Alternator A/C V questions

[Car guy in Virginia]

First of two sets of questions:

 

Why do alternators output ~1.x A/C Vs at the end of ignition STARTing, just after leaving the ignition switch START position and being in the ON position ... that is, when initially running? 

 

What, specifically sequence-wise, is happening inside the alternators that explains the initial ~1.x A/C Vs getting out(put)?  

 

I'm particularly interested in the exact sequence of explanation (considering the alternators are being turned and are having their field coils energized, and are having their V regulation to the brushes and slip rings and rotors, and their rotating magnets are inducing the current into the 3 stator coil windings, with the rectifier's diodes converting the A/C into DC for output.) 

 

Attached is a set of three A/C V o'scope pictures.

 

The first pic shows what happens during Start:  immediate V drop (in this case to -2.75 V), then immediate V rise toward 0V, then slight rise to positive V (the initial small hump).  The initial hump is when the ign sw has changed from START to ON.  

 

At the end of the first pic, the alternators are running (now in the ignition ON position).  And, this is where there is the graceful, larger 1.x A/C V ramp-up ... the second/main hump.  In this case, 1.386 A/C Vs.

 

The second pic shows where this 1.x A/C V promptly ramps down (on the far left) and then shows how the alternators' A/C V stabilizes around 0 V (rest of the pic).

 

The third pic shows where the alternators' running A/C V remains around 0V.

 

 

Second set of questions

Or ...

is there another explanation for the ~1.x A/C V ramp ups and ramp downs on the o'scope's A/C V readout - just after leaving START - while the corresponding multi-meter also indicates the same ~1.x A/C V (as the max)?

 

Thanks.

 

 

 

 

 

 

 

Edited August 23, 2021 by Car guy in Virginia
clean-up (see edit history)

[Car guy in Virginia]

Anyone?

[Larry Schramm]

25 minutes ago, Car guy in Virginia said:

Anyone?

 

My question is what does A/C-V stand for?  Is it for A/C voltage?  Is the first picture of the car starting or if not, then what is the scope sweep? Where are the readings being taken? On the output of the alternator or an internal reading?

 

Are the second row of two pictures of the car just after starting and after running a little time?  

 

What you are asking is not that clear to me and what we are looking at 

[Bloo]

I don't get what I am looking at here either, or what the question is.

 

Also what alternator? There has to be some magnetism for the alternator to bootstrap. Residual magnetism might not always be enough, so there will be some more positive way. Some alternators use the current through the idiot light. The behavior might be different depending on whether the particular car shuts the idiot light off while cranking. Some, maybe all GM alternators with an internal regulator have an extremely small parasitic current and do not depend on the idiot light to bootstrap. There are other ways too, like old Chrysler and Prestolite systems that simply turn on with the ignition switch and do not need to bootstrap at all.

[Car guy in Virginia]

Yes, A/C voltage.  Yes, from the very START of the cars.  Yes, the 2nd and 3rd pics are the (uninterrupted) continuation from the 1st pic.  Thus, the A/C V is being shown in its entirety from the very START through ON through the point where the alternator is now charging the battery and supplying the cars' electricity.  It is measured at bat (+) and bat (-), which are the same as the alternators' (+) and case ground.

 

What I'm asking is:  why is the o'scope showing 1.x A/C V at the end of the START/ON process - and only then ... and not again ... and not before then; and where is the 1.x A/C V coming from ... what inside the alternators is doing this ... and, in what sequence inside the alternators (where exactly in the sequence). 

 

I think the sequence is everything - it matters, especially since there are different sequences of execution inside.  The alternators begin executing one way, but then transition to executing another way.

 

I have walked through the sequences, but can't explain the 1.x A/C V rise and decline.

 

Thanks.

 

  

[Car guy in Virginia]

Yes, the alternators bootstrap (to use your wording).  The battery supplies the excitation when ignition is START or ON (but only ON, before the alternators are running).  Yes, the excitation occurs in/for the inner coils for the magnets; then when the magnets are rotating, they are inducing the current into the outer coils (stator).  The control of the inner coils is through the internal Vreg (a multiple nested transistor, multiple resistor, and multiple diode regulator).  The capacitor is inside the alternators but outside the Vreg.  The dash light is not involved in exciting the field (inner coils), as it happens. 

