[Sutripta]

Originally Posted by Jeroen

But have a look at these as they illustrate my point:

http://densoautoparts.com/alternator...rs/alternators.

http://densoheavyduty.com/alternators/light-duty

This is a bit generic, but it shows the output at say 1000rpm around 65A and at 2000RPM about 100A.

Curves seem to be mainstream, and certainly different from

Originally Posted by Jeroen

If I check a few diagrams from say Denzo, I would say maybe 20-25%.

and if you factor in the crank pulley/ alternator pulley multiplication ratio, even more.

If I were jump starting a car, I would rev the helper car engine to whatever its cruise rpm is.

The helped car is an unknown quantity (it has broken down, after all!) Keeping the helper car engine on is insurance. Might as well maximise your cover. Matter of philosophy.

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

Matter of philosophy.

Sure, you know me, philosophy and opinion over facts any day. I'm known for my opinions not for my facts necessarily!

As an afterthought. On car engines / alternators, do alternators typically spin faster or slower then the engine itself? I.e. you reference to the crank pulley/ alternator pulley multiplication ratio.

I have never given it much thought really. But I would say that typically the pulley on the the crank is larger then the pulley on the alternator? It also makes sense to have the alternator spinning faster then the engine looking at the output curve!

In which case the alternator would spin faster then the engine. Which also means the idle / high rev output moves toward the flatter part of the output curve, reducing the difference between the two?

Or is it the other way around?

Jeroen

[Aroy]

Yes Alternators spin much faster than the engine. That is how they can generate healthy power at idling speeds.

Traditionally the Dynamos (DC generators) had commutator. The brushes were connected to commutator. As it was segmented, the brush would wear out faster at higher speeds, that is why there was a limit to its speed, hence power output.

Alternators in contrast have slip rings which are continuous hence the wear at high speed is much much less than in case of a dynamo, so they can be run at higher speeds with less brush wear.

[Sutripta]

Originally Posted by Jeroen

Sure, you know me, philosophy and opinion over facts any day. I'm known for my opinions not for my facts necessarily!

As an afterthought. On car engines / alternators, do alternators typically spin faster or slower then the engine itself? I.e. you reference to the crank pulley/ alternator pulley multiplication ratio.

I have never given it much thought really. But I would say that typically the pulley on the the crank is larger then the pulley on the alternator? It also makes sense to have the alternator spinning faster then the engine looking at the output curve!

In which case the alternator would spin faster then the engine. Which also means the idle / high rev output moves toward the flatter part of the output curve, reducing the difference between the two?

Or is it the other way around?

Jeroen

Say (since you prefer numbers!) 2:1 ratio.
For every 1000 rpm increase in engine rpm, alternator rpm increases by 2000 rpm. Difference it makes will be more than for an increase of 1000 rpm.

What you are suggesting is output is close to maximum (saturation) even at idle. And will be even more so if alternator is spun faster at idle. Not so. (Actually that has been the crux of your argument all along. That there is no real difference in alternator output when the engine is idling, and when the engine is at full chat. Which is why I asked you for those curves.)
Seriously, how would you go about choosing/ sizing an alternator for a vehicle?

Wonder why the venting? The trials and tribulations of moving house?

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

Say (since you prefer numbers!) 2:1 ratio.
For every 1000 rpm increase in engine rpm, alternator rpm increases by 2000 rpm. Difference it makes will be more than for an increase of 1000 rpm.

What you are suggesting is output is close to maximum (saturation) even at idle. And will be even more so if alternator is spun faster at idle. Not so. (Actually that has been the crux of your argument all along. That there is no real difference in alternator output when the engine is idling, and when the engine is at full chat.

Wonder why the venting? The trials and tribulations of moving house?

I don’t think I said that the output is close to maximum even at idle. it was all related about it being better to rev the donor car engine to 2-2500 RPM during a jump start.

I have been saying that I don’t think there is a substantial difference between the idle output and the output an engine RPM of 2-2.500 which according to some would be “better".

Lets take the curve I provided earlier:

http://densoheavyduty.com/alternators/light-duty

And lets go with your 2:1 ratio.

Now lets look up what this means:

Say, idle at 1000RPM engine, means 2000 RPM alternator: output 105A
at 2000RPM engine, means 4000 RPM alternator: output just under 130A

To me that is only a 25% increase of alternator output from engine idle to engine revving at 2000 RPM? Or am I overlooking something?

