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Liquid cooling flow rates for proper cooling of an induction motor

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BigMotorGuy

Electrical
Jul 27, 2009
56
I need to determine the minimum flow rate of liquid required to cool a random wound 3 phase induction motor (150Hp, 575v). This will be a submerged pump which will divert a +40C fluid through the motor housing along with through the air gap. How can I relate the required flow per motor watt loss in order to maintain a class F temp limit? This fluid temperature is fixed and cannot be changed.
 
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You are probably into the world of CFD studies. The fluid path will be very turbulent, and the internals of a motor are not designed for efficient heat transfer to liquids.

Are you saying you want to adapt an air cooled motor and run fluid through it?


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Thanks ScottyUk.....we currently deal with submerged cryogenic motors where a much colder fluid (dielectric fluid fortunately) is pumped through the motor housing in order to cool it. Since the temps of the fluid are usually well below 0C, cooling is guaranteed without special attention to winding temps. It's trying to use the warm/hot fluids for cooling where things might get tricky. As far as the turbulence, yes, there is plenty of that, however, we employ several design changes from air motors to deal with the fluids.
 
The energy lost in the motor (1-eff)X kW rating should be converted to heat values (BTU/min perhaps)and equate this value with the heat transfer due to fluid flow around the motor casing. You need your fluid's specific heat capacity and since you already have your fluid ambient temp and Class F temp rise (135 deg.C), it should be easy for you. Your fluid velocity flowing around the casing should be equal to the volume flow you get from above calculations divided by the space/area you have allocated for your fluid to flow thru.

And yes, turbulence affects the cooling efficiency and you have to adjust for it.
 
I'm not a thermodynamics or fluids person, but here's a few things that they might consider when doing such an analysis:

As burnt2x stated, the specific heat capacity will affect the coolant's temp rise for some given amount of heat. ScottyUK pointed out that the fluid flow will be turbulent. Turbulence generally increases the rate of heat transfer across a boundary, although this effect is nonlinear and computationally non trivial. Turbulence is a function of velocity as well as fluid viscocity and, while it may improve boundary transfer efficiency, it will adversely affect the flow rate through the motor's gaps. Or require higher pumping power to maintain a given mass flow rate.

A thermodynamics person could probably dig up some empirical data comparing the performance of various cooling fluids that you can use to adjust your current experience.
 
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