 

Thanks.

[Car guy in Virginia]

Larry, Bloo, can I take it you have not seen alternator o'scope signals before, from START? 

 

When I posted the o'scope information and worked up the material onto the pictures, I was trying to simplify all the data to direct the question. 

 

The pictures and work took days to collect and to synthesize.

 

I also have all the parallel DC V o'scope pictures, as well as all the corresponding multi-meter A/C and D/C V data and pictures, as well as all the A/C loads data and pictures.

 

Several weeks invested, in toto.  

 

Last piece of the puzzle is the explanation of the 1.x A/C V ramp up and down.

 

Thanks.

[TerryB]

A/C in automotive is usually a reference to air conditioning. AC, no slash, is normally voltage or current, at least in USA.  That for me was part of the confusion in your question.  Good luck with your inquiry!

[Bloo]

So why AC Volts then? Aren't you just seeing a capacitive spike when the field comes on and the unit begins to charge?

 

My experience looking at alternator patterns are on what I suspect is a much shorter time scale, looking at the three phases to see if one is missing. Sorry to be dense, but I don't understand what you are looking for or why.

 

What make/model is this Alternator?

Edited August 24, 2021 by Bloo (see edit history)

[maok]

IMO, the AC ripple (a better descriptor) on the DC line-level is probably direct relationship to the speed of spin of the starter motor, which turns the motor and hence the alternator. The starter motor will hesitate based on the mechanical resistance of the engine and ability of the battery to supply current.

Edited August 24, 2021 by maok (see edit history)

[Ronnie]

1 hour ago, Car guy in Virginia said:

Last piece of the puzzle is the explanation of the 1.x A/C V ramp up and down.

 

Depends on what alternator you are talking about. I didn't see what vehicle you are doing the tests on. All alternators aren't controlled the same way even if they have internal regulators.

 

On my '88 Buick the regulator in the alternator is monitored and partially controlled by the Body Control Module (BCM) by sending a voltage signal to it. During startup the regulator is sending pulsed signal (PWM), not a constant voltage, to the field windings to create a magnetic field. That might account for the odd reading you are seeing on the scope. Once the RPM increases the pulsed signal to the field winding drops out and normal regulator operation begins.

 

I don't know if this answers your question because I really didn't understand your question to begin with. I just thought I would give it a shot.

[Car guy in Virginia]

58 minutes ago, Bloo said:

So why AC Volts then?

 

It was part of a larger process to measure (and determine) and baseline the health of all the alternators (across the fleet).  When I got to the A/C part of the process, the (A/C) ripple was measured, with and without loads, and during START and ON.  Everything measured as expected (and was fine), except I was surprised to see the A/C immediate negative drop (to -2.75V), followed by immediate A/C rise toward 0V, followed by the slight initial positive A/C V, followed by the (higher) 1.x A/C ramp up and decline just before the A/C V stabilized at 0 from then on.  And, I especially was surprised to see the exact same sequence of A/C events across every car measured. 

 

Plus, I was surprised because I was expecting the rectifier to cancel out any negative A/C V, and began wondering why the scope was showing negative A/C.  This raised a question about how it could be that the alternators' negative A/C gets out(put) during START.  It seems that it should not.  But, the scope showed (essentially) the same negative A/C V on every car measured.  It seems the diodes should prevent this very thing? 

 

Then, once the A/C V went positive, that made sense [from a couple different perspectives, release of the starter (release of that current draw), activation of the field coil, internal capacitor supplying/smoothing initial output V, etc]. 

 

But, then when the A/C V ramped up to 1.x and ramped down (to 0), then I was lost in the sequences ... and, again, there are different sequences internally in the alternators ... so, given the different possible sequences, which of the devices in the alternators were driving the ramp ups and which of the devices were driving the ramp downs ... and why.  And ... were they the same devices doing both? 

 

Or ... was none of the A/C on the scope actually A/C?  Was it actually A/C impact by the D/C starter draw down and recovery?  And was part of it A/C impact by simultaneous field coil draw down (among other simultaneous current draws across the cars)?  And, if so, when - along the signal path - did the scope A/C signal begin being actual A/C V.

