The alternator on the 2001 S'Crew is rated at 130 amps. Does anyone know at what RPM's this rating is made?

TIA

 

I would be reasonable to expect full power from the alternator at 55 mph and maybe a bit lower. Maybe even 45? In my Supercrew, 55 mph is around 2200 rpm. I bet you get full power as low as 1800 rpm. Just guessing.

 

InfernalCombustion is right. Max power is usually at about 1800-2200 RPM on most vehicles.

 

I'm also guessing but I will take a stab at it but with a technical twist.

To me, a 130A alternator should be able to deliver full amperage at much more than just one specific RPM. I would venture to say it's a range but mostly that the HP required to deliver 130A will always be the same regardless of engine speed.

The only change would be the required amount of torque to drive the pulley at a specific pulley RPM as opposed to a higher or lower RPM. The belt system can only handle a specific amount of torque before it starts to slip, therefore, based on that the belt drive could only allow full 130A output at a certain pulley speed and then any thing higher until the alternator self destructed.

Just curious, why would you need to know this information about your alternator anyhow?

DaveMan

 

What you say would be correct in terms of an energy input/output equation, but real world alternators are completely RPM dependent. Worse, the output of an alternator is not a linear function of RPM... the output is worse at low RPM's. Here is an example of an alternator output curve, thought not for the Ford.

http://www.balmar.net/71-seriesdimensions.htm

As you can see, at low RPM's the alternator doesn't put out much juice, and in reality, at higher RPM's its working output is in the 80 to 110 amp range.

So, I'm curious as to what Ford means when they say 130 amp alternator. Is this the absolute max that you can squeeze out of it? If so, then load planning ought to consider realistic amperages of 90 to 100. OTOH, if, as others have said, this is the mid range RPM output, then there is extra juice available.

 

Hey BeastRider,

I surely don't want to argue with you here but I'm sticking with my original statement 'cause there is no way one can say that an electrical device like an alternator can only produce one current at one specific RPM. I would agree to that if one can keep the load that the device sees at a constant. However, if the load can change, then so must the current at the same speed of the alternator. The current output must increase to the point that the belt starts to slip. After all just what is controlling the current here, surely not the VOLTAGE regulator, it's the load that is in the circuit.

I'm not an electrical Engineer, I'm a Mechanical, but I do design electric forklifts as an occupation. I don't design motors or alternators but you are going to have to do more at convincing me before I stand corrected.

I think the curve shown on the link is for a fixed specific load trying to maintain a specific voltage across it.

Question:
Ever hooked up a set of jumper cables from one car or truck to another while the engine was idleing? Do you recall what happens to the jumper vehicle, the alternator abruptly loads up and tries to supply whatever current is demanded by the disabled one. If the load is too high, the jumper car can stall or the belt will start to slip. According to graph, you will get possibly either too much current or not enough at idle. Everything I've ever know about electricity says that current in a circuit is however much is needed at the voltage and load connected, regardless of anything else.

Another example:
Take a portable welder that is idleing, set the welding current to max and disable the automatic fast idle after you strike an arc and I will bet the engine stops. Why, because the current was too much for the alternator to deliver at the specific RPM. According to that graph, the alternator would only produce one and only one current at a specific RPM. I just don't agree.

Prove me wrong and I will stop.

DaveMan

 

I'll respond this way. Obviously, a 50 amp alternator and a 100 amp alternator will put out different levels of current at a given RPM. And, the 100 amp alternator will require more engine horsepower to run it at a given RPM.

However, at least three things besides RPM affect the alternator output: The number of windings on the alternator rotor, the size of the wire, and the magnedtic flux generated by the stator. Of these three, only the flux can be readily changed "on the fly". One can put more and more current thru the stator windings, but at some point you reach a crossover... you are putting as much juice into the alternator as you are getting out. Thus, it is not possible to build an alternator that maintains a constant current across a wide range of RPM's. If it were possible, I don't think you'd be seeing the kind of output curves that are produced by today's alternators.

BTW: Alternator output versus load is balanced with the regulator. It cuts out stator coils to reduce total output. There is also a ground dump, the way it was done with a generator.

 

Hey, since we're on a related topic, can someone help me stop static electricity during the winter....I hate that happening..

 

Hey BeastRider,

I'm done, you're right. BTW, are you an electirical Engineer? Where did you get your wealth of knowledge? Fill me in so I can learn.

Hey Thefoyboy,
Had the same problem for years and finially figured it out, it's the cloth seats. The cloth seats generate static electricity while you slide out of the seat. I think too it depends on the kind or type of shoes you commonly wear, (more or less insulating). I found that when I wore my Rockports, it was much worse than when I wore tennis shoes.

One way to help reduce or eliminate the shock is to hold onto something that is made of metal and then slide out of the seat.

