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[Guest]

We are having issues designing a cooling system for a large motor. The existing system uses a modified heat exchanger (radiator, using water circulation), which is inadequate. As a result the motor runs extremely hot (145-deg C winding temperature). Several trail-and-error modifications have been carried out on the cooling system without much success. The present system is closed loop, with the cool air from the radiator being sucked in by axial fans on the ends of the rotor. It is blown over the end windings: partly over the back of the core, partly through the rotor spider, and through the vent ducts back to the radiator.

The axial flow fan blades angle adjustable are so we tried to initially increase the angle so more air volume would be sucked before considering other major modification. This did not work -- the volume of air the fans would blow did not improve with angle change. There is also limited space making increasing the depths of both fans (to increase air volume) impossible.

As noted above, the motor in question does have a built in heat exchanger (water cooled) mounted on top of the motor. Unfortunately, it is ineffective despite being in good working order -- no blockages, or leaks etc. We propose to independently mount the blower and heat exchanger, the original fans will be removed to avoid interference with the air from the blower.

How can one accurately calculate the volume of circulated air required to flow through the motor. How, also, can one calculate the heat transfer in the radiator required to provide effective cooling. All motor parameters are known.

We need help with the formula for calculating the required airflow (cubic feet per minute, CFM) in this case. We must select and fit suitable fans to replace the existing ones which we are not confident are the correct type.

[#1GearHead]

Determine kilowatt loss = (hp)(0.746)(1.0 - efficiency) / efficiency.

Then, cooling water requirements = 0.5 gpm of 80 deg F water for each kilowatt of loss.
Be sure that if there are two separate coolers that the are piped in parallel, not in series.
BTW: assuming a 25 deg C ambient room temperature, 145 deg C is very high and the machine is in danger of thermal runaway.

The volume of ventilating air within the motor can conservatively be estimated at 125 CFM of 40 deg c air @ 1/2 in of water pressure for each of the above kilowatt of loss.