 

 

    

 

58 minutes ago, Bloo said:

Aren't you just seeing a capacitive spike when the field comes on and the unit begins to charge?

    

I have thought about this, but it remains unclear:  is the immediate A/C V rise (from negative -2.75) toward 0 the field coil coming on (the signals there certainly appear A/C hump in nature) ... or is the initial positive A/C V the field coil coming on ... or is the 1.x A/C ramp up the field coil coming on ... or are all three of these the field coil coming on in fits and staggers (as the starter jerks the car/alternator over) .... ?

 

I'll pose that question.

 

And, I would ask in return ... 

On the 1st scope picture, can you point to when the field coil is coming on? 

Can you point to when the capacitor dumps? 

Can you point to when the zener diode (first) switches (on)?

Can you point to when the zener diode possibly re-switches off?

Can you point to when the battery excitation of the field coil ends and the alternator is self-driving the field coil? 

 

And, then there's the question of what explains the 1.x A/C V ramp down (to 0).  Because certainly by then the battery excitation of the field coil is done, and the alternator is self-driving the field coil.  So, which devices are ramping the 1.x down ... and why?

 

 

Thanks.

[Car guy in Virginia]

1 hour ago, maok said:

the AC ... is probably direct relationship to the speed of spin of the starter motor

 

Yes, earlier I had posted that I had all the corresponding DC V pictures and measurements as well.  And, indeed, the AC and DC signal patterns (not voltages) align seemingly perfectly ... until the 1.x A/C V ramp down ... at that point, the DC continues high (14.xx).

 

So, thanks, and I'm glad you posted and posted this very thing.

 

So, given your insight on this part, can you explain IF the scope's A/C signal IS or IS NOT actually A/C signal ... and can you explain if it IS or IS NOT alternator A/C output?  These both are critical.

 

And, can you explain why the 1.x A/C ramping down occurs when the DC already is continuing high (steady 14.xx)?

 

Thanks.

[Bloo]

So we are talking about the ripple?

 

That might even make sense. More ripple right after it starts charging?  There is more load on the alternator when the car first starts because although the voltage is regulated, the battery draws more current at first as it replaces the energy used to start the car. Then it tapers off, all by itself. More current draw during those first moments would cause more ripple. You may just be watching the battery charge. I wonder if a trace of the current flowing from the alternator would track your ripple trace. I'll bet it would.

 

Never mind the capacitive kick. I misunderstood what you were talking about.

 

 

Edited August 24, 2021 by Bloo (see edit history)

[Car guy in Virginia]

1 hour ago, Ronnie said:

All alternators aren't controlled the same way even if they have internal regulators.

 

Yes, I'm with you on this.  No PWM and no BCM and no ECU-controlled alternators in the fleet (of cars being assessed), as it happens.

 

 

1 hour ago, Ronnie said:

Once the RPM increases the pulsed signal to the field winding drops out and normal regulator operation begins.

 

Yes, this is part of the differing sequences I was referring to in a couple of the posts; so am in sync on this also. 

 

Glad you mentioned both points.

 

 

Oh, and for those following this, the reason I had asked Larry and Bloo if they had seen scope A/C V signals from START is that other technicians with whom I have spoken have said they only have looked at A/C V once the cars are running.

 

 

Thanks.

  

[maok]

Try doing the same measurements with the spark plugs removed, my guess is that you will get less ripple affect.

 

Why is this important to you, are you worried about sensitive electronics attached to the car?

[Car guy in Virginia]

19 minutes ago, Bloo said:

You may just be watching the battery charge.

 

Excellent! 

 

So, you are answering the original first question then:  the 1.x A/C V is the increased-loaded alternators responding ripple-wise proportionately to the drawn starting current which is then (during the 1.x A/C V duration ramp up and ramp down) being replaced?  And, this further explains why the 1.x A/C V only happens once - and only at the end of START to ON ... and not before then nor again when running.

 

And, this also explains the slight variation in the 1.x across all the cars.

 

And, this also explains all the signal patterns being the same across all the cars.

 

What's also interesting pre-charge-wise is being able to see the starter V drop and V recovery (as the starter overcomes the engine's inertia) on the D/C V scope pictures and the A/C V drop negative and negative V recovery toward 0 as the alternator simultaneously begins rotating.  Overlaying those scope pictures was cool, to say the least.

