Automotive Alternators. Constant Current or Constant Voltage Source?

Well, I can see both of you having valid points, but (and it's a big but )

His stance has been in the context of the OP's situation, which is; the alternator divorced of it's built-in or as-designed voltage regulator. Without the VR, you would never consider the alternator itself to be a constant-voltage source.

So it seems you are trying to compare apples to oranges; you can argue semantics if you are not talking about one-in-the same object.

My $.02, and only because you asked

Tom D.

Yea I thought that to over the whole thing and given the OP's very limited and specified description of what he wants to do I have serious doubts he knows what he is talking about either.

As far as battery charging goes there are only a handful of common large battery types that would fit into his description of being chargeable from a 12 - 55 VDC source with a current limit of 2 - 4 amps. Lead Acid, NiCad, MiMh and lithium variants are the main ones I know of and all of them primarily use voltage monitoring as their primary charge control methods and even then each one has its own very specific charge rates and voltage monitoring requirements.

Oh well until or if the OP guy ever comes back and gives more info on what he wants and how he plans to power the system its basically for me just a fun little educational debate on terminology!

(That and I just need a new mouse to play with for a while.)

OK, I'm again guilty of not following an offered link before opening my big mouth.

While that confession is true, so are my earlier big mouth comments.

In the other link, that OP has forgotten that an alternator has two circuits with current running through them, the rotor and the stator circuits. Controlling the current in the very inductive rotor circuit is ideal for pulse width modulation by a MOSFET.

It is a voltage source. Faraday's law of induction makes an Electro-Motive Force (voltage source) by a changing magnetic field cutting through a current carrying material.

I was hoping you would chime in!

That's how I learned the basic description of generators/alternators as being voltage producing sources where the current flow is determined by other functions/properties of the circuit or control system.

The guy had me wondering though being he was so insistent on calling them constant current sources that continually vary their current output to maintain a stable voltage.

Thanks!

You can save your colleague some face by telling him that he just got a few words out of sequence. The current flow in the rotor is adjusted to change the magnitude of the magnetic field (B) that makes the voltage source. Now your colleague may now question why would this current have to be adjusted if the output was a voltage source. A voltage source produces a constant voltage regardless of the load attached to it. (Within the linear range.) Well the magnitude of the changing magnetic field and the rate of change in the magnetic field establishes the magnitude of the voltage. The rotating speed of the rotor (motor RPM) will also change the voltage. So to maintain a constant (regulated) voltage out of an alternator that has constantly changing rotating speeds like a automotive alternator the rotor magnetic field decreases as the engine speed increases.

That's right, it's the voltage that comes first, and causes the current to flow. You can't have current without voltage (except in superconductivity, and then you need a voltage to get it started).

When I first got into AC circuit theory many moons ago, references to current leading the voltage (capacitive circuit) used to confuse me, as I couldn't see how the current could come first. I worked out that this was only after the circuit had settled down, and eventually saw in a textbook words to the effect "current leading the voltage is only after a number of cycles have taken place". Pity they don't all say that, I suppose authors get so used to the idea they forget it causes problems for beginners.

Yep! Me too.

Funny how so many do not realize that simple principle.

In 4 -20 mA instrument control output loops, the voltage magnitude is manipulated to control the current value of the constant impedance load.

It amazes me how many technicians as well as engineers do not realize it and therefore do not take advantage of applying voltage measurement troubleshooting techniques to active control circuits rather than opening the loop, interrupting the control and/or stopping the equipment to take current measurements.

Interesting.

I once had a friend with a motorcycle. He burned out the head lamp and the instrument lights also burned out. When he obtained replacements, he didn't want to chance burning out the new head lamp, so he replaced just the instrument bulbs, which promptly burned out.

Without the current load of the headlamp, the voltage was increasing with rpm (measured with a voltmeter). With the current load of the headlamp, the internal inductance of the alternator increases impedance with increasing frequency (rpm) and cancels out the increasing voltage. When the headlamp was replaced, there was no problem.

So in this case, the alternator approximated a constant current source.

Then your friend either had a generator with a permanent magnet rotor (a kick start only motorcycle might be configured this way) or the voltage regulator was failing.

To me that is a voltage orientated alternator as most are, probably PM, basically output voltage changes with RPM. Ones using a wound rotor are easy to design with a regulated output voltage....

