# Does an Alternator Really Take This Much Torque?

1. May 25, 2013

### thomasorange

Hi,

I'm looking around for a replacement alternator and came across the specs which state max current available at 6000rpm and requires 47nm of torque.

I was quite suprised with this needing that amount of torque. So I'm starting to imagine a typical diesel which most don't get up to 6000rpm. So i'm imagining there is some kind of gearing between the engine and alternator, perhaps 1:2, so at 3000rpm of the engine the alternator is 6000rpm. With the alternator needing 47nm of engine power (after gearing) it would now need 94nm before gearing.

Is this right? Does this alternator just swallow a lot of the engines torque just to run a flat out electrical system?????? Agreed, it shouldn't need this power all of the time hence run at lower torque but I'm suprised what the alternator takes?

Are these figures accurate?

Thanks

2. May 25, 2013

### Staff: Mentor

3. May 25, 2013

### SteamKing

Staff Emeritus
Most belt driven alternators in cars and trucks are not driven 1:1 at engine speed. The alternator pulley is usually smaller, making the alternator turn faster than engine speed. The alternator needs enough output at engine idle speed to keep the electrical system of the vehicle functional.

4. May 25, 2013

### thomasorange

It is an average car, the voltage 12v and the current is 150A max. For reference the datasheet I found was here

I took a look at the wiki page but I'm a little confused as to what I am looking for - I'm not sure what to apply to which formula

5. May 25, 2013

### SteamKing

Staff Emeritus
There are two methods:

1. In the wiki article, go to the section "Conversion to other units", toward the bottom of the article.
Select if you want power in watts "P/W" or horsepower "P/hp". For "P/W" select the formula for speed in RPM.

2. A rough estimate of the output is multiplying the voltage by the amps output, since 1 watt = 1 volt * 1 amp. This estimate will be high, since a power factor of 1 is assumed. For 12v and 150A, the alternator puts out 12*150 = 1800 watts or 1.8 kW. Not a lot, really. Compare this power with the engine output to see how significant it is.

6. May 25, 2013

### thomasorange

Thanks for the explanation. However when I plug in the figures into the formula, I come out with 10KW

$$P(w) = {\frac{τ(nm)~x~ 2π ~x~ω(rpm)}{60}}$$

I then plug my figures in:

$$P(w) = {\frac{47(nm)~x~ 2π ~x~2200(rpm)}{60}}$$

I get 10,828w.

where have I gone wrong?

Thanks

7. May 25, 2013

### SteamKing

Staff Emeritus
That seems OK. Method one is just a rough estimate. I wouldn't worry that you don't calculate 18 kW. You still have the HP and torque curves from the supplier, and these figures can be converted to N-m and kW.

1 HP = 746 watts.

8. May 25, 2013

### Danger

I'm going to rely up on Ranger Mike to clarify this, but one thing to keep in mind if you're using this in a real-world application is that the load determines how much power is required. We used to put cut-out switches in the output wires of our alternators to eliminate electromagnetic parasitic drag upon our engines while racing. The alternators still grabbed some horsepower to overcome bearing and pulley resistance, and air resistance from the intake fans, but it saved a few hp. (That's also why we got better gas mileage in the daytime; no headlights meant less drag.)

9. May 25, 2013

### jim hardy

Because power is product of RPM and torque, as RPM increases required torque drops off.

Is 47 n-m clearly stated to be at 6000 RPM alternator speed ? I would suspect it's at idle instead, perhaps 1800 RPM..

Here's a paper with an interesting description of alternator characteristics:
see fig 4.
http://www.rle.mit.edu/per/ConferencePapers/cpConvergence00p583.pdf

10. May 26, 2013

### OmCheeto

Where did you get 47 nm?
The graph you posted maxes out at just short of 14 ft-lbs @ 2800 rpm, which my converter says is about 19 nm, and my calculator says results in 5800 watts.

Which doesn't really make sense to me. 150 amps * 12 volts = 1800 watts = 2.4 hp.

Why does this alternator consume 3 times more power then it puts out? Do all alternators do that?
And it maxes out at 10hp! It's throwing away 7.6 hp at max rpm?!?! Ahhhhh!

Maybe I'll convert my lawnmower into an alternator experiment in the morning.

