12-11-2009, 10:33 PM
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#1 (permalink)
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EcoModding Lurker
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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
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12-11-2009, 11:24 PM
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#2 (permalink)
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...beats walking...
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...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.
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12-11-2009, 11:36 PM
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#3 (permalink)
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EcoModding Lurker
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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
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12-11-2009, 11:39 PM
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#4 (permalink)
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...beats walking...
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...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)
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12-11-2009, 11:50 PM
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#5 (permalink)
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EcoModding Lurker
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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.
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12-12-2009, 12:03 AM
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#6 (permalink)
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...beats walking...
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...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.
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The Following User Says Thank You to gone-ot For This Useful Post:
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12-12-2009, 12:38 AM
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#7 (permalink)
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EcoModding Lurker
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Thanks OTM!
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12-12-2009, 01:20 PM
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#8 (permalink)
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Master EcoModder
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Fan efficiency is a very complex topic.
Quick Shot review 11: Thermaltake Extreme Volcano 12
Quote:
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Cooling fan aerodynamics can get complicated. Swept area, blade profile, static pressure, free-air flow, RPM limits for acceptable noise levels; a lot can go into the design of a modern pentametric fan. The way the equations shake out for most 80 and 92mm computer cooling fans, though, means they have seven blades. Generally speaking, fewer blades mean higher efficiency; the same goes for propellers, which is what gave rise to the counter-weighted single bladed props used by some very strange aircraft. But it's my no doubt criminally over-simplified understanding that dropping a normal cooling fan design to, say, three blades, would mean you'd have to run the blades so fast to get the same air flow as a less efficient 7-blade fan that the resultant device, while efficient, would make too much noise and/or wear out too fast.
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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.
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