[DML] Re: Fan current

["spaceboy_2912" <seanmm@xxxxxxxxxx>]

Joe posted this at DMCTalk and there is some excellent discussion
being had off of this post over there:
http://www.dmctalk.com/showthread.php?t=7315

Some good numbers and scenarios are be discussed!

Sean Mullins
DMCTalk Moderator
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--- In dmcnews@xxxxxxxxxxxxxxx, "joekuchan" <josephkuchan@...> wrote:
>
> 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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