Calculation for fan draw?
 

I did a simple measure of how much voltage drop the battery experiences with my cars fan:

No fan - 14.45V
First setting - 14.40V
Second setting - 14.35V
Third setting - 14.31V
Fourth setting - 14.27V

I have this data but don't know how to apply it for determining power drain. The car is an '09 Focus. I tried googling to find alternator specs but was unsuccessful. On older Focus's the alternator was 105amps. I would suspect mine is probably close to this but even if I use this value, isn't this only true at high rpms?

Would the equation Watts = Volts x Amps be applicable or is there a change of amps with rpm?

I also wonder if I can further use this to calculate horsepower drain by Hp = (Volts x Amps x Efficiency) / 746 But I don't know what to use for efficiency and I once again wonder if efficiency changes with rpms?

Anyway, if some of you could help me make some useful information from the voltage measurements ... I'd appreciate it!

Doug

...you gotta know the AMPS and VOLTAGE being used by the fan, not what the alternator is rated at.

...take both a VOLTAGE and AMPS reading...that'll give you the wattage at that setting, then divide by 746Watts/HP and you'll have a HP value at 100% assumed efficiency.

Okay, I was hoping that somesort of assumption could be made but getting a direct measure makes sense. I also appreciate the comment that 100% efficiency can be interpreted for the simple equation.

Thanks!

Doug

...DC motors are fairly efficient (~85-95%) but their fan blades are NOT! So, total electrical power (HP.e) will be greater than the true mechanical power (HP.m) needed to move a given volume of air...that is, you're gonna put "in" more HP than you effectively get "out," ie:

efficiency = Power OUT / Power IN = (HP.m / HP.e)

Well, even if the 100% efficiency is off, the calculated power ratios from no fan to maximum fan should be correct even if the calculated power usuage from efficiency is a bit off. My concern would be more if there was a signficant variability with rpms, say ... 45% at 1500rpm to 95% at 6500 rpm.

Do you think it would be safe to assume fairly equivalent efficiency value across the full rpm range?

Doug

Edit: negate that, I re-read your post and find I'm confusing the ICE with the fans motor. This is correct isn't it - the efficiency parameter is in consideration of the electrical motor.

...a fictious example: it'll take 100HP of electrical power to "blow" 80HP of mechanical air movement.

...the biggest "loss" is due to the fan-to-air coupling, not the motor-to-fan coupling.

Thanks OTM!

Fan efficiency is a very complex topic.
Quick Shot review 11: Thermaltake Extreme Volcano 12
I actually have a small inverter drive fan with 3 blades, probably originally designed for computer cooling. I use it as a desk fan during the summer since it is very efficient.

The power draw is less than the rating of the blower motor fuse / relay.

http://ecomodder.com/forum/showthrea...olts-8908.html

MetroMPG's Metro is much thriftier than most cars, so your fan probably draws more power than his.

http://ecomodder.com/forum/showthrea...aseo-8767.html