I'll need to build one of these too I'm guessing.

11. May 26, 2013

### OCR

12. May 26, 2013

### SteamKing

Staff Emeritus
I did a little digging around on the mfg. website. There are several points for the OP to consider:
1. The mfg. lists the alternator model 8SC3110V as being superseded by a newer model.
2. It is not apparent where the torque value of 47 N-m is given.
3. The system voltage is listed as 24V, rather than the 12V on the data sheet.

13. May 26, 2013

### Ranger Mike

parasitic drag

Sitting here having coffee before the Indy race...please read post in General engineering forum ,,,,March 16-08 - Switching off alternator during hard acceleration
Danger and Brewnog are on it as usual

Since the 1950s race cars ran on battery only for drag races and short duration round track races ( where magnetos we not permitted). This saves about 4.5 Horsepower on typical V8 engine. Alternators and earlier generators are notoriously inefficient. It takes about 1 HP to make 25 Amps in a perfect world. So dynometer results since the 1960s say you gain about 4.5 HP with the alternator removed.
You gain about 7 horsepower when you eliminate the mechanical water pump.
Now you need a larger battery to go the same distance ( 20 lap race?) The weight of an electric water pump vs. mechanical is about the same. So you would ne adding to the total weight by changing to the bigger battery.

Couple of things to think about. When you take off the alternator on todays cars..major headache as everything is electrical. Life was simpler in breaker point days ( and early ignition system). You will reduce parasitic drag and increase your race reliability when you do this. You eliminate drive belts that can break and put you out of a race. You can take the 5 pounds off the front end of the car ( alternator is 5 #, water pump 7#) and locate it closer to polar moment ( bigger battery). You can run an electric fuel pump ( most all of todays cars have this) to reduce parasitic drag. Mechanical fuel pumps were mounted on the side of the engine block and vapor lock happened a lot on hot summer days because the heat caused by the internal combustion engine would heat sink into the aluminum fuel pump and cause vaporization of the fuel in the fuel line. Again you have the advantage of mounting the fuel pump close to the fuel call to keep the fuel cool before it got near the engine ( we wrapped thermo wrap around the fuel lines once it got close to the engine room. You do need to add a roll over switch when you do this. You can save a lot of HP by the radiator cooling fan used. At one time a steel multi blade fan was mounted on t he mechanical water pump and this cooled the radiator. Next evolution was the clutch fan that was “ on “ in traffic idle situations and kicked “ off” at speed but this was one heavy item. Next came a flex fan that weighs next to nothing and the fan blades flexed “ out” to suck in more air at higher rpm. Finally all this was eliminated with electric cooling fans but here we go again with a bigger battery.

14. May 26, 2013

### Danger

You've passed my level of experience here (which is why I always reference you). We were just Saturday street racers, on a quarter mile of rural road in front of my friend's farm. For us, it was stock gas tanks and mechanical pumps. (I don't think that electric pumps were even available at the time.) We did, however, use "chiller cans". That was essentially just a coffee can bolted to the fender well. A section of the fuel line was coiled up inside it, and it was filled with ice.
My Roadrunner came factory-equipped with a 7-blade clutch fan, and my buddy shelled out some shekels for an aluminum flex-blade. The flex is probably more efficient, but I'm hanging onto the clutch for nostalgic reasons. It makes one hell of a noise, which scares the competition.
I do have to ask for clarification about something: I think that I can surmise from the words what a "roll-over switch" is, but I've never heard of such a thing in an automotive context. Is it just a fuel shut-off that comes into play when inverted?

15. May 27, 2013

### jim hardy

Well, goodness i sure missed that datasheet link in post #4 !
http://news.prestolite.com/drupaldocs/C-3576-2_8SC3110V_150a.pdf

Datasheet Graph shows torque peak ~ 13.6 ft lbs at maybe 2750 RPM which calculates 7.12 hp,
just about what the dotted hp line on same graph shows .

and at output of 14.5 volts and 150 amps, which is 2175 watts, which is 2.91 hp, that's 41% efficiency.
41% seems a bit low if typical is 50 to 60:.
Datasheet says "heavy duty" so maybe it has an oversize fan. I have an old Leece-Neville truck alternator that has quite a large one compared to the one on a passenger car...
And a fan explains input power continuing to increase with RPM.

