[Sutripta]

^^^
Should have added:-
Lets keep things simple, and leave ECU controlled alternators aside for the time being at least.

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

My question: how are the current and voltage limits taken care of in an alternator.

Ok, now I understand. A pointer, these days all to do with (zener) diodes and transistors. That is on the current alternator mounted regulators.

Originally Posted by Sutripta

Lets keep things simple, and leave ECU controlled alternators aside for the time being at least.

Sure, but actually I would like to understand a bit more about ECU controlled alternators as well.

BR jeroen

[Aroy]

The voltage control is simple
. At less than battery voltage, the diode does not permit current from flowing to alternator from battery - Cutout
. Beyond the set voltage the voltage reference diode (zener?) will trigger the controller to modify the field voltage - Voltage Regulator

Current output is controlled by the controller by regulating the field voltage - Current Controller. The controller kicks in once the rated current is exceeded.

[Sutripta]

Originally Posted by Aroy

Current output is controlled by the controller by regulating the field voltage - Current Controller. The controller kicks in once the rated current is exceeded.

Sounds perfectly reasonable and plausible.
BUT have you ever seen such an item? I mean ACTUALLY seen one which measures and controls/ limits current. NOT "has to be/ obviously/ commonsense dictates/ stands to reason" etc, and therefore has to be present, whether one can see it or not.

Wonder why I haven't!

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

Sounds perfectly reasonable and plausible.
BUT have you ever seen such an item? I mean ACTUALLY seen one which measures and controls/ limits current. NOT "has to be/ obviously/ commonsense dictates/ stands to reason" etc, and therefore has to be present, whether one can see it or not.

Wonder why I haven't!

Regards
Sutripta

Give the man some credits on the voltage control. Pretty spot on! Current control: Current is the result of voltage and resistance according to Mr Ohm anyway.

[Sutripta]

Originally Posted by Jeroen

Give the man some credits on the voltage control. Pretty spot on! Current control: Current is the result of voltage and resistance according to Mr Ohm anyway.

Of course Mr. Roy is right about the voltage control part of it. That was never the issue.

Square one was about current limit being actively controlled. Can it be actively monitored and controlled? Absolutely. Is it? Your thoughts please.

Originally Posted by Jeroen

Ok, now I understand. .

Not previously. But NOW I think You'll understand what I am trying to say. Irrespective of whether you agree with me or not!

A pointer, these days all to do with (zener) diodes and transistors. That is on the current alternator mounted regulators.

Thanks once again for the pointer. But then again I do have some vague acquaintance with electronics.

Regards
Sutripta

[Aroy]

Here are some links explaining the working of Alternator and the regulators

http://www.carparts.com/classroom/charging.htm
http://www.my-cardictionary.com/elec...regulator.html
http://www.autoelectro.co.uk/how-it-works-alternators

[lapis_lazuli]

It's much more complicated, but made simple enough, to be used in almost all cars these days. Kindly read the datasheet of a garden variety alternator regulator: used in a wide variety of vehicles, yes in India too!

http://www2.st.com/content/ccc/resou...CD00055101.pdf

I could try to explain, and could go on for hours, but I thought it is better left for you to read and appreciate.


The output current is governed and hence coarsely regulated by modifying and or limiting the rotor field current. Each alternator has a design limit, governed by the maximum rotor excitation current. Any alternator would run dangerously high voltages at the output if load is minimal. Remember SST Scorpio bulbs blowing??

Just "zener" clamps would be inefficient and ineffective. The clamp is basically (not only) the battery as well. The main harness acts as a degenerative feedback, providing some hard limit too.

The fact that you can't see it, hence it ain't not there: is not true no more.

[Sutripta]

Originally Posted by Aroy

Here are some links explaining the working of Alternator and the regulators

http://www.carparts.com/classroom/charging.htm
http://www.my-cardictionary.com/elec...regulator.html
http://www.autoelectro.co.uk/how-it-works-alternators

Sure.
And where does it say that the regulator is monitoring and controlling/ limiting the current output? (ECU controlled alternators can be very different, which is why I said that these should be considered later.)