 

32 minutes ago, Bloo said:

Never mind the capacitive kick.

 

I actually was glad you mentioned the spike, because, if you look closely at the 1st scope picture, the 1.x A/C V ramp up actually begins vertical ... and rounds from there (rounds up and rounds down).  Though, now, it appears that vertical is the beginning of the current replacement.  Notice how it gets half way up the 1.x A/C V ripple at the immediate beginning of the hump.

 

35 minutes ago, Bloo said:

More ripple right after it starts charging?

 

Yes, but - for clarification - are you thinking the A/C V negative rise toward 0 (look closely at those initial humps in the 1st picture) is the alternator then charging (beginning its charging) ... or are you thinking the initial positive small hump A/C V is the alternator then charging ... or are you thinking the 1.x ramp ups and ramp downs are the alternator then charging ... or are you thinking all 3 are the alternator charging-away? 

 

This is a loaded question, but I don't want to bias your thinking/answer. 

 

 

46 minutes ago, Bloo said:

There is more load on the alternator when the car first starts because although the voltage is regulated,

 

This is interesting also because, initially, the regulated V appears to be below the zener diodes' threshold [given the -2.75 A/C V (drop)], so the regulation occurs through the combination of the path through the field coil and also not through the field coil (the 2 inputs, at this juncture, to the first transistor). 

 

One of the sequences ... 

 

Fascinating engineering.

 

1 hour ago, Bloo said:

I wonder if a trace of the current flowing from the alternator would track your ripple trace

 

Yes, I have that data ... though in another form ... and it does agree (now that I put thought to that set), as the current usually jumps to 13As and immediately ramps down to 9, 7, 5, 3, and lingers around 1.8ish As.

 

Huge thanks Bloo!

[Car guy in Virginia]

1 hour ago, maok said:

Try doing the same measurements with the spark plugs removed, my guess is that you will get less ripple affect.

 

Consistent with your thought, the cars with the oldest batteries had the most 1.x A/C V and those with the newest batteries had the least 1.x A/C V (now that I think all those through).  Also, one of the carbureted cars had sat for a couple days in high ambient heat, and the float bowl vacuum-lost some fuel.  Guess what its 1.x A/C V was when trying to start?  Almost as high as the oldest battery car (now that I think that through).

 

So, thanks for this thought.

 

1 hour ago, maok said:

Why is this important

 

All part of understanding and knowing the cars in the fleet.  I've electrically baselined nearly all the systems across them.  Just hadn't done A/C V from START before ... on the scope ...

 

Thanks.

[37_Roadmaster_C]

I'm going to jump in here as a party pooper. I have 40+ years in electronics and industrial electric fields and the same time in hobby automotive electrical/electronics. Bloo has the same in automotive technology and knows his stuff also. Now for the pooper part. PLEASE loose the incorrect alphabet soup of terms and spell out exactly what you are asking. A/C doe NOT mean alternating current! AC voltage does. What does 1.x A/C v refer to? Be specific. Another one that looses me is your reference to -2.75 A/C v drop. If you are saying there is a 2.75 volt AC drop from key off reference, you are incorrect. With the engine not running there is 0 volt AC in an automotive system. A battery is a DC voltage device, no AC voltage at all.

OK, I have beat on you a lot. My reason is that without proper and specific statements and questions we are all shooting blind at an invisible target trying to help.

In short, the AC voltage in an automotive system really only has two basic reasons. First is the effect of the diodes and/or the failure of one or more. The second is ripple and that is directly related to three things only. 1) resistave connections, both on the positive and ground sides. 2) the condition of the battery. The battery is the filter in an automotive system. 3) the load on the alternator. This is a function of the discharge state of the battery, the condition of the battery and the actual electrical load on the system (lights, electric motors accessories etc...). Battery DC voltage will drop during starting due to the extreme load presented by the starter. The pickup after Start is a function of several physical properties including residual magnetism, rotor (field) speed, field excitation which is a combination of field current (not voltage) and magnetic properties of BOTH the field rotor and the stator core.  All of these properties directly relate to the output voltage and current of the alternator.