The other way is to trade in your XLT on a Lariat and whala, problem goes away becuase of the leather seats. Plus you get a higher payment along with some stripes, Lariat decal and a few other bennies

Talk later,
DaveMan

 

Yah, yah, BSEE, Rice University, 1997, at the tender age of 47. Specialized in systems and controls with an emphasis on robotics and the actuators that control them. Did a lot of motor/generator work, a lot of which I have forgotten... prolly due to cheap scotch.

You are right on about the theoretical construction of an alternator, and although today's alternators can't do it, I can't help but wonder if more sophisticated control electronics couldn't drag more amps out at low RPM's... I've never seen a graph of stator excitation currents versus output currents... either that, or use a combination of permanent magnets in conjunction with a powered stator.

BTW: You are right on about the static electricity... I hate to drive my wife's car in the winter... fuzzy cloth seats... every time I slip my denim jeaned *** across them, it is a guaranteed zap, especially if I wear my rubber soled tennies. I'll take the leather, please.

 

I have tested my own truck on our Bear brand roll out tester. When I held the truck at 2000-2100 rpms with nothing on (heater,ac, radio, etc) I got a amp reading of 138 amps. I have not done anything special to my truck this is the stock alternator. Most of the time when I test an alt it never hits exactly what it's rated for, it's either +/- a few amps. I am sure there is a cutoff point where most alt's put out their max amps and then there is a drop off point where at higher RPM's the belt will slip or the alt will actaully lose amps. In fact I can remember a time when I tested a lady's car (a GM) and when I told her to rev it up to 2000 she held it WOT. I noticed as the RPM's climbed so did the amps and at about 4000 they started to drop a few amps. Not that any of this helps anyone but just my .02.

 

Good info, Mr. Freak... substantiates the estimates made by Infernal Combustion... also suggests that when I'm tooling along at 45 MPH and 1200 RPM, actual amps are prolly around 90.

 

Okay guys,

Hey Beastrider-You're pretty damn humble, you are alright in my book.

I thought I was done but, damnit I must comment here and as someone I know on this site has told me, they just can't resist.

My original point was exactly what KYFordFreak mentioned,where the he// did those 138 amps go if there was no load? I'm saying that the 138 amps he measured is only due to the load that was applied to the alternator, there must have been a variable load on the tester that allows the alternator to see a substantial load!

If the load was small, then the alternator's output would be small. If the load was big, then the alternator's output responds accordingly, more current, at the same RPM I must add.

All I was trying to say originally, when I stuck my nose into this thread, was that an alternator does not just output 138 amps at 2000-2100 RPMs if the load does not require it, but rather it gernerates exactly what is required to satisfy the circuit's requirements.

Of course there's a limit as to how far the alternator go to produce or match what is needed and yes that depends on lots of things like, field current (amount of flux gererated), winding size (wire dia.), losses like friction and insulation breakdown, and last but not least RPMs.

Finial:
What was the load applied across the circuit of the truck that was tested at 138 amps @2000-2100 RPMs?

What would the current have been if the load was reduced by 100%? Bet is was almost 100% less current with the lower load.

If what I just spoke of is true, then my original statement is true. An alternator can produce more, much more, than one specific current at any given speed. But of course, all that must be within the limits of the design of the device and maybe this last statment is what I assumed we all would understand.

As much as I hate to ask, correct me if I'm wrong but I know I'm not an idiot with this subject, of course unless someone can convince me otherwise.

Glad to hear the static electricity theory was farily accurate 'cause I had first hand knowledge on that one too.

Talk later guys,
DaveMan

 

Um, guys, the question about the static electricity was a sly attempt at humor to break up the very serious discussion. It was getting so technically deep my head was hurting and thus the attempt at humor. Guess I'll keep my day job..<grin>

 

Answer is I don't know will try to find out. The way the tester is hooked up there is a possibilty of a load but I am not sure if one is put on it or not. Looks like there would almost have to be to test the alt correctly.
Here's how the tester works. Select # of cyinders, then you hook up your + and - cables and then there is a sensor probe that goes around the neagative battery and the other little wire (field wire, ground wire, ?) to sense the RPM range. At this point with the truck started you would see a RPM of about 600-900, a voltage around 12-13, and amps at least 80 or so. Machine will ask you to rev it to 2000 and hold for 10 seconds. At that point volts should be at 13-16 (14.4 considered the best all around), amps should near the rated capacity of the alt. At end you get your final report of peak volts ( 13.9 on mine) , amps (138 on mine) and diode condition (good on mine).
I have seen very few cases of alternators putting out more than what they were rated but that was the case on mine. It was not overcharging as you can tell by the volts just putting out more amps than expected. IMO output has a lot to due with the mechanical state of the charging system.

I'm am out of ideas for now but, I think in I might test it again just to see what it does now as it has been about a year since that test above.

I can see where your coming from on the variable load thing with an alt. The higher the demand the higher the output.