Originally Posted by lapis_lazuli

It's much more complicated, but made simple enough, to be used in almost all cars these days. Kindly read the datasheet of a garden variety alternator regulator: used in a wide variety of vehicles, yes in India too!

http://www2.st.com/content/ccc/resou...CD00055101.pdf

I could try to explain, and could go on for hours, but I thought it is better left for you to read and appreciate.


The output current is governed and hence coarsely regulated by modifying and or limiting the rotor field current. Each alternator has a design limit, governed by the maximum rotor excitation current. Any alternator would run dangerously high voltages at the output if load is minimal. Remember SST Scorpio bulbs blowing??

Just "zener" clamps would be inefficient and ineffective. The clamp is basically (not only) the battery as well. The main harness acts as a degenerative feedback, providing some hard limit too.

The fact that you can't see it, hence it ain't not there: is not true no more.

In both the block diagram and the pinout chart don't see any current sense input. Nor is overcurrent prevention mentioned as one of its functions. (Response control is not overcurrent protection)
Missing something?
Links to application notes/ design briefs/ white papers would have been more enlightening.

Would like people to think over something else again.
AVR failures, in India at least, occur with some regularity. Leads to blown/ blackned bulbs, dried out and spoilt batteries, and a host of other problems. We can assume that there were situations when the alternator was at maximum output. How often have we seen a burnt out stator in such cases. And why not? For that matter, how often have we seen a burnt out stator in an alternator?

With the coming of ECU controlled alternators, the philosophy is changing, and with it the design. However, here in India, we are not yet at that stage, and as I said, lets keep ECU controlled alternators for another day.

Regards
Sutripta

[lapis_lazuli]

Originally Posted by Sutripta

Missing something?

Somewhat!

In simple words, it is not needed in a simple implementation; let me try to explain.

Assuming a constant rpm:

1) As you know, all alternators or any voltage source would have something called a short circuit current. Despite a dead short at the output, the current would be limited to that value Isc in the worst case.

2) Since, the o/p current of the alternator is a function of the field current, which in any case is limited to 7...9A, there is only as much the alternator can provide. This is decided by the automaker, and specs sent to the alternator maker, that way. No question of "drawing extra" from it: it would be automatically "OHMICALLY" limited. Meaning, the loaded voltage drops! When this happens, the regulator senses it, and decides there is an overload, and reduces the field current PWM.

4) A typical alternator stator would have a winding resistance of a few tens of milliohms, and an extended over-current scenario, causes a significant rise in temperature, sensed by the regulator IC + thermistor combo mounted in close proximity. The thermal protect feature starts by decreasing the PWM/excitation current of the field coil, till the temperature drops ( by way of lesser current). A hysteresis is provided.

This is the reason you don't quite see burnt stators: they were shut long before they could flare up!

If however the rectifier/s failed in SHORT CIRCUIT MODE (internally shorted), the battery would have fried the stator.

ECU controlled regulators, are not that far, I think in MHO. In fact, they are already here.

[Sutripta]

Originally Posted by lapis_lazuli

Somewhat!

In simple words, it is not needed in a simple implementation; let me try to explain.

Assuming a constant rpm:

1) As you know, all alternators or any voltage source would have something called a short circuit current. Despite a dead short at the output, the current would be limited to that value Isc in the worst case.

2) Since, the o/p current of the alternator is a function of the field current, which in any case is limited to 7...9A, there is only as much the alternator can provide. This is decided by the automaker, and specs sent to the alternator maker, that way. No question of "drawing extra" from it: it would be automatically "OHMICALLY" limited. Meaning, the loaded voltage drops! When this happens, the regulator senses it, and decides there is an overload, and reduces the field current PWM.

