and says that there should be no problem running this motor in Oil. Anyone with experience with motors in Oil? Remember, sealing the motor is not an option. It has to fully flooded with oil.

Any help would be awesome.
M.G

 

[electricuwe]

Why do you use a DC-Motor? With an Induction motor fed from a VFD-inverter you can have speed control without commutator or slip ring.

 

[cbarn24050]

Hi, the oil will indeed affect the comutation process. Years ago I worked on electromechanical computer which had a constant oil spray most of the motors were ac but there were some dc servo motors as well and these were allways giving trouble even though they were shielded as much as possible from the oil.

 

[buzzp]

It is important to make sure there are no air bubbles within the oil. This can have a major affect on the life of the motor.
If cleaning the brushes solved the problem, then maybe the oil was contaminated or it was not filtered using a small enough filter. In the AC motors, we used the same oil that we filled our large transformers with. Verify that the oil is filtered, by the mfg, to meet the requirements of the motor mfg, if they will help any. Good luck.

My experience with this is based on AC oil filled motors used in oil fields for submersible pumps. The oil will create a dielectric but this should have no affect because the brushes are contacted each time it rotates. I think your problem is related to improper oil or dirty oil.

 

[mgopalan]

Some more comments

1) THere are some severe size limitations in the application and as such I cannot use Induction or other fractional Horse power motors. Basically I am limited to 1.5" in diameter maximum and I need about 500nMN of Torque at 1200 RPM, all this at 150C ambient.

2) I also cannot use AC motors simply because it is battery powered and I cannot afford the losses in the dc-ac conversion process. Plus, with the DC motor, I get Dynamic Braking which I need in this application (Turn-Stop-Brake-Reverse-Stop-Brake)

3) The oil-motor assembly was evacuated with a Vacumm pump to 65 microns then injected with oil and then re-eveacuated to remove the air in the oil itself. So I am pretty confident that there are no air bubbles of any meaningful size inside the motor. However, what the actual size of whatever air bubbles that remain is a mystery to me, but I venture a guess that they are smaller than 0.01mm, definitely smaller than my eyes can see.

4) I read somewhere that Silicone as a high dielectric, will form a film between the cummutator and the brushes basically adding a extra resistance layer and therefore slowing down the motor and also reducing the current draw. This seems to agree with my observations. Anyone have any comments?

5) Also read in another thread/post on this forum that silicone itself will severly degrade dc motor performnce, even in quantities of a few parts per million (even vapors from Silicone RTV's etc). Is this for real? If it is then, the silicone oil was probably a bad idea.

Thanks for all the responses..

MG

 

[jbartos]

Suggestion. Can the motor be filled with a different media, e.g. inert gas?

 

[electrageek]

You are using a standard DC motor that is filled with oil. Has this ever been done by anyone? I agree with the idea that the oil may be increasing the resistance of brush to commutator connection. DC motors that have their commutators inadvertently coated with oil have sparking problems and poor performance. I have never heard of this type of application very interesting problem. I assume that the problem is the need to equalize the pressure throughout the motor. This must be a high ambient pressure application. Is it possible to use a brushless DC motor?

 

[mgopalan]

The need for the oil is predicated by the very high ambient pressure. Basically the motor drives a ball screw linkage that opens or clases a small valve. However, the ambient pressures can be as high as 20000psi!! Thats the hydrostatic pressure at the bottom of a 20,000 ft oil well.

A seal is needed to keep the drilling fluids and contaminants out but this leads to a high pressure differential on the valve shaft (20,000 psi across a 1/8" shaft is about 250 lbf). Of course the motor cannot be expected to provide enough torque to overcome these loads using just a DC battery pack. So we have to pressure compensate.

The option of using air is not practical simply because air is too compressible and the volumetric change required could not be handled. Actually the oil is evacuated to remove the dissolved air to make the oil harder to compress under pressure.

At the present time, I have basically three options.
1) Try a different oil (this is assuming the problem is only due to the silicone and not the oil itself) Is it possible that another oil will work ok with dc motors? I know of several people that are using this in their products (look up 6016288 at uspto.gov)but no idea as to what they have done to overcome this problem. Ive heard that using MObil 1 Synthetic might work?
2) Go with a Brushless motor, but that would require a little more smarts to run the motor in real time. Plus, I do not know if I can brake it electrically. Also all control electronics would have to repackaged and qualified to fit in our geometry (tubular and skinny) and run at 150C. Doable but time consuming and expensive
3) Use a Magnetic Barrier coupling. I know they use these things in hazardous areas stirring devices all the time. This way, the motor and planetary gear assembly can remain in a sealed air environment and the torque is just transmitted using a magnetic coupling into the high pressure area. The limitation here is one of size efficiency. To get the barrier wall thickness to handle 20000psi would mean I need a lot of permanent magnets to carry the torque at that air gap. This would definetely solve the problem, but adds a lot of extra stuff.

