I have just checked my book. The correct conversion is 746 watts per HP.
Therefore, 7.5 HP equals 5595 watts. That is still more than the juice
drawn by a 10 amp 14 volt motor (140 watts). I'd bet the generator has
even less than 50% efficiency (maybe 30%, maybe less). At that rate the
generator will draw at least 467 watts to drive the 120 watt fan. This
is 1/12 of the power drawn by the fan at top speeds.
I agree that the power consumption of the fan is non-linear with respect
to rpm. I very much doubt that it actually reaches a peak, then drops at
the upper range, but I do bet that the power draw curve flattens out
significantly.
Despite quibbles about the numbers, the take-home point is that a
mechanical fan delivers the best draw of air just when you do not need it
and sucks up the power just when you DO need it. It makes sense to use
some other mechanism to run the fan.
It is just that with a 20-something amp generator, it is quite an extra
load on the generator to add on the 10 amps of the fan. I always get
worried that my ammeter in my 4A goes slightly negative when driving at
night!!! I'd hate to have to add any extra electric draw to that! (maybe
my regulator needs regulating!)
Maybe we need to use a clutch on the fan similar to AC units. That way we
have the efficiency of direct drive of the fan, but only a little draw (I
hope) of electricity to run the clutch. Does anybody have experience with
AC unit clutches? I sure don't.
-Tony
ARhodes@compuserve.com
'67 TR4A CTC73336L
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