You also did not mention how old the bike was.....or its make or type!!! That would have given a clue as to why....

The current taken is the product of the voltage developed and the resistance of the load (bulbs in this case). So current will be variable.

Voltage output is also affected by the maximum power output of the alternator and some have a steep curve downwards as the load current climbs. In my limited experience only, this tend to be much older motorbikes (though I expect there are still some made even today with such problems.)

Any motorbike that needs the current draw of the headlamp to reduce the voltage output of the alternator to stop burning out the bulbs, can only be defined as primitive at best.....addition of some form of modern voltage control can only have a beneficial effect......But if it is PM, the only way to achieve voltage control is with a simple circuit and a couple of big transistors, MOSFETs be a good choice, to level off the high voltage peaks....The type will need to be sorted out once the maximum voltage/ current has been ascertained....

I found an interesting .pdf here:-

http://www.tedpavlic.com/teaching/osu/ece327/lab3_vreg/lab3_vreg_procedure.pdf

Also some further instruction here:-

http://wps.prenhall.com/chet_paynter_introduct_6/6/1665/426415.cw/index.html

That might give some possible ideas for someone with such a motorbike.

Just comment that current = voltage divided by load resistance, not product of.

Only for DC currents.

All right impedance (+ a few other complications)

We had a few old motorcycles and three wheelers on the farm over the years when I was growing up and I do recall that at least two of them just used a big zener type stud diode as a voltage limiter to keep the system voltages under control.

I have also seen a number old one and two cylinder electric start lawn and garden type engines that used a similar concept on their battery charging/auxiliary power circuits as well.

Simple highly effective and quite durable.

That's also another reason to avoid attempting to run a vehicle without the battery attached. Most automotive electronics are rated for supplies in excess of 18-20 volts. Without a battery, the alternator output voltage will shoot skyward, instantly burning lamp filaments as well as any electronics not rated to handle input voltages that high.

While on the subject, keep your battery terminals clean. Too much corrosion can open the connection to the battery thus exposing your car to the excess voltages decribed above.

Not all alternators have built in voltage regulators. If he has an alternator without a voltage regulator it may act as a current source. The voltage in this case will be speed regulated.

Any alternator that puts out more than a few amps generally has a voltage regulator/limiter of some kind although it may not be built into the units them selves.

It wasn't until around the 1970's that they started putting them inside, GM's Delco SI series units, although there were more than just a few manufactures like Ford and Dodge that still used external mechanical buzz box type regulators on their alternators, even ones rated up over 100 amps, well into the late 1990's and early 2000's even though by then the whole vehicle was relying heavily on computers and electronics to control everything.

What can I say I am a bit of a automotive and industrial alternator history and application buff.

I reread the posting at your original link.
It sure sounds like he is using an alternator with the voltage regulator either missing or disabled. He talks 14v to 55v into a load. That is just not possible if voltage regulator is attached and working per design.

My Harley only has a small alternator of 38 amps. Newer models have bigger alternators. For all, the voltage regulator is not part of the alternator. The alternator is inside the engine attached to the left side of the crank and the voltage regulator is in the wind in front of the engine. Harley also uses the headlamp as part of a snubber circuit for the starter motor. Took me awhile to figure that one out.

"Are stock form alternators unmodified alternators a constant current of constant voltage device within their designed working limits?"

Designed to provide current and voltage in a system with a certain power requirement. i.e. Lots of accessories or few accessories.

Designed to operate with a regulator that maintains a constant voltage under variable loads and variable current depending on the power needs of the electrical system and to prevent over-voltage and overcurrent to the battery.

There is an easy way to show you exactly how they work, attach a good voltmeter across a car battery. Start the car, see the massive voltage drop as cranking takes place, several hundred amps.

Then the alternator starts to replace the lost charge on the battery at a voltage usually several volts under the battery voltage, it will creep up and up as the charge is replaced and will usually level out at around 14.4 volts as charging shuts off....

But its not instant. The time taken/needed is a function of the maximum output current (power) of the alternator and the depth of the missing charge on the battery. Big battery, deep discharge, small alternator - could take 10-30 minutes.

The above is really only true for LA Batteries, with them it is easy to see......

