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

 

[gjones33]

Hi sdk

I suggest you look at our website.

http://www.sensorlessdrives.com

 

[MarkAthay]

As for oil, maybe you should try a good mineral oil. Transformer oil, which is just your basic mineral oil, is used all the time with electrical components. It's not so much a lubricant as an electrical insulator. It's cheap, pure, and works well.

Where no brushes are involved, maybe a good silicone oil would work well too?

Mark

 

[jbartos]

Suggestion: Please visit

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.

 

[gjones33]

Hi sdk,

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

 

[wvbht]

Mgopalan:

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.

 

[jbartos]

Suggestion: How about posting the cooling liquid problem under Chemical Engineers in this Forum?

 

[IRstuff]

Two thoughts:

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

 

[purplepete]

I believe the US Navy was amongst the first to identify that silicon compounds had a disastrous effect on DC motor commutation, for reasons as discussed above. A company here in Oz just a few years ago had a serious problem after silicone compounds used as a freeness agent in the wet end of a paper machine and vented to the building exterior (in the moist air) found its way back into the DC motor cooling air intake. The results for the 600 kW motor commutators was disater.

 

[mikeee]

Hi

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.

 

[mgopalan]

Gentlemen,

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.

 

[jbartos]

Comment to the last posting: Please, notice that the option "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." allowing the motor stay "in a controlled environment" was not stated or apparent in the original posting.

 

I am controlling only speed using an IC MC 33035 of Brushless DC motor. I am also using hall effect DCBUS current sensing for current limit. Any idea
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.

 

I am controlling only speed using an IC MC 33035 of Brushless DC motor. I am also using hall effect DCBUS current sensing for current limit. Any idea
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.

 

[electricpete]

Mr Dynamic Braking - could you please post this as a separate thread?

 

[jbartos]

Suggestions:
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