   Sorry to be so negative, but accuracy and detail will get you the answers you desire.  Also, when supplying test instrument data (scope pictures) ALWAYS provide ALL technical data (sweep speed, voltage per division, AC or DC coupling and input impedance if known). A scope picture without resolution data is so limited as to be almost worthless.

[Larry Schramm]

Car guy,  To answer your question about seeing scope readings, if I have seen them, it was years ago.  I have been in this business for decades and I have lost count the amount of diagnostic tools I have seen.

As for your images, what it appears to me is that in the first picture the key is on and the vehicle is being started.  The dip is the large current draw with corresponding voltage drop because of the start of the starter.  Think kind of like a direct short across the battery but really a large resistance.  Once the starter starts cranking the current requirements of a running motor decreases and eventually the load decreases and then goes away when the car starts. This is why the voltage recovers.  

 

The next picture is of the voltage increase to charge the battery from the start of the engine.  I would guess that the battery is a good battery and not with a low charge.   After the battery is charged back to the base voltage, the charge amount needed is going back to close to zero.   The need for a battery charging requirement is reduced and the only really current requirements are for the ignition and an accessories &lighting that are turned on.

 

Being AC or DC voltage, they are roughly correlated to load of the vehicle.  The alternator/generator generates an AC voltage that is run through a rectifier diode trio to change from AC to DC voltage and then a voltage regulator to charge the needed amount for the vehicle.   You have AC voltage in & DC voltage out.  They correspond to battery condition, system load requirements and follow in tandem.

Edited August 24, 2021 by Larry Schramm (see edit history)

[Car guy in Virginia]

Larry, thank you, indeed.  This is exactly the type of clarification and confirmation that syncs with all the data collected and syncs with the analyses of the various alternators' internal electrical schematics.  Thank you for taking the time to write it.  And, thank you for the being the first person to respond on the post.  I had submitted it Sat., but after 80 or so reads, no one had responded by Mon evening.

 

And, yes, the battery (from these scope pictures) is a good one, so you correctly determined that as well.

[Ben Bruce aka First Born]

SO, were you really looking for answers or were you testing us?

  Just curious.

 

  Ben

[Car guy in Virginia]

 

 

6 hours ago, 37_Roadmaster_C said:

The pickup after Start is a function of several physical properties including residual magnetism, rotor (field) speed, field excitation which is a combination of field current (not voltage) and magnetic properties of BOTH the field rotor and the stator core.  All of these properties directly relate to the output voltage and current of the alternator.

 

This helps.  Thanks.  So, to clarify, timing- and sequence-wise, looking back at the 1st picture, is your reference to the pickup the rise toward 0 V, the slight positive immediately thereafter (the small hump), or only the 1.x V ramp up (the large hump)?  (The 1.x in this case was 1.386V; it was on the picture.)

 

And, during the starter's execution - with the alternator rotating (during those 1.6 seconds), is the alternator initially outputting during the rise toward 0 V, during the slight positive thereafter, or only during the 1.x V ramp up?  Or, only during the latter two (and not the first)?

 

Understanding this would wrap up a lot of schematic loose-ends.

 

Thanks.    

[Car guy in Virginia]

22 minutes ago, Ben Bruce aka First Born said:

looking for answers or were you testing us

 

Hey Ben.  I recall your assistance from an earlier post a few years back (I think).

 

I had most of it figured out, but was questioning the scope's initial AC (thanks 37) -2.75Vs, apparent AC humps on the rises toward 0, initial small positive AC (small hump), and the 1.x AC ramp ups.  This led me to question how the diodes could be outputting anything negative AC-wise (all the alternators are fine).  That sent me deep into the alternators electrical schematics, to see if there was a sequence explanation therein.  I could fine none.  All the schematic sequences (not an easy task) revealed no explanation.  What I was missing - that bloo caught (huge thanks!) - was that the 1.x AC Vs were the starting loading ripple.  Deep in my subconscious was the threshold usually used for excessive AC ripple ... which none of the cars had demonstrated throughout all the normal accessories loads measurements.  Indeed, all the normal accessories loads ripple measurements were well below threshold, and the patterns looked great - exemplars.  Once bloo said what he did, the subconscious rose right up to consciousness.  The answer was instantly manifest.

