On Saturday, April 3, 2021 at 4:32:13 PM UTC-7, Frank Krygowski wrote:
On 4/3/2021 12:57 PM, Jeff Liebermann wrote:
On Sat, 3 Apr 2021 10:58:34 -0400, Frank Krygowski
wrote:

On 4/3/2021 1:45 AM, Jeff Liebermann wrote:
The first thing to do is throw out the "voltage regulator" that
reduces output (and decreases efficiency) at high RPM (high
frequencies). Replace it with an electronic voltage regulator.

It's not clear to me that the built-in voltage regulation represents a
loss in efficiency. That is, I don't think the power is being taken from
the rider's input by the coils and magnets, then converted to heat by
the inductive reactance. My understanding is that what would be
increased power at higher speed is simply never produced.

Maybe. I need to dig through the available literature and do some
testing to see what really happens. You might be correct as drag
produced by the dynamo is reduces at high RPM's by the increased
frequency which presents a higher inductive reactance. On an output
power vs RPM graph, that causes the curve to become "flat" at high
RPM's creating an effective output power regulator. In theory,
inductive reactances do NOT dissipate power. Only resistances
dissipate power. Since the inductance doesn't change, there would be
no change in efficiency. However, there are also saturation effects
in the inductor (windings) which do dissipate power once the current
exceeds the core saturation limit. That should produce heat at high
RPM's. If you have time, try spinning the bottle dynamo at a high RPM
driven by an electric drill and see of it gets hot.
Several times I've touched the outside of my Union bottle dyno after a
long ride. It was never more than very slightly warm. Of course, I was
moving so it was losing heat by convection.
Also, if you have
a motor that can drive the dynamo (or just a spinning bicycle wheel),
try shorting the load (headlight). My guess(tm) is that the motor or
wheel will spin quite easily.
I'm sure I did that some time in the past. I may have notes about it...
I know from private correspondence that David Gordon Wilson (of MIT and
_Bicycling Science_) said he was once working on a scheme to produce a
constant rpm bike dynamo, but once he understood how they worked, he
stopped thinking about his scheme.

Cool. Anything you can share?
No, it was just a brief remark - something like "Oh, I thought they just
wasted all the power above 3W. I'll stop thinking about a constant
velocity drive."
I can think of a variety of CVT
(continuously variable transmission) designs that might be suitable.
I'm not so sure he gave up because he thought it won't work. My
guess(tm) is that it didn't provide a sufficient improvement to
justify the cost and complexity.
I think that's what we're up against in this entire discussion. The
relatively simple systems we have now work very well. It's hard to see
how a lot more complexity and expense will pay off.
Please note that a hybrid battery plus dynamo contrivance would be a
far better proposition. At low speeds, the light operates off the
battery. At higher speeds (i.e. downhill), where there is power
available, it switches to dynamo. In it's spare time, it charges the
battery.
From Sheldon Brown's site: https://www.sheldonbrown.com/dynohubs.html

" I used to have a Dynohub on a tandem, and the bulb consumption was
unacceptable. I solved the problem (and some others) by running the
Dyno's output through a full-wave bridge rectifier and then hooking the
DC in parallel with a 6 volt (5 x 1.2v cell) nickel cadmium battery.
This not only provided light when I was stopped, the Dyno would
re-charge the nicads, and, when we went so fast that the voltage rose
above 6 volts, the low internal resistance of the nicads sucked up the
excess, gaining a bit of extra charge and saving the bulb.

"The rectified output of the Dynohub was always connected to the lights.
There was no way to turn the light off while in motion. It would have
been easy enough to rig a switch for that purpose, but I didn't see the
need. The Dynohub has _very_ low drag."

(There are other articles there on generators, lights, etc. )
I strongly, strongly dispute the need for 800 lumens for riding on the
road at night. To me that's very similar to saying bicycles should all
have motorcycle-duty drive chains and spokes.

I think it was Jay Beattie who suggested the 800 lumen number. I
merely stole it from him. I agree that 800 lumens probably too much.
However, if such a high power dynamo product ever arrives on the
market, there will surely be a lumens war among vendors to see who can
advertise the largest number. At that time, 800 lumens will reserved
for purists and regulatory agencies.
Yep, safety inflation is real.

Since when is being able to see "safety inflation"? Let's go for a night ride sometime, you and your bottle dyno and light, and me and my whatever light I chose. I'll wait for you at the bottom. On flat roads and the bike path through South Waterfront I can get by with a little flea-watt flasher or a clip on flashlight from 1968 -- or my old Wonder Light. But that is not where I do (or did pre DST) most of my riding. Everything involves a descent, often on old broken concrete roads. I've done those on dyno only, and its inadequate except at a creeping pace.

-- Jay Beattie.