If it is externally regulated alternator, I would use the Field wire from regulator
to trigger a relay to feed a high power brushless computer fan.
fans have the focusing steering
vanes on the hub support, and instead of air exiting them at wide angles in 4 hotspots of flow, there is a dense speedy narrow column of air which stays compact and dense several feet away. Its really impressive. Aimed precisely there I no need for a shroud
with such fans. Most options in the 120Mm x 38mm size format.
FFB1212EHE is rated at 3 amps, but that is start up surge, it is about 1.8 amps at 12.8v and rated at 190 cfm, and a very high static pressure rating for that cfm.
It is my current
favorite 120mm fan and I have tried many. At 14.7v it is equal to the 252 cfm version(Delta TFC1212DE) at 12.7v, which does not respond nicely to voltage as speed control, shutting off at 7v while still consuming close to 2 amps and being incredibly loud and powerful.
I just got some 140mm Delta fans, the 24v version 51mm thick, is a beast among beasts. @ 26v it sucks up about 50 watts, and it slows perfectly to a silent crawl and consumes about 0.15 amps at 4.4 volts through the bucker. New favorite!
These 12v Delta fans are LOUD when Fed 12 to 14.7 volts but most can be tamed nicely via voltage. I use XL4005 or XL4015 based 5 amp voltage buckers with great success as speed control, the 4005 drops 0.27v across it the 4015 drops 0.18v. You can dial in a desired airflow/noise level/amp draw via them. Some of these come with current
limiting pots too. I've not had good luck with these, longevity wise, and recommend against them.
Not much use for lowering the speed of these high rpm
Delta fans in alternator cooling duty I guess. I do recommend covering the wire entry into the hub with some dielectric grease or similar and spraying the circuit board with Deoxit shield s5 spray with the red hose getting inside the guts. I've not had one of the Delta fans fail from corrosion
, but almost all fan failures of other brands have been caused by it.
Spinning a 11 blade external fan alternator at ~2500 rpm
for truing the slip rings, I was amazed at just how little air the fan moved radially. Its a tiny fraction of these computer fans. One of these high power delta computer fans forcing all its flow through a shroud
into the back of the alternator, would be 10x more effective than the external fan, in my opinion
A lot of the Deltas are PWM 4 wire fans. I have successfully controlled a few of them via the PWM 4th wire, but they use more juice than using a voltage bucker as speed control at maximum and minimum speeds, so I just use Buckers, or in the case of some 24v fans, buck/boost converters.