Dear Oso! > You are attempting to draw on your car experience (watching a > voltmeter) and trying predict how the alternator will react in a > totally different usage.
Why do you think that you know how I did this test? It certainly was NOT like you think! It seems that your crystal ball needs some polishing! :-) I made this test with an accurate DC/AC true RMS clamp meter placed over the alternator output wire, in addition to a digital multimeter connected as voltmeter across the alternator output, and switching on enough loads to depress the system voltage enough to allow the alternator to run in current-limited mode while measuring its maximum current at different RPMs. The only instrument from the car's panel that was used was the RPM gauge. A friend did "slave" duty throttling the engine to different RPMs while I read the meters. > You would have a much better understanding > if you were watching a bi-directional amp meter. The clamp meter IS bidirectional, but of course it will never show current going INTO the alternator, because of the rectifiers! And I think I do have a pretty good understanding of car alternators, having designed and built external regulators for a few special uses such as wind turbines, and also some for cars back in the days when cars came with electromechanical regulators and many people preferred the stability of electronic ones. > You stated "I fear that a 70A alternator running at half its rated > speed will definitiley NOT produce 50-55A, but rather will end up > with insufficient voltage to put ANY current at all into the battery > bank! It depends on the specific alternator, of course, but if it's > rated at 6000rpm, I would not count on it being usable at all at > 3000!" Yes, I stated that, and I admit that it may lead to misinterpretation. So, let me explain a little, and then clear this up: Neglecting the effects of resistance, iron loss and some others, the voltage induced in alternator coils is directly proportional to the product of excitation current and RPM. The frequency is directly proportional to RPM, of course. If a car alternator is running into a significantly discharged battery, the regulator will apply full excitation, which will then be constant at the system voltage, roughly 12 to 13V in this situation. This will cause an essentially fixed excitation current. So, both the induced voltage and the frequency will be proportional to RPM. In this situation, and if the RPM are high enough, the output current is limited to an almost constant value by the inductance of the windings, because as the RPM increase, so does the frequency and thus the inductive reactance of these coils, compensating for the voltage increase and resulting in a constant current. This self-limiting of the current is the reason why alternators don't need a current regulator to protect them against overload, while dynamos do. In a perfect world, a car alternator would then generate an essentially constant maximum current at ANY speed above a certain threshold, this threshold being the RPM at which the induced voltage gets equal to the system voltage plus the rectifier drop. But our world is imperfect: There is the resistance of the coil wire, there is iron loss, there is flux dispersion, and so on. The result is that this ideal constant current curve with a sharp cutoff gets distorted into a smoother shape. But the basic facts remain that, firstly, at high RPM the maximum current varies little with RPM, and secondly, that at a certain minimum and nonzero RPM the alternator completely stops delivering current into a 12V system. And now I return to my original paragraph, to clear it up: I would guess, and I stress that this is just a guess, that a manufacturer would specify his alternators at some low to midrange RPM at which the output starts getting into the plateau. If this is done, for a good, relatively low loss alternator, then at half that RPM it will most likely fall below the threshold of zero output current. That's why I vehemently opposed the idea that an alternator rated for 70A at 6000 RPM would generally produce 50-55A at 3000 RPM. It could be true for a specific alternator, but if I were the manufacturer of that machine, I would then rate it at a lower RPM! > If you go to: > http://www.balmar.net/PDF/Alternator%20Drawings/60- > seriesdimensionaldrawing.pdf > > They have a wonderful drawing of their alternators in various > amperage ratings. I couldn't see this PDF file, my computer reports it as being corrupt! Is it perhaps in some new version of PDF which my Acrobat 5.0 can't read? But I browsed around on the Balmar web site. Please note that these are NOT car alternators, but marine alternators! There are differences between teh two kinds in RPM operating range and sometimes in the regulation scheme. If you use the data for marine alternators to analyze what I wrote about car alternators, then we are comparing apples to oranges. The principles are the same, but the implementation varies quite a lot! > In The lower left hand corner are the output curves > for them. The lowest line is their 70 amp alternator (also shown are > 100, 120 and 150 