Heat transfer on Large Electric Motor windings.
2

Question

Heat transfer on Large Electric Motor windings.

r2d2 (Electrical)

(OP)

15 May 00 15:18

I have a problem of designing a cooling system for a Large Electric Motor. The existing system involves a modified heat exchanger(Radiator type with water circulation) that is inadequate. As a result the motor runs extremely hot (130 Deg. Celcius winding temperature). Several "hit and miss" modifications have been carried out on the cooling system without much success. The present system is a closed loop, with the cool air from the radiator being sucked in by axial fans on the ends of the rotor, blown over the end windings, part over the back of the core and part through the roror spider and through the vent ducts back to the radiator.

Please advise a good proven method of calculating the volume of air required to be circulated through the motor and the heat transfer in the radiator required to provide effective cooling. All parameters of the motor are known.

motorman (Electrical) · Answer 1 · 2000-05-16 15:46:06

determine kw loss = (hp)(0.746)(1.0 - efficiency) / efficiency.

then, cooling water requirements = 0.5 gpm of 80 deg F water for each kw of loss.
make sure that if there are two seperate coolers that the are piped in parallel, not in series.
comment: assuming a 25 deg ambient, 130 deg C is very high and machine is in danger of thermal runaway.

r2d2 (Electrical) · Answer 2 · 2000-05-18 13:59:03

Thank you for your helpful formula "motorman" it was most appreciated. Could you possibly help me with the formula for calculation of the required airflow (cubic feet per minute, or cfm) in this case. I will have to select and fit suitable fans to replace the existing ones which I am sure are not right either.

motorman (Electrical) · Answer 3 · 2000-05-23 14:14:38

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 kw of loss.

r2d2 (Electrical) · Answer 4 · 2000-05-24 14:16:56

Thank you once again for your help it was most appreciated. Preliminary calcs on the existing system so far suggests the cooling capacity of the heat exchanger is inadequate.

bozz (Electrical) · Answer 5 · 2000-06-10 02:39:18

Are the axial flow fan blades angle adjustable...if so you may initially increase the angle so as more air volume being sucked before considering other major modification.

motorman (Electrical) · Answer 6 · 2000-06-14 15:14:34

question: is the adding of this water cooler a "field modification"?

r2d2 (Electrical) · Answer 7 · 2000-06-15 09:55:40

Thank you for your suggestion "Bozz" this was considered at the initial stages of modification, but did not prove effective from the volume of air the fans would blow even with angle change. There is also limited space making increasing the depths of both fans(to increase air volume)impossible.

Yes "Motorman" the adding of the water cooled heat exchanger and auxiliary blower is a "field modification". The motor in question does have a built in heat exchanger(water cooled)mounted on top of the motor, that is ineffective, even though it is in perfect 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.

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Heat transfer on Large Electric Motor windings.
2