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Alternator output vs RPMs?
By zmoz, Section Homebrewed Electricity Posted on Wed Mar 10, 2004 at 01:26:18 AM MST
What happens at lower RPMs?

I've already made a different post about this, but I will be putting a lawnmower engine with a 50 amp one wire alternator. What happens to an alternator like this at low RPMs? Do the amps just get lower? What about the voltage? Also, what would be the best RPMs to run this alternator at for maximum output?

Alternator output vs RPMs? | 9 comments (9 topical)

Re: Alternator output vs RPMs? (none / 0) (#1)
by drdongle on Wed Mar 10, 2004 at 05:02:11 AM MST

Auto alternators want to see between 1000 and 2000 RPM before they produce a usable out put. I would recommend finding you engines "sweet spot" in the power curve
( best power out put vs fuel use) and then pick the appropriate pullys so as to ensure that the alternator will have enough speed to function. of course if the sweet spot is 1800 RPM a 1 to 1 may be perfect, or you might need to do a 2 to 1. You may may have to find out experimentally.

Dr.D
Carpe Vigor, Dr.D

Re: Alternator output vs RPMs? (none / 0) (#2)
by RobD on Wed Mar 10, 2004 at 05:11:01 AM MST

We can not really determine how your specific alternator will function without know things like the coil resistance and power.
Basically the voltage drop will be related to the internal alternator resistance and as the load increases (impedance drops) the alternator output voltage drops. the lower the coil resistance the less drop providing on whether the alternator has the power capacity to meet the load.
RobD

Re: Alternator output vs RPMs? (none / 0) (#3)
by dave123 on Wed Mar 10, 2004 at 09:50:01 AM MST

What size engine will you be using? I built one using a 3.5 hp vertical shaft lawnmower engine, with a 50-some amp delco alternator. If it is hooked up to too heavy of a load, it will kill the engine even wound up to max rpm. The engine is older and probably tired, but still something to think about.

When you say 1 wire, do you mean only the battery cable? or do you mean the battery cable and a voltage sense wire? I would recommend the latter, as then you can easily cut off the alternator to start the engine. Also, I made a pulser circuit to turn the alternator on and off repeatedly so that it can charge under a load that is heavy enough to kill the engine if on full time. A bigger motor running at lesser rpm would be a better way to do it, but my engine was basically a garbage pick.

[ Parent ]

Re: Alternator output vs RPMs? (none / 0) (#4)
by finnsawyer on Wed Mar 10, 2004 at 11:26:24 AM MST

The regulator varies the rotor current as rpm changes and battery voltage changes in order to keep the battery charged. As rpm drops a point is reached where maximum allowed rotor current is not enough to provide sufficient output. The regulator then keeps battery current from flowing backwards through the alternator. In reality as long as there is rotor current the field windings will provide an output voltage. At low rpms this voltage would just not be enough.
GeoM

Re: Alternator output vs RPMs? (none / 0) (#5)
by zmoz on Wed Mar 10, 2004 at 12:34:04 PM MST

I will be using a 6.5hp engine with a heavy flywheel, so it shouldn't even know the 50 amps are there. I would like to keep the RPMs on the engine relatively low, because it will have alot of power to spare. So how many RPMs do you think it should be turning for maximum output?

Re: Alternator output vs RPMs? (none / 0) (#6)
by jubalearly on Wed Mar 10, 2004 at 01:28:15 PM MST

To answer your questions first, a 6.5HP engine will easily drive a generator for 50 amps (14v x 50a = 700W - may as well say 1HP). If it weren't for the regulator, volts would vary by the ratio of the RPMs. No current will flow until you exceed battery voltage (plus losses).

Normally at least 1500RPMs (alternator RPM) is required for any useful output, and somewhere between 5000-6000RPM is needed for full output. I am setting mine up for 6000-7500RPM, using a 2.625 alt. pulley and a 6" engine pulley. Actual belt ratio may vary, but I can get full output somewhere between 2400-2800 engine RPM.

