Hey Randy,

Since you are also using servo motors as generators,
why not characterize one for performance vs RPM?
They are almost perfectly cylindrical, and it's
easy to jury-rig (no pun intended) a test setup.
Once you know the generator characteristics, you'll
have a much better idea of what sort of rotor you
need for your wind conditions.

Just put two of the motors, shafts facing one another,
in the "V" of a piece of angle iron. A couple of
wraps of duct tape will hold them in place, and the
shafts will be in nearly perfect alignment. Then
couple the two end-to-end shafts with a few wraps
of duct tape and you're all set to use one as the
power source to drive the other as the generator.

You will find that the output voltage of the servo
motor is practically a linear function of RPM (for
a constant-resistance load), and the output resistance
of the generator is practically independent of both
RPM and loading. Therefore the entire generator
can be reduced to a Thevenin equivalent circuit,
a voltage source in series with a resistor. The
the voltage source will be a constant times the shaft
RPM. You can then use this equivalent circuit to
figure the RPM needed to provide an arbitrary voltage
output into an arbitrary load.

In theory, you can determine both the generator
constant and the output resistance from only two
data points, but I tested over a range of
RPM and load just to check my results and get a
good average.

You will need a way to vary the voltage applied to
the driving motor so that you can vary the RPM of
the generator to take data. I used a variable-
voltage power supply, but you could also use a
couple of batteries (one vs two in series). You
will also need a way to measure the shaft RPM. An
electronic tach is ideal, but if you don't have one
you can use a stopwatch and a long piece of string.
Time how long it takes to wind up a certain length
of string on the motor shaft, and use the average
circumference of the winding to figure how many
revolutions occured during that time. Any good
meter can be used to measure output voltage, but
you may want to put a filter capacitor in parallel
with the output to smooth the measurement.

If you test open-circuit voltage (no electrical
load on generator) vs RPM, you can determine the
generator constant in volts per RPM. I'd check
this value for at least two generator speeds.
Then you can determine the generator's output
resistance by putting a known load on the generator,
measuring the output voltage and RPM, and calculating
what the internal resistance must be in order to
produce the measured reduction in output from what
you would have expected at that same RPM with no
load. Again, it is advisable to take several
measurements at different conditions of RPM and
load so that you can average out measurment
uncertainties from your results.

By the way, the servo motors I have (bought from
Surplus Sales in Lincoln, Nebraska) produce a
no-load voltage of 0.0377 volts per RPM, and
have an output resistance of 1.24 ohms.

How do you use this information once you have it?

For example, suppose I want to be able to supply
5 amps charging current at 14.6 volts (13.8 volts
battery plus 0.8 volts across my blocking diode).
First, I calculate that the 5 amps will cause a
loss of 5 x 1.24 = 6.2 volts across the generator's
internal resistance. I therefore have to produce
an internal generator voltage of 14.6 + 6.2 = 20.8
volts. At 0.0377 volts per RPM, this means I
have to come up with some driving arrangement that
will spin the shaft at 20.8/0.0377 = 552 RPM.

What happens if the shaft spins slower, say,
450 RPM? The internal voltage becomes 450 x 0.0377
= 16.965 volts. The current supplied into
the battery is the difference in charging voltage
and internal voltage, divided by the internal
resistance. So, the charging current (if we assume
that the battery and diode voltages are the
same as in the previous example) now becomes
(16.965 - 14.6) / 1.24 = 1.91 amps.

I can also calculate the cut-in RPM for the
generator in this charging application. It is
simply the RPM at which the generator's internal
voltage just equals the voltage needed to start
supplying charge to the battery. For a 14.6-volt
charging requirement, this is 14.6/0.0377 = 387
RPM.

I have not measured the mechanical-to-electrical
conversion efficiency of my servo generator, as
I don't yet have a method to measure torque at
the shaft while it is running.

best regards,

Marty Jones
mjones@...

ps - I may be driving through West Texas this
coming spring, on my way to a pool tournament
in Las Vegas. I wouldn't mind seeing your
plastic-barrel blades, I really think that's a
neat idea.

-----Original Message-----
From: randy young [mailto:judgeyoung@...]
Sent: Wednesday, December 29, 1999 11:12 AM
To: awea-wind-home@eGroups.com
Subject: [a-w-h] multiple turbines


Happy Holidays, all!

Has anyone ever had experience with using multiple gennys on one tower? My
little home made 5' unit is light and doesn't produce much power, but two
might charge the battery for my treehouse cabin. Is there any advantage to
multiple small units(other than my small units only cost about $30 ea., a
larger unit might cost more to build)? Could they be wired in series for an
increase in low wind performance? Any other advantages/disadvantages? Could
they be set up to switch from series to parallel at higher wind speeds to
prevent to high of a voltage?

Another question: If my generator is not properly sized to my blade, is it
possible that a smaller blade would make more usable energy by virtue of
increasing the rpm and, consequently, voltage?

just one more... Has anyone used a "golf cart" differential as a gear box
for a slightly larger generator? The units are light, efficient and rugged
and have a ratio of about 5-7 to one. I think that the unit, combined with
an alternator might be feasible with about a 9 foot or so prop (pvc pipe, of
course!) with a belt drive to increase the ratio further. Do you think that
this will be too much friction to start in relatively light winds?

I planned to try to attach the prop to one wheel hub of the cart rear end
and use that brake to stop the prop and simply lock the other brake, but the
differential spider gears may make the ratio double? Any brainstorming
ideas?



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