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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / October 2005how many hp to turn an 80A alternator at full tilt?
anybody know? Im trying to make up a generating/charging system for some electric motors to push a boat, i can do it cheaply and efficiently with lots of redundancy in the system with a series of ganged car anternators, but im having trouble figuring how to size the motor to drive them.... would a small engine, lets say an averge 125cc 4 stroke drive a single 80A alternator at full draw? Any approximate equations for this, or rough feelings? Would appreciate any advice at all! Thanks Shaun 80 amperes at 12 volts is 960 watts. Electrical equivalent of horsepower is roughly 750 watts, so you would be drawing about 1.28 horsepowers worth of electricity at more or less peak power. But don't assume 100% efficiency in converting mechanical horsepower to electrical. I would derate by 50% or more, and use an engine in the range of 3.5 to 5 horsepower at least. You can use the exact conversion factors if you want, but it won't affect your basic engineering issue. > 80 amperes at 12 volts is 960 watts. True enough for 12 volts, but unless you "fiddle" with them, every "12 volt" automotive alternator I've ever encountered did its level best to keep the output peg at somewhere between 14 and 15 volts, usually about 14.6, to be exact. Which changes the equation slightly... Assume 14.6 volts (seems to be the most typical) on the output, and you've just jumped the whole mess up to almost 1200 watts, which means... > Electrical equivalent of horsepower is roughly 750 watts, > so you would be drawing about 1.28 horsepowers worth of > electricity at more or less peak power. ... is actually going to end up higher than the 1.28HP (without actually doing the math, I make it something pretty close to 1/2 HP or higher) per alternator that you're coming up with. > But don't assume 100% efficiency in converting mechanical > horsepower to electrical. Amen to that! > I would derate by 50% or more, and use an engine in the range > of 3.5 to 5 horsepower at least. Absolutely. And that's assuming just a single alternator being spun - Start "ganging" them like he was talking, and "things is gonna git ugly" in a fairly short hurry due to mechanical losses before the HP even gets a chance to be applied to the alternator. Agreed that the details don't change the basic problem, though.  SignatureDon Bruder - dakidd@sonic.net - New Email policy in effect as of Feb. 21, 2004. Short form: I'm trashing EVERY E-mail that doesn't contain a password in the subject unless it comes from a "whitelisted" (pre-approved by me) address. See <http://www.sonic.net/~dakidd/main/contact.html> for full details. > > 80 amperes at 12 volts is 960 watts. > [quoted text clipped - 6 lines] > the most typical) on the output, and you've just jumped the whole mess > up to almost 1200 watts, which means... I thought about going into it a little deeper, Don, but felt the 12 V approximation was close enough for this approximation. In fact, he could bugger with the regulator, or even bypass it totally, and really get some higher voltages. Back in the 70's, I guess, people sold little receptacle type boxes you could install under the hood of your car and plug in drills, saws, etc. And they worked. They were riveted shut, but of course I had to open one. It was nothing more than a switch that put the regulator out of the circuit and put the alternator on full voltage output...And it WAS enough to run power tools with AC/DC (brush type) motors. I never saw them cause any problems, but I would not recommend using an alternator in this manner. > -- > Don Bruder - dakidd@sonic.net - New Email policy in effect as of Feb. 21, 2004. > Short form: I'm trashing EVERY E-mail that doesn't contain a password in the > subject unless it comes from a "whitelisted" (pre-approved by me) address. > See <http://www.sonic.net/~dakidd/main/contact.html> for full details. > anybody know? Full tilt would be about 14 volts. So: Power=voltage*current, therefore its (14*80) watts or 1120 watts. Allow for about 85% efficiency (belt loss, diode and wire heating, fan losses), so .85(input power)=1120 watts, or input power is about 1318 watts. 1 horsepower=746 watts, so in horsepower the alternator needs 1318/746, or about 1.8 horsepower. >> anybody know? > [quoted text clipped - 7 lines] >1 horsepower=746 watts, so in horsepower the alternator needs 1318/746, >or about 1.8 horsepower. 85% efficiency is a very optimistic figure. Most 12-volt automotive alternators operate at between 50 and 60% efficiency. Another important consideration is the fact that the nominal output rating of an automotive alternator is not intended as a continuous rating. Battery charging under normal conditions requires a fairly heavy output for a short time, but then it tapers off as the battery approaches a full state of charge. Try to run an "80-amp" alternator at a continuous output of 80 amps, and it will get hot enough to melt the solder connecting the stator windings to the rectifier diodes. Sizing the alternator(s) for the load, it would be prudent to count on about 1/3 to 1/2 of the nominal rating for sustained running. >>>anybody know? >> [quoted text clipped - 10 lines] > 85% efficiency is a very optimistic figure. Most 12-volt > automotive