I don’t think that a 25% additional output is going to make a huge difference during a jumpstart. It might do something, but not much I would think?

I looked at some images of other brands/types of alternators, using the same logic. The pulley ratio pushes the output at engine idle and subsequently at 2-2.500 RPM toward the right of the graph and thus the flatter part of the curve. So the absolute output increases, but the difference between the idle and 2-2.500 RPM output reduces further.

On some curves I see, using your 2:1 ratio about 40-45% increase in output, in some I see about 20% increase.

I’m not a hundred percent sure about the 2:1 ratio. I thought, but I could be wrong, it is typically a higher ratio. Which means you shift even further to the flatter part of the curve.

We have moved so often, it is just a tedious process we need to get through. Sold my 1975 Royal Enfield Bullet, so that is now part of my fond memories of India!

Jeroen

[lapis_lazuli]

Originally Posted by Jeroen

Or am I overlooking something?

Jeroen and others interested, I request you to kindly read section 2, of this paper, spending a couple of minutes on Fig 7. The behavior and properties, of the typical Lundell, is clearly explained, before moving on to the actual subject matter.

The problem is when you crank: the battery voltage dips to 7 or so, forcing the alternator to follow it as well: at that operating point, the I vs RPM curve from Denso(or any manufacturer) is INVALID, as it is characterized at 13.5V output NOMINAL operation. Hence the Power delivery curve is important. Note the line just after the Fig 7!

(Don't be misguided by the word "predicted", in there, it is as close as it can get to the real measured figures!)

[Jeroen]

Originally Posted by lapis_lazuli

Jeroen and others interested, I request you to kindly read section 2, of this paper, spending a couple of minutes on Fig 7. The behavior and properties, of the typical Lundell, is clearly explained, before moving on to the actual subject matter.

The problem is when you crank: the battery voltage dips to 7 or so, forcing the alternator to follow it as well: at that operating point, the I vs RPM curve from Denso(or any manufacturer) is INVALID, as it is characterized at 13.5V output NOMINAL operation. Hence the Power delivery curve is important. Note the line just after the Fig 7!

(Don't be misguided by the word "predicted", in there, it is as close as it can get to the real measured figures!)

Thanks, I read through this paper, interesting. so now I'm trying to apply it to the question at hand. To re-iterate: Is it 'better' to rev the engine to 2-2500 RPM of the donor car during a jump start.

There are I would think two main scenario's to this question:

1 What happens during the actual cranking sequence (lower voltage), what effect does the alternator of the donor car have over the donor cars' battery

2 Prior and after cranking, (more stable voltage) what effect does the alternator of the donor car have over the donor cars' battery


If I take your earlier point and look at figure 7 and also figure 8, what does it mean for the two scenario's. Lets stick with the 2;1 pulley ratio, so lets consider it for engine idle 1000RPM, alternator 2000RPM and engine 2000RPM, alternator 4000RPM.

Could you explain, thanks

Jeroen

[lapis_lazuli]

Originally Posted by Jeroen

Could you explain, thanks

Jeroen

If we go by figures for this particular alternator: at the 7..10V point of the Fig 7, irrespective of the engine rpm, you see the current hovers around 140A. This 7...10V is the crank voltage. So even if the donor car, was revved at 2000rpm, (4000 at the alternator or any such) the LOADED voltage seen by the alternator (now out of regulation, due to saturation) is still 7...10V, because of the crank.

<Recall my statement made elsewhere: There's only as much it can provide, again, governed by the field excitation!>

Answer to 1:

So, you do pump in some energy to the battery, but that's irrespective of the rpm. A much higher rpm above idling, is not necessarily beneficial because the alternator operating voltage is clamped at 7...10V, during the time of crank, in which region, the current delivered is more or less constant.

Answer to 2:

Now, 2 alternators feed two paralleled batteries! Complex! One battery is deep discharged. I would think a bulk of the charge would be absorbed by the discharged battery: it is a bigger sink now! The source being Alt 1+ Alt 2 + Battery 1! At this point, prudent to remove the jump cables. Mix and match and charge of 2 unequally discharged batteries, is detrimental!

[Jeroen]

Originally Posted by lapis_lazuli

Answer to 1:

So, you do pump in some energy to the battery, but that's irrespective of the rpm. A much higher rpm above idling, is not necessarily beneficial because the alternator operating voltage is clamped at 7...10V, during the time of crank, in which region, the current delivered is more or less constant.

