Heat Transfer in Electric Motor

[KllrWolf]

I am working with electric submersible motors filled with a mineral oil. The oil acts as both a cooling fluid and lubricant for the motor. The motors operate in sea water with a normal ambient rating of 30°C. The motor shell material thickness depends on the material being used, but is no more than .25" thick. The three main materials that we offer as normal options as a shell material are NiAlBronze, 316 Stainless Steel, or 6061-T6 Aluminum. Motors can range from 5 Hp to over 800 Hp in size.

My question is about doing the thermal calculations. For simplicity, we assume the surrounding water moves only due to natural convection, and the ocean is a large mass so the ambient never warms. Is it way off base to use the fact that the oil has the worst thermal transfer rate by far compared to either the metal shell or surrounding water, and therefore use the same heat transfer coefficient for a motor no matter the shell material? I do not need it to be spot on, but within about 3-4 degrees to pin down minimum dimensions for the motor.

 

[frusso110]

Had to read your problem a couple of times before I understood what was going on... Are the shell materials coated with anything on the outside that might affect heat transfer? Probably not relevant.. But curious anyway.

When you say "oil has the worst thermal transfer rate" are you referring to thermal conductivity?
What minimum dimensions for the motor are you talking about?

 

[frusso110]

I'm wondering that since the thickness is so thin, it might be a valid assumption to assume the outer surface is at the same temperature as the inside surface of the wall - thus yes, the housing material would not matter. Not really sure though. Would like someone else to comment.

 

[KllrWolf]

Sorry I was not clearer. If there is a part that is causing the confusion I will gladly clear it up.

The oil does have a lower thermal conductivity than the shell material and water.

The minimum dimensions are those such as length and diameter that produce surface area for the fluids to exchange heat.

 

[TD2K]

I wouldn't think the metal would be a significant factor in your heat transfer coefficients. what I would do is estimate my heat losses assuming the dT across the metal shell is 0. When you have a heat loss at the end of your calculations, go back and see what dT you need across the metal shell for that heat loss. That should tell you if your assumption is valid or if the metal needs to be included. I'd be interested to hear what you find.

 

[MintJulep]

The equations are the same, it's only a coefficient that would change.

So really, how hard would it be to use the correct coefficients?

 

[KllrWolf]

We have a few motors of the different materials and various sizes being built currently and will run heat tests on them. Currently we have no data on the NiAlBronze or the Aluminum as far actual numbers, just that it has worked when they tried it in the past. The reason is that I am trying to come up with something real simple that our salespeople can use to try to stop them from selling motors in too small of a frame. When I say simple, I mean it has to have as few choices as possible to keep them from getting confused. Your responses help me feel like I am on the right track, and can get better precision after testing.

Thanks for the help.

 

[KllrWolf]

All calculations came within 1.5°C of each other. Now to get the tests done to confirm the theory.

 

[chicopee]

Question, from the power input to the motor, how much of that power are you estimating to be transformed into heat removed by the oil?

 

[racookpe1978]

Is the oil (within the motor case) agitated or circulated through the motor? Or is it "static" like an insulator?

I'd think as a reasonable first-order approximation, that a circulating oil could be assumed to be steadily increasing temperature passing through/around the motor coils, then only loses heat as it goes through the oil passage(s) next to the 1/4" shell.

Then add the bearing heat load. See what effect that increased heat energy has.

Then add a assumed rotor heat load.

 

[KllrWolf]

Question, from the power input to the motor, how much of that power are you estimating to be transformed into heat removed by the oil?

I am assuming all power input into the motor that is not ouput on the shaft is heat to be removed. Thus if the motor is producing 100 kW at the dyno, but it takes 110 kW of power to achieve, I am assuming I need to remove 10 kW of heat from the motor.

Is the oil (within the motor case) agitated or circulated through the motor? Or is it "static" like an insulator?

The oil is circulated through the motor. It goes up the center of the rotor shaft from the aft up to about the forward bearing. It then leaves the rotor and flows past the forward bearing. It then flows back to teh aft end of the motor through the air gap, windings, and oil passages. Then it flows past the aft bearing and completes the circle.

I'd think as a reasonable first-order approximation, that a circulating oil could be assumed to be steadily increasing temperature passing through/around the motor coils, then only loses heat as it goes through the oil passage(s) next to the 1/4" shell.

I have been treating it as the oil is all one temperature and it loses the heat over the surface area of the inner part of the shell, which should account for heat directly lost from the laminations to shell to the outside water as that has a greater heat transfer rate than the oil. I am assuming it makes up for the areas where the oil does not circulate decently as well as the oil being cooler in areas.

 

[PHovnanian]

Do you have any values for the mass flow rate of the oil through the motor? That and its specific heat capacity should give you a good idea of the temperature rise in the oil. If it is significant, you may have to do some more detailed analysis of the heat transfers. But if it is small compared to the average temp. you can probably use that.

 

[KllrWolf]

The flow rate will depend on the size of the motor. The smaller ones may take 2-3 minutes to circulate the internal volume, while the larger circulate faster. It also depends on the synchronous speed of the motor. But thanks for that info, it does explain some of the minor variations seen so far between tested motors. Should have a major block of heat testing finished near the end of next week, and I will update as to the results.

 

[KllrWolf]

Finally got the results from the tests, so here is the basics of it as promised. Most motors were within 2°C of their calculated operating temperature (internally). The material type did not have any noticable difference on the final temperature. We did have two motors of the same size that were about the middle of the motors in terms of horsepower and size that were actually more than 10°C cool than expected. The design and materials of the motors was the same basic design as the others - both bigger and smaller - close to them in horsepower. I am looking into why it worked out that way and hope to discover the reason.