Alternator speed

We have a 7 foot water wheel in a mountain stream and would like to hook up
an alternator, and light up the fish pond. Can anyone tell me what RPM it
needs to turn to run one headlamp?
Reply to
E. Walter Le Roy
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I would think that would depend on the alternator. 'Twere me, I'd run a simple bench test, then you'd know.
Reply to
Grant Erwin
The alternator, or the wheel? What sort of headlamp?
Given that most automotive alternators seem to be driven at about 2x the crankshaft speed, and the engines idle at least 500 (probably more like 750 or 1000) rpm I'd say you need to give it at least 1000 rpm.
I'm not sure how your average automotive alternator would do without working into a battery -- I'm sure a regulator could be built that would allow you to run without one, but it'd be a bit of a science project.
Lessee --
(10A)(12V) = 120W = 120 Joules/sec
Assuming you can extract all the energy out of a 7' drop: (7 ft)(0.305 m/ft)(9.8 m/sec^2)(1kg/liter) = 21 Joules/liter
which means that you need a flow of (120 Joules/sec) / (21 Joules/liter) = 5.7 liter/sec
which in the US would be 5.7 liter/sec = 343 liter/min = 94 gallons/min.
That's assuming 100% efficiency, you'll probably be lucky to get 50%, so say 200 gallons/min at a bare minimum.
Unless I've slipped a digit it needs to be a healthy stream, and that wheel is either going to need to turn fast or it's going to need to be fairly wide. I'd try to find an old book on efficient water wheel design, to see about building efficent wheels.
Reply to
Tim Wescott
The alternator requires voltage to activate regulator, it's designed to recharge a battery. You need a generator to operate alone. An alternator running at 500 rpm can produce 12 volts. Hope this helps..
Reply to
John D
Just offhand, with a motor idling at 1000 RPM, and the alternator overdriven at least 3 to 1, I'd say you are at 3000 RPM
Reply to
Rex B
Probably about 1-3K rpm. They usually turn about 2c the engine speed and won't provide enough power for a lighted car at idle although they are close. Consider idle speed to be about 500 rpm. For a single car headlight you may be able to get by wit a bit less speed. Probably the best thing is to hook it up to a drill press or lathe and see what works.
-- Why isn't there an Ozone Hole at the NORTH Pole?
Reply to
Bob May
Since all the OP wants is some lights in the "pool" how about a different approach. Small DC motors make good generators. Perhaps they would work better at low speeds and low power than a fairly large car alternator.
I recently saw some plans were a guy made his own alternator for a direct drive wind generator so that he didn't need any gears or chains to increase the rotor speed to make the generator work.
Reply to
Chuck Sherwood
Get your local electrical shop to assemble a low-amp, single-wire 10SI GM alternator with a big pulley
- - Rex Burkheimer WM Automotive Fort Worth TX
Tim Wescott wrote:
Reply to
Rex B
| We have a 7 foot water wheel in a mountain stream and would like to hook up | an alternator, and light up the fish pond. Can anyone tell me what RPM it | needs to turn to run one headlamp? | Thanks | Walt
About 1500-1700 rpm to get most to "bootstrap" themselves on. You'll need some source of power to let it think there's a battery there. Any 12V lead acid battery, even motor cycle or lawn tractor sized, will give it what it needs. I was trying to figure out if you could use diodes to keep the battery from driving the load, but it isn't coming to me. I'd have to draw it out, I guess. I did a project a few years ago for my kids' science fair project where I drove an alternator with an AC motor and pulled the three phase out to drive the project. I got 14VAC and with a transformer bumped a pair of phases up to 50V, which is as high as they let me go anyway. At the time I just used a wall wart for 12V and had a pushbutton to "start" the alternator.
Reply to
carl mciver
Actually, most alternators are not self-energizing and as such, without a battery, you don't get anything back out (in a normal alternator, the battery voltage is applied to the rotating field coil and the resulting output is rectified and fed back into the battery). There are some self-energizing alternators (I have seen them in a marine environment), but I don't think you will find them on cars. That is a big difference between the alternators and the older cars with generators -- if the battery was totally dead, you could push the car up to speed and the generator would kick in and you could drive. With an alternator, if the battery is totally dead - you're dead.
Reply to
Mike Fields
Thanks to all who replied. Got a lot of good info.
Reply to
E. Walter Le Roy
Actually, most of the cheap DC motors I am familiar with require MORE speed than an automotive alternator. Also, with these DC motors it is very hard to regulate the output voltage- you have to control speed or put in a regulator in series with output, an inefficient method.
I'd go ahead and add a battery- small cycle or lawn tractor one- to the alternator. If it has an internal regulator, fine, otherwise add one. You need that to keep from burning out the light. You also need to add some sort of switch so that the light only comes on when wheel is turning, to keep from running the battery down, but that shouldn't be too hard to rig up
Reply to
Don Stauffer
Here is a URL for some people that built wind machines complete with the direct drive alternator:
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It is very cool how they did it. The alternator arrangement probably could be modified to work with a water wheel.
Good Luck, Bob
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