So to OP: yes, they take substantial torque.
Please check my arithmetic, and my interpolation on that graph you linked.

At idle you can hear the engine slow down from the extra load when you connect the battery terminal.
On the old electromechanical regulators you could manipulate the contacts for a quick system check. Nostalgia, sigh.

old jim

Last edited: May 27, 2013
16. May 27, 2013

### OmCheeto

Thanks! I just dropped by there also, and read up on that alternator. They posted a graph of current output vs rpm.

http://www.prestolite.com/productinfo/alternators/8SC3110V/8SC3110V_curve.jpg

That explains the shape of the hp curve in the original graph.

This alternator is special use, designed to be run at one speed, and is not to be hooked up to a battery.

ref

The original graph was posted erroneously by Prestolite in 2011 and should probably be ignored as far as numbers go. The shape of the graphs are probably still valid.

Wow. Prestolite is pretty slow. The question was posted in 2009!

hmmm..... Who makes 12HP trucks?

17. May 27, 2013

### Ranger Mike

excellent Jim... ( how old is old??) Experience is the fountain of youth...and yes Danger..you are right. switch activates on impact or when vehicle roll over..disconnects juice to fuel pump

18. May 27, 2013

### Danger

Tonka.

Thanks for the info, Mike. That thing is a great idea, but I've never heard of it before.

19. May 27, 2013

### OmCheeto

Not to me.

When my alternator fell off that one day and I powered my electrical system for the 12 mile ride from work to home with solar panels, I decided that my car consumed roughly 9 amps of current, as the deep cycle backup battery voltage changed very little from start to finish. The deep cycle was of course to get me through the shady spots. They line every stinkin' street in this town with trees!

But anyways, the voltage and power graph only shows the potential of the alternator. It doesn't really show what the alternator, in real life, would be doing.

And comparing the output of a standard automotive alternator, with this special use alternator, it's obvious to me, that they are different beasts.

Automotive (diesel) alternator:

50 amps at 200 rpm? Good god. (note to self: get one of those for your windmill project)

------------------------------------------------------------------------------

8SC3110V special use alternator:

Zero output at 1400 rpm!
Good god. It's spinning 7 times faster than the automotive beast, and putting out nothing! And the automotive beast is capable of putting out 9 times what I need just at idle.

Gear this one up to 700 rpm, and with a 8000 rpm limit on the alternator, you get a 4000 rpm limit on the engine. Definitely designed for a diesel.

--------------------------------
hmmm....
This might make a good topic in the Electrical Engineering forum.
What makes these two alternators so radically different?

Last edited by a moderator: May 6, 2017
20. May 27, 2013

### jim hardy

That was a guess on my part , when I thought from OP's linked datasheet curve this was a general purpose alternator.

9 amps sounds reasonable for electric fuel pump, electronic ignition and alternator field.
Lucky it wasn't one of those with electric power steering and radiator fans , & headlights that turn on with ignition switch..

you're exactly right of course, the alternator is equipped with a voltage regulator and in normal use operates well below its capability curve.

look closely at that graph - on horizontal axis it is engine rpm
and other one is alternator rpm.

I just measured two old Fords in my yard.
Both have 2.5 inch alternator pulleys.
The gas engine(1976 F-100 pickup truck) has a 6.5 inch flywheel crankshaft pulley,
and the diesel(1983 Ranger compact pickup truck) a 7 inch one.
(Now, that little diesel is rated to 4000 rpm.)

So 600 engine rpm idle would yield 1560 and 1680 alternator RPM respectively.

Still, that is mighty near the low end of the Leece Neville capability curve
so I suspect it is a higher RPM alternator intended for use with a bigger engine having a large diameter crankshaft pulley.
Big diesels have a narrower RPM range; a GM 6-71 like in old Greyhounds is typically 550 to 2300 RPM ~4::1 range, while a little 4 cyl car like Ford Escort might be 600 to 6000RPM ~10::1 range.
So the bigger slow turning diesel with its narrower operating range is less apt to overspeed the alternator.

So I suspect the two alternators are not so radically different.
Still, I never heard of a "batteryless alternator" before and am curious what would be different about one.
I'm outside my knowledge base there..

Now THERE'S a good idea.