4) A typical alternator stator would have a winding resistance of a few tens of milliohms, and an extended over-current scenario, causes a significant rise in temperature, sensed by the regulator IC + thermistor combo mounted in close proximity. The thermal protect feature starts by decreasing the PWM/excitation current of the field coil, till the temperature drops ( by way of lesser current). A hysteresis is provided.

This is the reason you don't quite see burnt stators: they were shut long before they could flare up!

If however the rectifier/s failed in SHORT CIRCUIT MODE (internally shorted), the battery would have fried the stator.

ECU controlled regulators, are not that far, I think in MHO. In fact, they are already here.

The way you posted it, one thought that this module did measure current, and took action accordingly. Quoting from your post

The output current is governed and hence coarsely regulated by modifying and or limiting the rotor field current.

So you are saying that current is not measured, but temp rise is sensed.
Have to go through the entire literature, but seems fair enough. Whether the module you linked to has it or not, I don't know, but is likely. A thermal cutoff is an almost no cost feature to implement in an electronic module. And is good engineering. But it is a slow reacting process. And is used as a last line of defence when everything else has failed. But it is not monitoring and controlling output current. Which is what this discussion is about.

Let me repeat it: The basic alternator AVR DOES NOT MEASURE/ MONITOR OUTPUT CURRENT, and therefore does not control it. It does not monitor because it DOES NOT NEED TO. (As to why it does not need to is the interesting part, which I'm sure people will get to on their own. See if you can find out why.) Which is why in all (old/ simple) cases you will find the AVR monitoring only the voltage, and controlling it. It is oblivious to current.

Re: blown stators: Take a robust old world alternator. Remove the AVR, and connect the rotor (field) across output and ground (ie rail to rail). Connect a battery, and as much load as you want, and spin it up.
An output diode or two might fail, but the stator is very very unlikely to burn up. (People might say that an output diode failing open acts like a fuse, protecting the stator, and the arguments will keep going on and on!)

You are right that diodes failing short, the stator would have been fried. However here we are talking about current/ power produced by the alternator, not consumed by the alternator.

Regards
Sutripta

[Sutripta]

The way this thread is going, I think I should put up some backside protection!
Thank God I mentioned that we are not discussing ECU controlled alternators. In the interests of the safety of my backside, I think I should expand that.

There is likely to be a line of alternators which are spinoffs of the ECU controlled alternators, a bridge line if you will. These will require more sophisticated controllers, essentially because the alternators themselves are somewhat different, different philosophies, different goals.
So for these alternators one is going to find regulators, and regulator components, which are rather different from the ones we are discussing.

Lets stick to our basic alternators for this discussion.

Regards
Sutripta

[lapis_lazuli]

Originally Posted by Sutripta

The way you posted it, one thought that this module did measure current, and took action accordingly.

I tried to explain in a matter of fact way, indeed. That is exactly the sequence of normal operation

Originally Posted by Sutripta

So you are saying that current is not measured, but temp rise is sensed.

Temperature rise is sensed in case of an extended over current event, like I had written.

Originally Posted by Sutripta

A thermal cutoff is an almost no cost feature to implement in an electronic module. And is good engineering. But it is a slow reacting process. And is used as a last line of defence when everything else has failed.

You are absolutely right, but things are not as trivial. This temperature sensor is more than just a thermal cutout in this case. It is a true closed loop correction of the field current, with low end regulators having a simple single pole single zero loop compensation. In a catastrophic scenario, a 30 degree temperature rise above the last accumulated average of say 90degrees, is a matter of a couple of seconds. This would cause a drop in AVR voltage of approximately 200mv, enough to reduce the field and output current to safe park zone. I think it is important for readers to understand the field current concept than to stick to the literal meaning of some terms like "monitor" and "control". It only drags the discussion. Again, system behaviors, by design, has to different in normal running and fault cases.

Originally Posted by Sutripta

Let me repeat it: The basic alternator AVR DOES NOT MEASURE/ MONITOR OUTPUT CURRENT, and therefore does not control it. It does not monitor because it DOES NOT NEED TO.
Sutripta

Originally Posted by lapis_lazuli

In simple words, it is not needed in a simple implementation; let me try to explain.