alternators operate at between 50 and 60% efficiency. I'd actually be surprised if any well-built modern rotating electrical machine is that poor. 85% might be high, but 50% is RIDICULOUSLY low for a well-designed electrical machine. That's more like what you'd expect from a heat engine, not an electric motor or generator (alternator). > Another important consideration is the fact that the nominal > output rating of an automotive alternator is not intended as a [quoted text clipped - 4 lines] > enough to melt the solder connecting the stator windings to the > rectifier diodes. Mabye for some cheap Asian alternators and Delco CS-130s, but I'll guarantee you that there are plenty of alternators that can run at 100% rating indefinitely. Battery charging isn't the ONLY thing the alternator has to do, and on a modern car with multiple electric radiator fans, powerful cabin A/C blowers, heated seats, A/C, power-hungry engine and transmission management systems, resistive window and mirror defoggers, etc. the steady state load on the alternator can be quite large. I agree with your basic intent, which is that over-sizing the alternator substantially is a good idea in this case, but I think you're being WAY too pessimistic. >I'd actually be surprised if any well-built modern rotating electrical >machine is that poor. 85% might be high, but 50% is RIDICULOUSLY low for >a well-designed electrical machine. That's more like what you'd expect >from a heat engine, not an electric motor or generator (alternator). Not ridiculous at all. You're generating AC, then chopping it to a semblance of DC with a rectifier bridge. Delco/Delphi alternator specs are typically in this range. So are Mistsubishi (Asian, but NOT "cheap"). If you make quantum leap to a brand like Niehoff, then it gets better, but they're WAY too expensive for automotive production. >> Another important consideration is the fact that the nominal >> output rating of an automotive alternator is not intended as a [quoted text clipped - 13 lines] >window and mirror defoggers, etc. the steady state load on the >alternator can be quite large. Nowhere near the nominal rating. Just like automotive engine power output. Run it at the "rated" power continuously, and the machine's life is measured in minutes, not hours. >I agree with your basic intent, which is that over-sizing the alternator > substantially is a good idea in this case, but I think you're being >WAY too pessimistic. Just recounting experience from a number of years in the engine industry. > Nowhere near the nominal rating. Just like automotive engine > power output. Run it at the "rated" power continuously, and the > machine's life is measured in minutes, not hours. If by "minutes" you mean something like 12099 minutes (201 hours = 8.375 days). In August 13 ??? 21, 1983, Mercedes decided to beat the 50000 km speed record (something that nobody had bothered to do for some years (Mercedes itself with the C111 of the 1970s, I think)). From these pages: C111: http://www.germancarfans.com/classics.cfm/classicid/5040823.001/page/2/lang/eng/ mercedes/1.html http://www.pistonheads.com/doc.asp?c=103&i=6730 http://www.mbspy.com/c111_III.htm Mercedes 190 E 2.3-16: http://www.germancarfans.com/classics.cfm/classicid/5040331.009/page/2/lang/eng/ mercedes/1.html http://www.benzworld.org/news/news.asp?id=238 "After 201 hours, 39 minutes and 43 seconds, two of the cars had clocked up 50,000 kilometers. The replacement parts carried on board in compliance with the regulations had not been required ??? the cars had been running perfectly smoothly despite the extreme strain. The third car was laid up for three hours by a broken distributor rotor arm ??? an item costing just a few cents, which the pit crew were not allowed to replace but had to repair." So, 22 years ago a good car was capable of running its engine at full power (or very nearly) for 201 hours almost continuously: "Every two-and-a-half hours, the cars came in for refueling and a change of driver during a 20-second pit stop. The heavily strained rear tires had to be replaced every 8,500 kilometers and the front tires every 17,000 kilometers. During these five-minute tire change breaks, the oil and oil filters were also replaced and the valve clearance was checked." I think that later other manufacturers were also interested by this record. I found mention of Saab at Talladega (1986 and 1996): http://www.thecarconnection.com/Vehicle_Reviews/Luxury_Cars/1999_Saab_9-3.S183.A 800.html http://www.saab.com/main/GLOBAL/en/awards_9000.shtml http://www.swedecar.com/saab_history.htm http://business.fortunecity.com/bechtel/441/17.html 1986: 100,000 km with an average speed of 213.299 km/h = 468.825 hours 1996: 226.45 km/h over 25,000 miles/40,000 km. = 176.639hours And this year another record attempt by Mercedes with a Diesel: http://www.germancarfans.com/news.cfm/newsid/2050502.003/mercedes/1.html Mercedes E 320 CDI 100,000 miles (160,934 km) Standing start 224.823 km/h ??? 139.699 mph*** World record ** 715.825 hours = 29.82 days ~= 1 month ! It is likely that these endurance records can be beaten, but it would be quite boring and expensive. Signaturehttp://www.mat.uc.pt/~rps/ .pt is Portugal| `Whom the gods love die young'-Menander (342-292 BC) Europe | Villeneuve 50-82, Toivonen 56-86, Senna 60-94 |
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