amp alternators) > You have to interpolate between the lines, but at alternator rpm (not > engine rpm) their 70 amp can produce about 32 amps at 1800 rpm, about > 38 amps at 2000, about 60 amps at 3,000, and finally reaches 70 amps > at 5,000 rpm. This is a fairly typical curve for many alternators. Yes, I agree it is typical. I tried to draw the curve from your data above, not being able to see the PDF... From what I can extrapolate, this alternator would be down to zero current at roughly 1500 RPM. So, if you take the maximal spec of 5000 RPM at 70A and then half the RPM, indeed you end up at 50-55A. But if you take the useful spec at the beginning of the plateau, roughly 3000 RPM and 60A, and then half the RPM, you and up at zero current! This illustrates my point: It is dangerous to take a single Ampere/RPM rating and try to infer the current at other RPMs from it. Instead, it is necessary to look at the real curve for the alternator one intends to use. > To me, it is ridiculous to spin the alternator the extra 2000 rpm to > pick up 10 amps, in this example. But maybe the better cooling from the higher fan speed would be beneficial! There are so many factors to consider... > That extra 2000 rpm will > significantly cut down the brush life. Probably, but I wouldn't bet on that. Brush wear is not proportional to RPM. There are many other factors, such as current through the brush, and against logic intuition, lower current does NOT always cause longer brush life! There is an optimal current per contact surface of a brush, and strong deviations from it to either side will reduce the life! The current through the brushes will be adjusted by the regulator to maintain the nominal voltage, if possible. So, a higher RPM will make the brush (excitation) current go down. For this reason, the curve of RPM versus brush wear can take pretty weird shapes! > Note that this is what the alternator can do at those rpms, not what > the regulator is telling it to do at any given moment. Yes, that's clear. It is assuming maximum excitation current, probably at 14.4V applied to the brushes, and the alternator output current being delivered across that same voltage. > "The regulator in a car alternator is designed precisely to fully > charge 12V batteries. Typically, the alternator output is regulated > to 14.4V or thereabout." Yes, that's how it is. > It is not designed precisely to fully charge 12V batteries. It is > designed to fully charge ONE 12V battery of a specific size. It will > vary the current based on the current state of charge of the battery, > to recharge it as rapidly as possible without doing harm. Dear Oso, I absolutely hate word wars (and other wars too!), but here I have no choice but to state that you are wrong! After starting, a car alternator has to do TWO tasks: Recharge the battery, and power all electrical loads in the car. But the alternator has only ONE output, and no external sensor to measure how much current is going into the battery, and how much into the other loads! So, it just cannot control the current going into the battery. The only thing it can do is regulate the voltage, either to a fixed value near 14.4V in the case of simple regulators, or to some time-controlled voltage shape in the case of more complex (two-stage, three-stage) regulators. It may also shift the fixed voltage, or the entire voltage curve, according to temperature, if it has a sensor located such that it can measure the air temperature in the engine/battery compartment. But only few internal alternator regulators have this sensor. And the voltage/time curve, or the fixed voltage, stay exactly the same regardless of battery capacity or number of batteries in parallel. Only the current has to vary in that case, but the alternator can't control the current because it simply doesn't know how much of its total current is going into the battery... > Higher > current rates used when the battery is in a lower state of charge. > Lower current rates when the battery is at a high percentage of > charge and is being "topped off". That's true, but it's not the alternator which is defining the current! It's the battery itself. If you apply a fixed charging voltage of around 14V to a partially charged 12V lead-acid battery, it will just by itself take a healthy charging current, and taper it off as it nears full charge! Only when it is at a very low charge state could it take an excessive current. But the charge current a car battery can safely take when very discharged is very high, far exceeding the rating of the alternator used in the car. So, usually the alternator runs at full current, depending on RPM, until the battery has come up to 14.4V or whatever the regulator is set to. At that point, the charging current starts tapering off, getting close to zero after some hours driving, while the alternator current is the charge current plus whatever current the other loads in the car are taking. If you use a larger battery, or several batteries in parallel, the alternator will stay in current limit for a longer time to bring them up, and then the taper current will be higher. But the full charge state