DC Motors in Oil
DC Motors in Oil
(OP)
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
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





RE: DC Motors in Oil
RE: DC Motors in Oil
RE: DC Motors in Oil
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.
RE: DC Motors in Oil
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
RE: DC Motors in Oil
RE: DC Motors in Oil
RE: DC Motors in Oil
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.
RE: DC Motors in Oil
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.
RE: DC Motors in Oil
RE: DC Motors in Oil
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
RE: DC Motors in Oil
RE: DC Motors in Oil
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.
RE: DC Motors in Oil
Brushless is the only way. Braking is not difficult,
Use permanet magnet motor but check magnet for 150 C.
<nbucska@pcperipherals.com>
RE: DC Motors in Oil
RE: DC Motors in Oil
RE: DC Motors in Oil
Possible solutions are sensorless pm brushless motors with ferrite or samarium cobalt magnets to withstand the high temperature.
Cheers,
G
RE: DC Motors in Oil
Best Regards,
dlfarrow@surenet.net
RE: DC Motors in Oil
RE: DC Motors in Oil
RE: DC Motors in Oil
I suggest you look at our website.
www.sensorlessdrives.com
RE: DC Motors in Oil
Where no brushes are involved, maybe a good silicone oil would work well too?
Mark
RE: DC Motors in Oil
http://www.loher.de/en/products/motors/6_19.htm
http://www.offshore-technology.com/contractors/pumps/hayward_tyler/index.html
for deep sea motors
It appears that a motor for 20000ft would have to be custom-built.
RE: DC Motors in Oil
Our drives can drive standard stepper motors as both stepper motors and efficient, sensorless, high speed, high torque bldc/servo motors. Our drive is a lot simpler and a smaller package than bldc hall effect drives and does not use pwm or variable supply voltage. Dynamic braking (not shorting motor) and reversing on the fly are standard features.
Another advanatge for your application is that a very wide range of stepper motors are readily available including motors designed for high pressure applications by specialist manufacturers.
Cheers,
G
http://www.sensorlessdrives.com
RE: DC Motors in Oil
I think that your problem with the silicon oil is that the silicon oil breaks down during the normal arcing at the commutator. The silicon (and silicon oil vapors, too) break down and deposit silicon (glass) on the commutator. The glass deposit will build up until the proper contact is lost.
This will also affect the operation of relay contacts in an environment with silicon oil vapor.
I come from a telephone company background and there was an engineering bulletin sent out about this problem, mostly with relay contact malfunction.
From this standpoint, silicon oil is the worst possible choice.
You could consult the silicon oil manufacturer to see if this is the case.
I see adds for heatsink compound that tout being "silicon oil" free.
RE: DC Motors in Oil
RE: DC Motors in Oil
1. The simplest answer is probably the best, e.g., the oil is simply increasing the resistance between the brushes and the commutator. Note that oil, used for lubrication, fills in the surface pits, thus further reducing the ability of the brushes to make good electrical contact with the commutator.
2. The oil soaking into the graphite brushes and then experiencing heat expansion will further degrade the contact resistance. Graphite brushes are essentially sintered together, thus depending on intimate physical contact between grains to keep resistance down. But, sinters are less dense, allowing oil to fill voids. Since oil, like water is relative incompressible, oil expansion will cause the graphite grains to separate, thus significantly reducing the resistance.
TTFN
RE: DC Motors in Oil
RE: DC Motors in Oil
I work in the underwater vehicle industry, this is a problem we had 20yrs ago. The main problem here is with the aqua-planing of the brushes (not the die-elctric effect). Most subsea motors are brushless these days. There are a few manipulators that use commutators but they are used intermittantly only.
i suggest you move to a electronically commutated design.
RE: DC Motors in Oil
Thank you very much for the support and comments. As suggested, I tried mineral oil and several pure synthetics to try and solve this problem all to no avail. The consensus here at our facility is that the oil is interfering with the electrical contact between the brushes and the commutator leading to a purely electrical problem as opposed to any mechanical problems.
The solution? Well, I went to a magnetic barrier coupling that withstands 20000 psi differential between the linkage and the motor and carries 32 Oz.In of torque. The solution works great and this allows the dc motor to remain in a controlled environment while allowing us to use an "oil" with excellent lubrication properties for the pressure compensation.
The final result? A 350% increase in force capacity with a 45% decrease in power consumption!
I'm pleased..
M Gopalan.
RE: DC Motors in Oil
RE: DC Motors in Oil
how to implement current limited dynamic braking? I have limitation in using current sense resistors in dcbus return path or in any phase. During normal braking, all three lower IGBTs are shorted and Kinetic energy of the motor is dissipated in the motor which heats up the motor.
RE: DC Motors in Oil
how to implement current limited dynamic braking? I have limitation in using current sense resistors in dcbus return path or in any phase. During normal braking, all three lower IGBTs are shorted and Kinetic energy of the motor is dissipated in the motor which heats up the motor.
RE: DC Motors in Oil
RE: DC Motors in Oil
1. Check the controller manufacturer for application notes
2. Visit
ftp://ftp.maxonmotor.com/Public/Download/catalog_2001/pdf/01_033_e.pdf
ftp://ftp.maxonmotor.com/Public/Download/catalog_2001/pdf/01_223_e.pdf
for different electronic controls
3. Visit
http://www.msha.gov/S&HINFO/PAPER3.HTM
for theory