[DML] Fan current

["joekuchan" <josephkuchan@xxxxxxxxxxx>]

I just uploaded two pictures to the photos section of the DML in a 
new folder titled "Fan current". With all the discussion lately 
about what various fan designs draw I decided to find out for myself 
what stock DeLorean fans really draw. I used a calibrated and zeroed 
clamp-on DC ammeter to measure the current of each of the two fans 
in 6195. These are original, never been replaced, 26 year old fans, 
by the way.

For this test the Joe Cool was removed and replaced by a fused fan-
fail jumper and a fan relay. The ammeter was clamped first on one 
leg of the jumper then the other to measure each fan independently. 
The car was started and the AC engaged to run the fans. The photos 
show the readings with each fan at its stabilized running current. 
Note that one fan draws 9.1 amps and the other draws 9.4 amps. 
That's a total of 18.5 amps, considerably less than the 28 to 30 
amps they have been rumored to draw.

Of course a defective fan, say one with a broken blade, bad 
bearings, or shorted windings could draw more, but a normally 
functioning fan draws nowhere near the 28 to 30 amp figure that has 
been bandied about.

As for current surges on start up, yes, this is a normal occurrence 
with motor loads. Depending on the motor's design it might draw 
anywhere from 4 to 6 times it's normal no-load running current on 
start-up. This is completely normal and is the result of what is 
known as back-EMF (back electromotive force) that is generated in 
the motor's windings during startup. The 4 - 6 times rule of thumb 
applies to the "no load" current demand of a motor. The fans on a 
DeLorean, even in their steady state running condition, ARE under 
load by the way. The fan blades are moving air and it takes real 
work (i.e. current) to make this happen. So we might not expect the 
start up current of a DeLorean fan to be somewhere around 4 ? 6 
times the 9.4 amps running load that I measured on 6195. It will 
probably be somewhat less as the no-load current of the motor would 
actually be somewhat less than 9.4 amps. 

All of this is the real reason why 15 amp fuses typically won't blow 
on normally functioning fans. The fans don't draw 15 amps normally 
and the startup surge is of too short a duration for the fuses to 
blow.

The only reliable way to measure the actual current on start up 
would be to use a strip-chart recorder, storage scope, or some other 
fast-response recording type of current measuring device. Trying to 
read the swinging needle of an analog ammeter won't work for this 
job as the mass of the meter movement, frictional losses, and needle 
ballistics are such that it won't accurately follow the current 
profile. A digital meter has none of these problems, but they suffer 
from a sampling rate that is too slow to follow the current profile 
and the rapidly changing digits are hard to read in any case.

Hopefully this helps clarify the true situation with respect to the 
amount of current stock DeLorean fans really draw.

Joe




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