Power Consumption for Genny
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Category: Allis Chalmers
Forum Name: Farm Equipment
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URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=32699
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Topic: Power Consumption for Genny
Posted By: Dave(inMA)
Subject: Power Consumption for Genny
Date Posted: 22 Jun 2011 at 7:42pm
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I am wondering how much hp is consumed by the typical 6v generator that was used on Orange tractors in the 40's and 50's. Someone is telling me that changing to a 12v alternator will consume a lot more hp. I'm not ready to believe it! I did find that a 120 amp alternator at full output will consume 2-4 hp, but data for generators is pretty hard to find.
Does anyone know how much hp it takes to spin a genny at full output? Thanks!
Also posted this on the Shops Forum but figure this might need the larger audience that's on this forum!
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WC, CA, D14, WD45
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Replies:
Posted By: Stan R
Date Posted: 22 Jun 2011 at 8:11pm
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Not an expert, but wouldn't the power consumption be based on the wattage (volts X amps) draw (lights, radio,...) that is on the generator and not the voltage only? No draw would be just the drag on the bearings, etc... and I doubt any difference.
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Posted By: Stan R
Date Posted: 22 Jun 2011 at 8:16pm
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Regarding 120 amps, multiply by 12 volts= 2400 watts. This equates to about 3.2 hp.
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Posted By: Dave(inMA)
Date Posted: 22 Jun 2011 at 9:41pm
Stan - maybe so. Not sure but wouldn't 12 x 120 yield 1440 watts? Is required hp determined by output? Not my area of expertise!!
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WC, CA, D14, WD45
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Posted By: ky wonder
Date Posted: 22 Jun 2011 at 9:51pm
i have never noticed any power loss when converting a old 6 volt to a 12, and while i have not done it on a old allis ,i have done close to a hundred on the other major players
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i like old tractors of all colors
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Posted By: Brian Jasper co. Ia
Date Posted: 22 Jun 2011 at 10:09pm
The 120a alternator is going to have the potential to consume more engine power because it's capable of more amperage. The original 6v generator is only capable of about 10 amps. Watts is watts.
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"Any man who thinks he can be happy and prosperous by letting the government take care of him better take a closer look at the American Indian." Henry Ford
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Posted By: Dave(inMA)
Date Posted: 23 Jun 2011 at 6:24am
So would this mean that a 12v, 120a alternator would consume 1440 watts compared to 60 watts for a 6v, 10a generator, both at full power and ignoring friction losses? Does the pulley size affect the calculation? Alternator pulley might be 1/2 diameter of the generator pulley.
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WC, CA, D14, WD45
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Posted By: Brian Jasper co. Ia
Date Posted: 23 Jun 2011 at 9:16am
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Pretty much. If the alternator is only having to produce 10 amps to bring the voltage up to the regulator's set point, the engine power consumption would be pretty close to the generator's hp draw. The gen might be a little less since the armature is smaller if we're comparing to a Delco 10si. Newer units would likely be different.
I used to use a 70 amp Ford alternator to break in small engines after I overhauled them. Full fielding that alternator would really work a 5hp engine.
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"Any man who thinks he can be happy and prosperous by letting the government take care of him better take a closer look at the American Indian." Henry Ford
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Posted By: Gerald J.
Date Posted: 23 Jun 2011 at 10:15am
746 watts to the horsepower. Small generator probably has about 80% efficiency, so 6 volts 20 amps puts out 120 watts, probably takes 120 /.8 = 150 watts on the shaft (neglecting belt losses) and that's practically 0.2 or 1/5th hp. But when charging its going to be putting out 7.2 volts not 6 if the regulator is perfect so we need to compute all over. 140 watts out, 175 watts on the shaft or 0.234 hp, nearly 1/4 hp.
Take the 140 amp alternator, if it can survive at full current for a while its going to be putting 140 x 14.2 volts = 1988 watts. Efficiency is probably not quite as good as the generator (so saith those who have built their on electric wind mill) say 0.7 for a shaft input of 2840 watts, or 3.8 hp.
