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Discussion Starter · #21 ·
As the voltage to the fan drops the fan slows down, uses less power (watts), and draws less current (amps)

As the voltage to the fan rises the fan speeds up, uses more power, and uses more amps.

This is only relevant to your problem with blowing the fuse. You are more likely to blow the fuse when the alternator is charging the battery.

IMO, a smaller alt pulley will still fix your stalling/discharging problem. Even though the fan draws more when the alternator is charging the battery. Your engine rpms will still drop less with a smaller pulley because the alternator will charge much more efficiently at idle and take less engine power to turn.

If you have a one wire alternator like me you might barely be charging at idle right now. One wire alternators are triggered by rpm. There’s a reason they don’t use one wire on anything but tractors anymore.
Thanks Knorm I will give it a try. Actually I don't have a fuse blowing problem now that I went from 30 to 40 amp fuse. If this doesn't help I'll have to change the fan to one with a much lower draw. Thanks for everones input eh.
 

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Opel Rallier since 1977
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IMO, a smaller alt pulley will still fix your stalling/discharging problem. Even though the fan draws more when the alternator is charging the battery. Your engine rpms will still drop less with a smaller pulley because the alternator will charge much more efficiently at idle and take less engine power to turn.
Well, I'll have to say I disagree to this too... sorry! Any higher rate of charging is more load on the alternator and thus the engine; any marginally better effiiciency will not change that. So that part will make the RPM drop worse. Having said that, if the faster alternator speed increases the voltage in the system overall, and IF that makes the ignition system work better at those low RPM's, then that MIGHT help. But this is getting in the realm of experimentation with IMHO very unknown results.

And, with a smaller alternator pulley, there are other downsides:
- Fan belt loading WILL go up
- Bearing and brush wear in the alternator will increase due to higher RPM's
- If you rev the engine, you will be pushing the belt and everything harder

IMHO, PO, your better choice of things to try is to connect the fan more closely to the battery as suggested, so it can do a better job of sourcing your fan load current and keep the current load off of the alterantor a bit better. That is an easier change too.
 

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Opel Rallier since 1977
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In other words:

Let's say, you have a 12V DC motor with 80 Watts. This means, the motor has a power consumption of 80W at 12V. Since power consumption P (Watt) = U (Voltage) x I (Current), as a result your current draw is P / U or 80W / 12V = 6.67A.
Also, the electrical resistance R (Ohm) = U / I, so your motor has a resistance R = 12V / 6.67A = 1.8Ohm. Since the resistance is constant (within our idealized example), you can see that you can rearrange the equation to I = U / R --> Since R is constant, if the voltage drops down, the current drops down in the same relation which will bring the power consumption down even further.
If the voltage drops by factor 2 (to 6V) and the current drops by factor 2 as well (to 3.33A), the power drops by factor 2[SUP]2[/SUP] = 4 from 80W to 20W (6V x 3.33A).

Dieter
FWIW.... You're on the right track!

Winding resistance is part of what limits motor current but is the smaller part. When a motor runs, it is a rotating winding in a magnetic field. We all know what other form that kind of machine takes..... a generator. What goes on is that the 'generator nature' of a motor actually generates a reverse voltage internally as it spins; this is called 'back EMF'. This back EMF in essence partially cancels the input voltage and that is what mostly controls the current flow. As load increases, the back EMF drops, giving more net voltage across the windings and that increases current flow. That added current times the supply voltage represents the extra power needed to turn the added load. (That is a bit simplified, but it gets you the idea.)
 

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Can Opeler
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I’m going to agree to disagree based on my experimentation with my one wire alternator.

There is a huge difference in efficiency of an alternator that is running at a bit too low idle rpm vs ideal idle rpm. If you underdrive the pulley too much it does become less efficient too, but not as much.
The extra wear is a non issue unless the pulley is ridiculously under driven. We aren’t driving 200,000 miles with our GT and we certainly don’t have the optimal crank to alt pulley ratio to begin with because we are using aftermarket alternators made for other cars.
Wire length to a cooling fan being shortened is only going to reduce resistance by hundredths of ohms.


I’ll post this picture to support your side. As losses do increase with RPM


But efficiency is also important. If our alternator has too large a pulley (which mine does) and it’s rpm below optimal that is a huge efficiency slope below optimal rpm.

*note both of these pictures are from different alternators. Every alternator is designed with different specs. My one wire drops efficiency significantly when rpm is below optimal as it struggles to catch up with even the slightest of loads.
At idle I’m drawing roughly 12 Amps with my AC on
My alternator is only supplying 1.3 Amps when my engine rpm is at 750
Bump my rpm up to 1200 and I’m charging over the 10 Amps my multimeter can handle.

Again I’ve been wrong before. You seem like you might have some practical and educational experience with DC circuits above the ohm law for dummies I have. These are just my observations from what I’ve tried on my own cars.
 
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