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

E. Walter Le Roy wrote:

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?

I would think that would depend on the alternator. 'Twere me, I'd run a simple
bench test, then you'd know.

GWE

E. Walter Le Roy wrote:

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


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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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..

"E. Walter Le Roy" wrote in message
link.net...
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

E. Walter Le Roy wrote:

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?

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

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?

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.

chuck

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:
E. Walter Le Roy wrote:

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

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.

"E. Walter Le Roy" wrote in message
link.net...
| 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.

"Tim Wescott" wrote in message
...
E. Walter Le Roy wrote:

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


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.

[snip]

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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.

mikey

Thanks to all who replied. Got a lot of good info.
"E. Walter Le Roy" wrote in message
link.net...
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

Chuck Sherwood wrote:
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.

chuck

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

E. Walter Le Roy wrote:
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

Here is a URL for some people that built wind machines complete with the
direct drive alternator: http://www.otherpower.com/trips1.html It is
very cool how they did it. The alternator arrangement probably could be
modified to work with a water wheel.

Good Luck,
Bob