will be attained just the same, given enough time. > The regulator for a specific car will be based on the amp hour > capacity of the battery that the manufacturer puts in the car. Absolutely not! In fact, many cars give a choice of battery size, while the alternator and regulator stay the same. > The > Toyota Camry comes with a 65 amp hour battery. So if we are talking > about bringing the battery from 95% to 100 percent charge and doing > it at a 10 percent rate, the regulator will instruct the alternator > to deliver 6.5 amps to the battery. Does the Camry have a current sensor on the battery wire? Otherwise that's just not possible. What will really happen is that the regulator regulates to 14.4V, and the battery will take a current that starts at perhaps 20 or 30A, quickly goes down to around the value you say, and then continues tapering down until reaching almost zero. If you used the parking lights, and your battery is low, the charging current will stay high much longer, and not because the regulator sets a higher current, but simply because at the fixed 14.4V the battery takes the higher current! Any lead-acid battery can easily take a 30% rate when significantly discharged, and car batteries can take much more than that without any damage. In my car, when I have used the electric winch to pull someone out of the mud, and then drive off, the charging current excedes 40A, into a 75Ah battery. My last battery provided service for four and a half years before starting to become weak, so I guess it didn't feel particularly badly treated! > 5 percent > of a 65 amp hour battery is 3.25 amp hours, the 6.5 rate will have it > fully charged in half an hour. Yes, that would be right IF you had a constant current charge, which isn't the case... In truth, the current will taper off, and so a real full charge takes very long! But you get above 90% rather quickly, and that's what matters. > When you take that same alternator/ regulator and couple it to a 650 > amp hour battery (or 10 batteries of 65 amp hours wired in parallel) > the regulator cannot tell the difference. True. > You are now 32.5 amp hours > away from full and trying to fill it at 6.5 amps. It will take 5 > hours of charging without any additional draw on the system to bring > the battery to 100 percent. No. The regulator will regulate to about 14.4V, and each of the ten batteries will try to draw a moderate current, which might indeed be near 6.5A (depends on charge condition, temperature, battery age...), and that will most likely put your alternator into its current limit, so its voltage will gop down a little until the batteries have become less charge-hungry! > And because you are filling it at 6.5 > amps, you are not generating the additional 30 plus amps that you > could be generating with a smarter regulator, or one better sized for > your battery bank. Wrong. The most basic, simplest car alternators with internal regulators will always try to regulate to approximately 14.4V, and so the current will be higher if the battery bank is larger and at a given state of discharge. > One solution is disable the internal regulator (or remove it ) and > use another regulator or controller that is more appropriate for the > larger battery bank. Try to buy a car alternator regulator based on battery capacity!!! I don't think you will find any! > Another option is to lock the regulator into a > 100 percent charge rate and then use a dump load controller and dump > load to burn the excess generation beyond that which will fit into > your battery bank and current usage at the moment. Yes, using a dump regulator is a good possibility, and is in fact what I would do if I used a car alternator in a hydro system. The internal regulator just isn't a good choice, because when the load is low the speed will increase. What I would do is using a dump load with its controller to regulate the battery bank voltage, and then manually set the alternator excitation current to the value that provides the optimal turbine RPM for best power output. With the system voltage held constant by the dump regulator, you can nicely control turbine speed with the excitation current! I did that with some of my wind turbine regulators, using electronic control of both the dump load and the excitation current, but still needed additional brakes on the turbines, because wind is so tremendously variable! Hydro is heaven in comparison, with water pressure being constant in most installations! > Anyway, I hope that this explanation helps. I think our exchange will help many readers here to better understand these things! Bye, Manfred. -------------------------- Visit my hobby homepage! http://ludens.cl -------------------------- ------------------------ Yahoo! Groups Sponsor --------------------~--> Get fast access to your favorite Yahoo! Groups. Make Yahoo! your home page http://us.click.yahoo.com/dpRU5A/wUILAA/yQLSAA/FGYolB/TM --------------------------------------------------------------------~-> Does your company feature in the microhydro business directory at http://microhydropower.net/directory ? 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