Electric Fans - This issue just doesn't make any sense at all....
01-15-2017 | 11:16 PM
#22
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http://www.electricaleasy.com/2014/0...-dc-motor.html
so depending on the actual type of electric motor in your Derale 18212 fans,
if the armature of the motor (i.e. fan blades) are stalled then technically yes a higher input voltage to the motor would result in higher current draw on first startup. Once the motor is spinning then current draw is reduced, and it's anyone's guess as to how the manufacturer accounts for this difference in their published numbers. It's been a while since i messed with DC motors but like article describes there's a starting circuit in most to limit inrush current, if these motors are cheap being auto aftermarket they may have no such circuit which is to stay if you physically held the fan blades stationary and fed the motor power it would be a simple I=V/R calculation and a continually higher than expected current and the motor would burn up since it expects to start rotating immediately.
I suppose, ballparking motor resistance at 0.5 ohm, that at a rated 12.0 volts it would be 24 amps, and real world at 14.5 volts it would be 29 amps.
you would need to get an accurate measurement of the electric motor resistance, and that of your electrical system. I suppose it might just be enough, if armature resistance is lower than 0.5 ohms and electrical system could be higher than you think- if you measure 14.2 volts at battery terminals but alternator could be putting out 14.6 volts at its terminal to make up for voltage drop in the system and you are seeing closer to that 14.6 volts versus 14.2 at radiator fan.
also my point of oem control starting fans in series on low speed, inherently reducing inrush current, tends to favor my memory of why they do it... because they use a less expensive DC electric motor not having any kind of starting circuit... and the 1 OEM fuse for powering both fans was 40A expecting the two fans to start in series.
so depending on the actual type of electric motor in your Derale 18212 fans,
if the armature of the motor (i.e. fan blades) are stalled then technically yes a higher input voltage to the motor would result in higher current draw on first startup. Once the motor is spinning then current draw is reduced, and it's anyone's guess as to how the manufacturer accounts for this difference in their published numbers. It's been a while since i messed with DC motors but like article describes there's a starting circuit in most to limit inrush current, if these motors are cheap being auto aftermarket they may have no such circuit which is to stay if you physically held the fan blades stationary and fed the motor power it would be a simple I=V/R calculation and a continually higher than expected current and the motor would burn up since it expects to start rotating immediately.
I suppose, ballparking motor resistance at 0.5 ohm, that at a rated 12.0 volts it would be 24 amps, and real world at 14.5 volts it would be 29 amps.
you would need to get an accurate measurement of the electric motor resistance, and that of your electrical system. I suppose it might just be enough, if armature resistance is lower than 0.5 ohms and electrical system could be higher than you think- if you measure 14.2 volts at battery terminals but alternator could be putting out 14.6 volts at its terminal to make up for voltage drop in the system and you are seeing closer to that 14.6 volts versus 14.2 at radiator fan.
also my point of oem control starting fans in series on low speed, inherently reducing inrush current, tends to favor my memory of why they do it... because they use a less expensive DC electric motor not having any kind of starting circuit... and the 1 OEM fuse for powering both fans was 40A expecting the two fans to start in series.
That explains the situation 100% right there.
These fans are not designed to be able to be run in series, at least according to the reviews of people that attempted to do so (if they knew what they were doing). That is why I went the route I went.
I am considering changing the entire thing up and having Fan 1 trip both fans low speed, and Fan 2 trip both fans high speed. Maybe doing it this way since I am starting the fans out slower they won't pull as much initial draw as opposed to going from stalled to high immediately. Plus I'm not really liking the working noise volume of these fans on high speed. A single fan on high will cool the motor from 184 when it trips on to 178 in about 60 seconds so it's not like I need the CFM that high is putting out. 2 on low should be quieter and still adequately cool the engine.
01-15-2017 | 11:37 PM
#23
Sleep on it overnight, wake up, use ohms law and do the math - and then you'll see that a device that consumes constant power will need less current with a higher voltage.
This is the whole reason why many parts of the world use 240V in houses and why the US has dual voltages in houses - so appliances can run at higher voltages and lower amperages. (Than means less wire.)
This is the whole reason why many parts of the world use 240V in houses and why the US has dual voltages in houses - so appliances can run at higher voltages and lower amperages. (Than means less wire.)
Last edited by wssix99; 01-16-2017 at 12:05 AM.
01-15-2017 | 11:43 PM
#24
I agree but why would it not blow with the engine running and alternator unplugged?? You would think that would pinpoint the alternator as the issue but its obviously charging with a system voltage of 14.2 so it can't be shorting anything. And I think if it was the alternator it would have blown when the fan was running and I plugged the alternator back in but it didn't, it just sped the fan up when the system voltage raised from the charging system.
YOur problem could be with induced current between two wires, or your short may express itself only when the system voltage is higher than battery voltage or the PCM may be triggering something only when the alternator is providing a healthy output. As pointed out above, the current should go down when the voltage comes up.
01-16-2017 | 07:24 AM
#25
you were correct,
I was looking at it from an incorrect perspective & typing too fast.
For something like his electric fan motor yes higher voltage will also mean more current draw as the motor spins faster.