In most installations, the alternator is turned faster than the generator. And with the larger diameter rotor in the alternator, the internal windage losses are higher in the alternator, but the brush area on the slip rings is smaller and smoother than the brushes in the generator. All DC motors and generation schemes perform better the faster the turn, generating more voltage per turn in the winding with high speed. The limits are bearings, windage losses, and throwing the windings out of the slots in the generator. The opposing fingered clam shells over the alternator rotor winding that is wound around the shaft makes it much stronger so centrifugal force won't break it as soon as windings would be thrown from the generator armature. In modern cars and trucks, alternators routinely run 2 to 3 times crankshaft speed. In my VW redline on the engine is 6500 RPM or so, and its alternator runs at least 3 times faster than the crankshaft, or nearly 20,000 RPM top. It takes hard work to keep the windings in the slots of a generator at 10,000 RPM and usually the speed up is limited to maybe 1.5 to 1 in a car, more in a vintage tractor where the top engine speed is 1600 to 2200 RPM. But the generator speed is often limited to 4000 RPM. I might to the two cylinder expo tommorrow, it would be interesting to compare crankshaft and generator pulley sizes on a variety of tractors though all green. I've used a green tractor to pull and orange plow and a red and silver tractor to pull a different color orange plow.
Unless you are running some auxiliary load, the charging machine is not going to stay at its maximum current long. If regulated properly and the wires are fat enough it will hit a peak current right after starting and the battery current will taper off to zero in 15 or 20 minutes. The charging equipment will still have to supply lights and ignition power.
When a Ford garage replaced my dead alternator, they put a big battery charger on the battery so the new alternator didn't see a nearly dead battery. They don't expect it to really do that 140 amps very long and don't want to push it or smoke it in the first minute of operation.
Gerald J.
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Posted By: Dave(inMA)
Date Posted: 23 Jun 2011 at 12:52pm
Gerald, thanks for all the analysis and explanation! Bottom line is that a 6v genny consumes a fraction of 1 hp while a robust alternator might consume 2-3 hp.... so the fellow claiming that switching to an alternator robs power is more right than wrong! Imagine you don't notice it much on a CA as it has plenty of power to absorb the hit and keep on tractoring.
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WC, CA, D14, WD45
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Posted By: Stan R
Date Posted: 23 Jun 2011 at 6:08pm
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Dave: so right (that is what I get for having a glass or two of wine and trying to multiply simple numbers...
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Posted By: ScooterBuck
Date Posted: 23 Jun 2011 at 6:38pm
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Not really an issue IMO. As stated above the ALT will not be putting out full amperage very often. The real gain in horse power may come with a better ignition spark if switched to 12 volts.
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Posted By: Dave(inMA)
Date Posted: 23 Jun 2011 at 6:56pm
Thanks, everyone. Always interesting to launch one of these topics on here.
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WC, CA, D14, WD45
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Posted By: Reeseholler
Date Posted: 24 Jun 2011 at 7:43pm
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That was interesting. I had never given a thought about it
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Posted By: DaveKamp
Date Posted: 25 Jun 2011 at 3:56pm
Yes, one can calculate the full load based on capacity, and pad it a bit for loss, just as Gerald pointed out.... but regardless of what you install, the amount of load it imposes is based on 1) static losses (friction, windage, etc) and 2) how much you're asking of it.
If your battery is full up (so no charging load), just add up the total wattage of all loads... the ignition system, the lighting... etc., and then convert that to horsepower.
Ain't much.
Actually, if you calculate the amount drawn during cranking, and consider that a well-tuned magneto-ignited B needs about two full rotations to start, there isn't even much recharging to be done to recover from the cranking event...
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Posted By: Dave(inMA)
Date Posted: 26 Jun 2011 at 6:21am
Makes total sense to me, Dave. Thanks.
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WC, CA, D14, WD45
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