It is exactly what I had written in my opening line. It is NOT NEEDED! I am glad you affirm!

Originally Posted by Sutripta

(As to why it does not need to is the interesting part, which I'm sure people will get to on their own)

Unlikely! All downstream loads are fed OFF the battery, with their own fuses and protections. The AVR output is too dirty without the battery to be used for anything meaningful. The battery charge current will taper off as it reaches the max charge. So no governance here. For deep-discharged battery? What happens Sutripta da? This time, quiz time for you!

Now the Voltage is about 10V at the battery end, while the alternator tries to pump 13.xx V. The regulator will try to maintain a 13vish. Mathematically 100s of amps now tries to get into the battery! What happens?

Originally Posted by Sutripta

It is oblivious to current.

Like I said, it can only provide as much! Governed by how much excitation the field coil has. Period. A controlled citizen, cannot be oblivious of the government, but of the surroundings, yes!

Originally Posted by Sutripta

Re: blown stators: Take a robust old world alternator. Remove the AVR, and connect the rotor (field) across output and ground (ie rail to rail).

What excites the rotor in the first place?? Is the battery connected? If the rotor has no excitation, even a 12000rpm will not destroy any alternator: robust or not. Except the bearings perhaps. I definitely missed something here.

However if the battery is connected, which excites the field coil, a field coil with typically 1.2...1.8ohms DC resistance will draw about 8A current. Fair. This might correspond to a maximum Alternator output current of say 70A. Even if you short it out, say, given the typical total stator winding resistance of 15 milli-ohms, a dissipation of I x Ix R = 73.5Watts which is no big deal to be dissipated by way of natural / forced convection cooling. Hence it never fries. Nothing else to it!

Originally Posted by Sutripta

However here we are talking about current/ power produced by the alternator, not consumed by the alternator.

Question of alternator consuming does not arise Sir. I said, if the rectifier stack fails, by mode of a short circuit (for readers, semiconductors fail either OPEN or a SHORT mode), the coil will fry in a few seconds.

[lapis_lazuli]

Originally Posted by Sutripta

The way this thread is going, I think I should put up some backside protection!

Absolutely not, Sir.

Originally Posted by Sutripta

essentially because the alternators themselves are somewhat different

Minimal. Cost reduction aims.

Originally Posted by Sutripta

So for these alternators one is going to find regulators, and regulator components, which are rather different from the ones we are discussing.

Automotive grade parts, have a really long life cycle. The legacy is the state of the art ! So a part used today, has been designed, typically 10 years back. The one on design bench today, will hit the market 2 years hence and last a decade or more.

[Sutripta]

Though it did not seem so at first, glad to see that you agree that current AVRs don't measure current because they don't need to. (Though I have a very strong feeling that we arrive at this conclusion through two very different thought processes.)

For others who still think that AVRs measure and control output current, I leave it to you to convince them. (Posting more than a few lines, esp. if it comes to saying the same things over and over again, irritate me.) This discussion has gone on (needlessly, in my opinion) long enough. For such people you can use the datasheet you posted to highlight the fact that a manufacturer gives an AVR chip/ module/ solution which does not measure output current.

Regarding some of the other points you have raised, some can be discussed, some debated, some I totally disagree with. But those are not relevant to the current discussion. Let this discussion be settled. Then if we both feel like it, we'll revisit those.

Don't take it otherwise, but your posting a data sheet, mention of poles and zeros etc I feel is a polite probing to see how much the other person knows. I could mumble some jargon like transfer functions, loop gains, stability, etc, but I'd rather reiterate something which I have said it before, and I'll say it again: On this forum of experts, I'm just a layman. And for almost all technical aspects discussed on TBhp, it can be answered by an informed layman, in language which can be understood by other laymen. And understood by other laymen should be the goal. (My posts don't follow this. Those are meant to stir thought. But I do steer clear of jargon.)

Regards
Sutripta