Answer to 2:

Now, 2 alternators feed two paralleled batteries! Complex! One battery is deep discharged. I would think a bulk of the charge would be absorbed by the discharged battery: it is a bigger sink now! The source being Alt 1+ Alt 2 + Battery 1! At this point, prudent to remove the jump cables. Mix and match and charge of 2 unequally discharged batteries, is detrimental!

So can we conclude that revving the engine of the donor car, doesn't really do much, if anything at all for scenario 1 + 2?

[lapis_lazuli]

Originally Posted by Jeroen

So can we conclude that revving the engine of the donor car, doesn't really do much, if anything at all for scenario 1 + 2?

Yes. It could be a feel good factor, revving it, but "placebo"!

[Jeroen]

Originally Posted by lapis_lazuli

Yes. It could be a feel good factor, revving it, but "placebo"!

thanks, that was my thought all along, albeit on somewhat, maybe incorrect, assumptions.

MYTH BUSTED, unless somebody comes up with new additional insights

Jeroen

[Sutripta]

Lots of posts since I last logged on (last night).
Will take me some time to go through them. Hold on!

Regards
Sutripta

[Sutripta]

@LL
See if you agree with me in characterising an alternator. If you agree, we can proceed further.

How I see it depends on what is important to me at that time. A voltage source, or a current source, (or increasingly nowadays, a noise source. Irrelevant for our discussion here.)
If the AVR has kicked in, it comes close to the proverbial voltage source (Constant output voltage, irrespective of current, or the other way of looking at it, very low output impedence.)
If the AVR has not kicked in, it has no role to play, and for our understanding of how the alternator works, we can forget about it. At that point, its just a power source. Rather imperfect voltage or current sources. All further discussions are with the AVR not having kicked in.

Because of the battery across its output, and field terminals, it will behave differently from a self excited power source, driving passive loads.
The output voltage will vary from the voltage at the battery terminals without the alternator being present, to that (voltage + a few tenths of a volt per cell).

@ Jeroen: As said earlier, its a matter of philosophy. As long as we don't try to convert each other, we can both live with our own philosophies. We will discuss the merits of each, if we feel like it!

Regards
Sutripta

[Sutripta]

Note for the Mods: Could we please split the alternator heavy posts to another thread?

[lapis_lazuli]

Originally Posted by Sutripta

characterising an alternator

It is a 3 phase voltage source, in series with a significant loss component.

Originally Posted by Sutripta

If the AVR has kicked in, it comes close to the proverbial voltage source (Constant output voltage, irrespective of current, or the other way of looking at it, very low output impedence.)

Yes, even without the AVR, it is a voltage source.
(A voltage source is a funny thing: a significant high impedance in series with it and BANG : you have a current source!)

Originally Posted by Sutripta

If the AVR has not kicked in, it has no role to play, and for our understanding of how the alternator works, we can forget about it. At that point, its just a power source. Rather imperfect voltage or current sources.

Yes, again! The AVR is a necessary convenience, at most. It is sometimes, a current source, as a side effect of certain events: a heavy load (as in crank), or when the terminal voltage is grossly lower than the open circuit voltage. Just a power source: yes, it is a power source unconditionally! It delivers measurable power under most operating conditions!

Originally Posted by Sutripta

Because of the battery across its output, and field terminals, it will behave differently from a self excited power source, driving passive loads.
The output voltage will vary from the voltage at the battery terminals without the alternator being present, to that (voltage + a few tenths of a volt per cell).

.

That is the tricky part, I have been stressing all along and you questioning it all along: What it means is, the field current itself is limited by the terminal voltage, which puts a cap on the output current. The terminal voltage is essentially the battery voltage!

Classic Faraday's law : V = 4.44 x N x B x A x F

Its the B which is made variable, by way of variable field current, to control V (assume fixed rpm)! And since it is an EMF equation, it is basically a voltage source at the end of the day! In nominal conditions, the PWM to the Field may be altered to limit current; under duress, the terminal voltage itself falls and self limits it!

Under nominal conditions, the AVR maintains the 14ish, which is enough: necessary and sufficient condition, as we put it. So an active measurement of the current at the alternator output IS NOT NEEDED.

It is saturation limited and in the worst case, self governing!

The "Plus a few tenths": is needed to push current in to the battery under NOMINAL (cruise) conditions.

A separate thread, is called for, definitely!