Gentlemen, I've read this thread from yesteryear. It pertains to an issue I've got and need some help with. Hopefully some of you genius's can help me if your still here.

I just bought a new 2015 Cadillac and it has a digital voltmeter on the instrument cluster. When you start the vehicle it reads approximately 14.2V-14.5V. After driving a short distance, 4-5 miles the voltage begins to slowly fluctuate and bottoms out at 12.5V-12.7V. You can stop and put it in park and it may or may not return to 14V's. The system has been tested by a dealer and no codes show up and no warning messages appear on the instrument cluster. I've personally tested the voltage across the battery terminals with a DVOM and the dash voltmeter is reading the same as my DVOM. The system voltage does fluctuate and my readings are all over the place. I've read as high as 14.5V and as low as 12.5V and in between. The dealer technician says that this is normal. I'm not buying it. Your posts here doesn't buy it if I'm reading this correctly. The built in voltage regulator should be maintaining some what of a constant voltage, around 14V's. OH BTW, you can turn on or off all the electrical assets in this vehicle and it doesn't appear to effect the output voltage. I know it is, but it's not readily apparent. I'm wondering if my best bet is to check current flow at each different voltage reading to see if there might be a short. My mind tells me that if there were a short in a circuit it should be constant and would show up. But I'm just a shade tree kind of guy. Thanks in advance for any and all advice.

First of all, congratulations on being able to afford the car!

My initial thought was that it sounds like a faulty voltage regulator, then I remembered that on modern cars the alternator is usually controlled by the engine management computer. I'd guess this is the case with your car. Maybe somebody can confirm, but my guess is the computer reduces the alternator output voltage (by adjusting the field voltage, as with a built-in regulator) when it thinks this is appropriate, e.g. when the battery is fully charged, probably with other inputs, like outside temperature, length of time the engine has been running etc. This is to avoid wasting energy by charging more than necessary. The voltages you give are consistent with this, as a fully-charged battery, off charge, has a voltage a bit above 12 volt, but not as high as 12.5 - 12.7. Voltage setting with a built-in regulator is in range of about 13.6 - 14.5.

If that's the case, what you're seeing could be normal, as the dealer says (but he might have explained it). Does the car show any sign of being under-charged, e.g. slow cranking? If not, I'd be inclined to stop worrying, at least until some symptoms show up.

I haven't seen a dash voltmeter on a car for ages. In UK there were some around about 20 - 30 years ago. If I'm right above, it seems crazy for Cadillac to fit one, giving rise to customer concerns and queries. Does the driver's handbook say anything relevant?

Thanks Codemaster,

It doesn't appear to be under charging. I've been all over the owners manual and can't find any specs at all. I've also tried finding a schematic but that is nearly impossible. I've even researched the alternator and it appears it has an internal voltage regulator. AC Delco list the alternator and voltage regulator as one (part #22988006). I would like to see a schematic to be sure. I understand what you are saying about conserving energy but I have always been taught that big swings in voltage, in this case in excess of 1V can be harmful to any electrical system. I know that some variance in voltage is necessary to excite the alternator to increase or decrease output but to vary the voltage almost 2V is troubling. It never exceeds 14.5V's which leads me to believe it has a limiter preventing excessive voltage. With the voltage reading 12.5V's, I've turned on just about every accessory and it doesn't increase voltage at all. I've increased RPM's and it doesn't change it either. If it is in an energy saving mode these should stimulate the alternator to increase output but it doesn't. I had another dealer tech check another vehicle exactly like mine and the voltage never varied off of 14.2V according to him. Hopefully you are right and I'm fretting over nothing. Thanks again.

Hmm interesting.

I have an Ford Mondeo 11 years old and according to the Haynes manual the alternator has a built-in regulator, but the (rather rudimentary) wiring diagram shows leads going to the engine management system, so it's possible that is adjusting the regulator setpoint. I've never had occasion to remove the alternator.

Interesting about the other dealer, which does suggest something is wrong.

All I can suggest is replace the regulator and see if it cures it, if it does you have a case with the dealer. Only problem is I expect that is a fairly big job, it seems it's a day's work to remove/replace the alternator on the Mondeo (remove a wheel and wheel arch liner and get it from under the car) compared with 20 mins on an older car. Or maybe a trial replacement of the engine management black box.

I don't think the voltage variation by itself will cause any damage to the system.