Heat Transfer in Electric Motor
Heat Transfer in Electric Motor
(OP)
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.
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.





RE: Heat Transfer in Electric Motor
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?
RE: Heat Transfer in Electric Motor
RE: Heat Transfer in Electric Motor
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.
RE: Heat Transfer in Electric Motor
RE: Heat Transfer in Electric Motor
So really, how hard would it be to use the correct coefficients?
RE: Heat Transfer in Electric Motor
Thanks for the help.
RE: Heat Transfer in Electric Motor
RE: Heat Transfer in Electric Motor
RE: Heat Transfer in Electric Motor
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.
RE: Heat Transfer in Electric Motor
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.
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 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.
RE: Heat Transfer in Electric Motor
RE: Heat Transfer in Electric Motor
RE: Heat Transfer in Electric Motor