 

And, I also would say to bear in mind that to collect comparable START scope data along with multimeter data while simultaneously video recording the results across various cars took days and repeated setup and teardown work, not to mention all the comparison work that ensued.

 

So, yeah, I needed help ... but, also I understand everything everyone has said, except for the few clarifying questions I have posed back.

 

 

 

 

[Ben Bruce aka First Born]

LOL.  Too deep for me.  I will stick to mechanicals and just know an alternator works or does not.

 

  Ben

[Ronnie]

5 hours ago, Car guy in Virginia said:

And, I also would say to bear in mind that to collect comparable START scope data along with multimeter data while simultaneously video recording the results across various cars took days and repeated setup and teardown work, not to mention all the comparison work that ensued.

 

I had a friend who works at Oak Ridge National Laboratory tell me that ORNL would spend thousands of hours and thousands of dollars doing a study to figure out which brand of peanut butter is best. I think the study you are doing falls into that category. It's a lot of effort to answer a question that has little value in the real world much less in an antique car forum.

 

 

[Larry Schramm]

ORNL = Oak Ridge National Laboratories.  

 

It began in the World War II Manhattan Project, which developed the first atomic bombs

[TerryB]

8 minutes ago, Larry Schramm said:

ORNL = Oak Ridge National Laboratories.  

 

It began in the World War II Manhattan Project, which developed the first atomic bombs

You need the best PB & J sandwiches in the world when you are working on a project like that.  Yes they are more well known for their nuclear endeavors.

[Ronnie]

50 minutes ago, Larry Schramm said:

ORNL = Oak Ridge National Laboratories.  

 

It began in the World War II Manhattan Project, which developed the first atomic bombs

 

That's the one. It's located in the same county I live in. A lot of people around here work there.

[37_Roadmaster_C]

17 hours ago, Car guy in Virginia said:

 

 

 

This helps.  Thanks.  So, to clarify, timing- and sequence-wise, looking back at the 1st picture, is your reference to the pickup the rise toward 0 V, the slight positive immediately thereafter (the small hump), or only the 1.x V ramp up (the large hump)?  (The 1.x in this case was 1.386V; it was on the picture.)

 

And, during the starter's execution - with the alternator rotating (during those 1.6 seconds), is the alternator initially outputting during the rise toward 0 V, during the slight positive thereafter, or only during the 1.x V ramp up?  Or, only during the latter two (and not the first)?

 

Understanding this would wrap up a lot of schematic loose-ends.

 

Thanks.    

 

OK, I will try a little... First off, please do not call the middle of your scope picture 0 volts. It is not 0 it is whatever you set it at as a beginning reference.  I will assume it is around 12.8 volts just to please myself! Now on to the point...

  A moving field generation device (Generator or Alternator) is incapable of generating any output until the field is moving, spinning. There is also a minimum speed needed and magnetic field density needed to generate usable output. This is why many early generators did not work at idle speeds. In your case the engine must be turning at a minimum speed for the alternator to generate a useable DC output. I am sure this will not happen before the engine is at least at idle speed, possibly higher. With that in mind all data before that time is irrelevant to the alternator operation. After output minimum speed is reached, then and only then can the alternator begin to replace the battery charge lost due to cranking and also provide any excess output to powering other loads. The recharge can happen quite fast with modern engines needing much less cranking and modern batteries being more efficient at storing energy. In short, there are just way to many variables to develop a profile to adequately answer your questions.  Sorry....

[Car guy in Virginia]

Fantastic explanation.  All of it.  Thanks!

 

I think it was Bloo who suggested looking at the current trace.  Overlaying that data with both your explanation and all the AC scope data [and actually also all the other DC scope data, where the AC and DC perfectly align right up to the point of AC ripple ramp down and DC 14.xx V continuation (fascinating to see that point of divergence)] reveals that the AC ripple, the 1.x Vs caught simultaneously by the scope and multimeter, after leaving START (to ON), is when your criteria are met (on the various cars).  And, once walking through the Darlington and Zener again (something took me back into those parts of the alternators and those portions of the schematics), the (alternators' internal electrical) sequences firmed up.