Stock, an auto alternator is rarely more than 50% efficient. You have additional losses in any belts gears or chain drive, as well as some small loss even direct drive (bearings). A 3.5HP lawnmower engine will only produce 3.5HP at 3600 RPM on the best day of its life, if then. If you are running more than 3000RPM, a lawnmower engine is doing very well to last 300 hrs. It needs to run fairly fast to get enuf cooling. So its best to run 2400-2800RPM and use a 2.5 or 3 to one pulley ratio. At 2400RPM (torque peak?) a 6.5HP B&S or Tecumseh engine will give you about 4HP, or say 3kW, maybe 1500w and 100 amps after you figure the losses. You can look at torque vs RPM curves on the Briggs & Stratton web site for more detailed info.

A one wire alternator has a built in regulator that is not suited for charging deep cycle batteries. By that, I mean that you will have to run your engine 2 or 3 times as long to get a full charge if you use an ordinary regulator. There was a good article in Homepower magazine about this about 2 years ago that is probably still on their website. Part of the problem is that they maintain a set voltage, depending on the regulator (they go from 13.8-15.5v), usually 14.8v, which is only suitable for the mid range of charging (say from 40-80% charged).

I believe that most one wire alternators (internal regulator) were 85 amps or more. How do you expect to keep it below 50 amps? If you have converted an old 55 or 65amp alternator to one wire, how do you expect to keep it below 50 amps, or control the output at all? You need more than the stock regulator. If you think controlling the speed of the motor will do this, it ain't likely to work. You can find other good references by searching using the terms in the answers you've gotten so far. HTH, -RussH

Re: Alternator output vs RPMs? (none / 0) (#9)
by zmoz on Thu Mar 11, 2004 at 02:02:00 AM MST

Got a link to the torque curves on briggs website? I can't seem to find them...

[ Parent ]

Re: Alternator output vs RPMs? (none / 0) (#7)
by nothing to lose on Wed Mar 10, 2004 at 05:01:30 PM MST

About 8-9yrs ago when I first bought my place I did something like this. Then I was dumb and had the grid installed, now I am wising up and see how dumb I was and getting back off it! :)

I had a 12hp riding mower. I simply took the deck off, made a bracket to connect where the deck was supposed to mount and then ran a belt to the motor pully. I never checked for details like actual rpm's, but it was rather a good setup!
A fast idle got me about 40amps or more acording to a standard auto amp gauge. Fast idle, I mean it was not even half throttle as I recall. Maybe 1/4 or 1/3 ?

I am sure I could have got alot better, but at the time I was only using one auto battery and a 300watt inverter. I had two batteries, one to use and one to charge. I mostly charged the second battery while driving my van, it was daul battery set up anyway, but when I did not need to go anywhere for long periods I used the riding mower setup.

It worked great other than I could not mow the yard sometimes because I was charging a battery. I'll be doing about the same thing again, but trying for more power and home made fuel.
.
nothing to lose

Spelin and tpying are my strong points, not electronics.

Re: Alternator output vs RPMs? (none / 0) (#8)
by kell on Wed Mar 10, 2004 at 05:48:50 PM MST

If you make the alternator go full field, you can control the ouput by setting the rpm of the drive. This might work for charging a battery bank. For a variable load you would need a mechanical rpm governor.
I don't know what you're going to use it for exactly, but a voltage regulator might just be getting in the way. You can disable the voltage regulator. Some car alternators have a tab you can ground out to make the alternator go full field, that is, full output. The common Delco alternators, for example, have an aperture for sticking a screwdriver in to ground a metal tab inside the case for diagnostic purposes, and you could simply ground this tab (you would probably have to open the case to do a proper job of making it a permanent modification).
You will get the best power out of it this way.

Alternator output vs RPMs? | 9 comments (9 topical)

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