Long story, but thanks for the enquiries and conversation.

 

[electricuwe]

To my mind the size restrictions do not make it impossible to use an induction motor. Of course an of the shelf motor wouldn't be suitable for the job, but I'm sure that AC-servo-motors could be found for that application.

Also regenerative breaking wouldn't be a problem in a baatery operated drive.

The problem with the AC-motor would be that it is quite difficult to design an inverter suitable for operation at 150°C.

 

[BJC]

Have you looked at the drive motors on the deep diving submarines like Alvin? They have DC motors ( or at least they are powered by batteries) and they run under pressure. It may not be the answer but they may have some helpfull information.

 

[MarkAthay]

Have you thought of filling the motor with somthing besides oil? I have seen motors filled with freon, all residential refrigerators are this way. They have some special oil mixed in for lubrication, but you wouldn't necessarily have to do that.

I think the fundamental problem you have is the carbonization on the armature. Oil, especially petroleum oils are well known for this.

Maybe I have given you some ideas that will help?

Mark

 

[jbartos]

Suggestion: It appears that any liquid will become more likely contaminated from the motor graphite brushes than when using any gas. However, some gases liquefy under 20000psi. This would leave one option viable, namely, to build the motor to withstand the 20000psi pressure. What if you happen to need a light fixture to withstand 20000psi? Will you fill it with any liquid?

 

[wsmith]

You have discovered why most electric-driven autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs) use oil compensated brushless DC motors for thrusters. One of several manufacturers of brushless DC motors that has been used in ROVs can be found at

http://www.thomsonind.com.

Another place to look to find a motor manufacturer with the right torque and form factor is in ads in Sea Technology, Hydro International, or similar marine or offshore oil and gas publication.
As noted in an earlier posting you must ensure that you get rid of all bubbles in your compensation oil, which doesn't have to be anything fancy. Telus 22 will do for a few hundred feet depth, but I don't have any experience to 20,000 feet. Given the cost (and cost of failure) of putting something that deep, I assume you don't want to scrimp on material or testing.

 

[nbucska]

Barrier is out of question.
Brushless is the only way. Braking is not difficult,
Use permanet magnet motor but check magnet for 150 C.
<nbucska@pcperipherals.com>

 

[jbartos]

Suggestion: Certainly, the brushless motor is a potential candidate; however, the series winding dc motor is usually not brushless. This means that the dc motor torque-speed characteristic is at a question.

 

[rhatcher]

did you say silicone???????????? Silicone and DC motors do not mix. I do not have any experience with a motor filled with silicone based oil, but I can confirm that silicon and DC motors do not mix. This is based on the more common case of someone using silicone sealant to seal a motor. In this case the problem arises with the brushes. Off the top of my head I believe that the silicon inhibits the formation of a proper film on the commutator and the result is high brush wear and commutation problems. Also, as you said, at this point we are talking about ppm contamination in the air.

 

[gjones33]

I suggest the problem with the oil is caused by arcing at the commutator brushes. No matter how good the commutation there will always be a certain amount of arcing especially if the motor is cycling through a start, stop, reverse, stop, start. The arcing causes a local &quot;hot spot&quot; which breaks down the oil and leaves fine carbon particles which attach through stiction to any current carrying component such as the commutator in this case.

Possible solutions are sensorless pm brushless motors with ferrite or samarium cobalt magnets to withstand the high temperature.

Cheers,
G

 

Don't use a synthetic oil. Use SAE-30 grade or better Non Detergent oil. When it flows it should be almost clear in colour, almost like a heavy baby oil.


Best Regards,
dlfarrow@surenet.net

 

[sdk_imported]

We are avoiding this problem by using a brushless dc motor with hall sensors. Does anyone know if we should be concerned about oil damaging the hall sensors or the circuit board?

 

as per my previous reply. Not only &quot;synthetic silicone&quot; degrades the performance but also any additives in the oil (detergents). The sort of oil you need should be readilly available but you may want to buy a small quantity from several manufacturers and compare the colour. There is a difference depending on manufacturer. Use the oil which is most visibly clear. Oddly enough this is often the oil which has the best price !