 

So, we're there.

 

And, this closes the loose ends for me from the schematics.  Much thanks for that.

 

And, I'll leave you with this, as it's just too funny ... the Darlington B-E V (Vbe) would appear to be 1.3 ... which is right around the 1.368 AC ripple Vs.  What a great way to end the project.  :- )  

 

Thanks.

 

   

 

 

[37_Roadmaster_C]

Ok. I am having lunch with Bloo and he has opened my eyes to your question much better. I will address this in more detail this evening when I am at a real keyboard.

[37_Roadmaster_C]

I think the question you are asking is "Why the AC ripple after start"? If this is the case the answer is actually quite simple.  Immediately after start there is a high load because of the battery discharge from the high starting current. In the case of ANY "bruit force" power supply, brute force meaning an AC source, diode rectifier and filter, the ripple voltage is determined mostly by the current capability of the source , the type of rectification (half or full wave) in the case of an alternator it is full wave three phase, and the filter type and efficiency (in the case of an automotive system the filter is the battery and it is actually a fairly poor filter). As the load increases the filter effectiveness reduces. As the load goes down the effectiveness goes up. This is shown in your scope pictures. With a discharged battery (even slightly discharged) the load is very high until the battery voltage approaches full charge. This is also amplified because the alternator output capability is greatly reduced at low rotation speed. Since the voltage regulator sees a lower voltage it increases the field current, which increases the output current, but also approaches magnetic saturation, which open a whole new basket of worms that I am not even going to delve into. In most cases the design of the field windings and core prevent total magnetic saturation. The output voltage is limited by the battery  and a little by the wiring and connections. Since the alternator is able to output a fairly high current (normally 65 or more amps), but still limited by the magnetic saturation and rotating speed, you are actually somewhat rapid charging the battery, but HIGHLY loading the alternator. As the battery voltage increases approaching full charge voltage the current draw reduces. As the load on the alternator is the current load, not voltage, as the charging current reduces the load reduces and therefore the filter system (battery filter properties) efficiency increases.  This is exactly what your scope traces show.  There are still many more variables that effect the overall system, but this answers your main question.  Now if you are trying to develop a baseline for preventative maintenance purposes you will need data for EACH type of alternator AND the specific systems they are in. A lot of work and time for, in my opinion, not much use. Your mileage may vary...... I hope this helps.

   An interesting experiment would to take one of the systems you have data for and repeat the scope and meter data collection with one change: As soon as the engine starts, increase the rpm to say 2000 to 2500 rpm. The effect of this is to increase the rotation speed of the alternator field, therefore increasing the output current capability. This will charge the battery much quicker and also reduce the system load on the alternator. I am not sure exactly what you would see, except for a quicker DC voltage rise and quicker AC ripple reduction. Just something to think about 🤔.

Edited August 26, 2021 by 37_Roadmaster_C (see edit history)

[Studeous]

Very interesting stuff. But I will go load the wagon. Let you guys worry about the mule. Or maybe watch some flat earth debates on you tube.

[Car guy in Virginia]

WOW 37, if we all were together and I could lay out all the data and videos and pictures and schematics across a couple tables, I (we) could point to everything you just mentioned.  Indeed, as I read your additional explanation, I could see each sentence's point in the traces and schematics.  There, right there it is, there, right there is that, there, right when it's supposed to be, there, see when that peaks, there, see where those diverge, there, see how that is now gone and this is continuing, et al.  I think at one juncture (during the posting) I had raised a series of questions focused on can we point to when ..., and we can.

 

You're amazing.

 

Glad Bloo and you had lunch ... : - )     and sorry I missed it ... 

 

Much thanks for getting back.

 

With respect.

 

 

 

[37_Roadmaster_C]

You are very welcome 😀!  There are so so so many things about everything that it is often hard to see the forest for the trees.  One very big thing in power generation, that is very often overlooked or misrepresented, is the fact that EVERYTHING is based on current and magnetism, NOT voltage!  Now do not get me wrong, voltage is represented as it is a physical property of current and resistance and everything that conducts current has resistance, even if it is very small. Even nitrogen cooled superconductors have resistance however very little.  I think you have a good idea about what you are looking at and will do fine!

Best Always!