Eyc headlight problem

On Fri, 2 Apr 2021 16:45:38 -0700, "Mark J."
wrote:

On 4/2/2021 4:13 PM, John B. wrote:
On Fri, 2 Apr 2021 12:32:51 -0700, "Mark J."
wrote:

On 4/2/2021 11:59 AM, Ralph Barone wrote:
sms wrote:
On 4/2/2021 9:00 AM, jbeattie wrote:

snip

I don't love it. I would love a dyno light with a solid 800 lumen
output, a little more upward spew and a stand light that was stronger
than the light on my give-away key chain from Wells Fargo -- and one
with a battery so I could use the light off the bike. We
transportational cyclists often need a light for use off the bike. A
flasher would be nice for dusk and dawn, but not required. And while
we're wishing, how about something lighter and more efficient than a
bunch of magnets whirling around. There must be some other way of harvesting electrons.

Such a light would be wonderful but it would be a stretch with a 6V/3W
dynamo, even at higher speeds where you can get more than 3 watts out of
it. Some LED makers are claiming 300 lumens per watt, at least in the
lab, but 200-250 lumens per watt are what is available commercially at
this time.

A 12V/6W hub dynamo (or even a 9V/4.5W hub dynamo) would make dynamo
lights with sufficient intensity more practical, including a beam
pattern where some upward spew would be possible. DRL flash capability
is trivial to add, as are internal batteries to be able to use it off
the bicycle. But there is just not much of a market for any of this.


You seem to have it stuck in your head that the internal impedance of a hub
generator is some immutable quantity and not a design parameter. Why not a
6V/12W hub dynamo? Hell, if you were willing to do frequency dependent
series capacitor switching, you can get a lot more than 3W out of a 3W
labelled hub.

And yet virtually all the commonly available bike dynamos come out with
a half amp nominal design. (So that wattage is half of voltage). I'm
told their coils "saturate" (or something like that) at a half amp.

I've only been using dynamos over a span of 50 years. I've owned 6V/3W
and 12V/6W generators, and once I saw an 8V/4W claim. If there was a
big improvement to be had with some other amperage design, I would think
someone would have tried to market it by now. Dynamo design might have
some complications, but surely they are well understood at this date.

Mark J.

It is because cyclists are such puny power supplies. It would be no
problem to design a more powerful generator into the hub of a bicycle
wheel if the power supply were great enough to power it.

No doubt this is why the *wattages* of all these generators are pretty
low. We had a 12V6W bottle generator on our tandem for years. It was
not an efficient model, but still. Even with double the cyclist power,
you could really feel the drag with that thing.

That doesn't explain why the *amperage* is consistently 0.5A on
virtually all the bike generators of the last ~30 years. I'm just
saying this consistency probably has a reason behind it.

Mark J.

Well, how much amperage does one need to power a head and tail light?
Or perhaps more accurately how many watts?
--
Cheers,

John B.

Mark J. wrote:
On 4/2/2021 11:59 AM, Ralph Barone wrote:
sms wrote:
On 4/2/2021 9:00 AM, jbeattie wrote:

snip

I don't love it. I would love a dyno light with a solid 800 lumen
output, a little more upward spew and a stand light that was stronger
than the light on my give-away key chain from Wells Fargo -- and one
with a battery so I could use the light off the bike. We
transportational cyclists often need a light for use off the bike. A
flasher would be nice for dusk and dawn, but not required. And while
we're wishing, how about something lighter and more efficient than a
bunch of magnets whirling around. There must be some other way of harvesting electrons.

Such a light would be wonderful but it would be a stretch with a 6V/3W
dynamo, even at higher speeds where you can get more than 3 watts out of
it. Some LED makers are claiming 300 lumens per watt, at least in the
lab, but 200-250 lumens per watt are what is available commercially at
this time.

A 12V/6W hub dynamo (or even a 9V/4.5W hub dynamo) would make dynamo
lights with sufficient intensity more practical, including a beam
pattern where some upward spew would be possible. DRL flash capability
is trivial to add, as are internal batteries to be able to use it off
the bicycle. But there is just not much of a market for any of this.


You seem to have it stuck in your head that the internal impedance of a hub
generator is some immutable quantity and not a design parameter. Why not a
6V/12W hub dynamo? Hell, if you were willing to do frequency dependent
series capacitor switching, you can get a lot more than 3W out of a 3W
labelled hub.

And yet virtually all the commonly available bike dynamos come out with
a half amp nominal design. (So that wattage is half of voltage). I'm
told their coils "saturate" (or something like that) at a half amp.

I've only been using dynamos over a span of 50 years. I've owned 6V/3W
and 12V/6W generators, and once I saw an 8V/4W claim. If there was a
big improvement to be had with some other amperage design, I would think
someone would have tried to market it by now. Dynamo design might have
some complications, but surely they are well understood at this date.

Mark J.

Pretty much all dynamo design dates back to a time when electronics didn’t
exist. Since light bulbs don’t put out much light at low voltage and tend
to die at high voltage (I seem to recall bulb life being inversely
proportional to the fourth power of voltage), it was important to try and
find a way to produce a constant voltage from a variable speed generator.
The solution that was found was to build the generator with a large leakage
inductance. This inductance provides more and more resistance to current
flow as the frequency (ie: speed) goes up. Since the voltage also goes up
with speed, the two effects cancel each other and you get a reasonably
constant output into a constant resistance load. This became its own
standard (with a 6V output because that was the voltage of three lead acid
cells or four carbon cells).

It is the adherence to this 6 V 500 mA standard which is inhibiting
progress. With modern electronics, it is possible to produce a regulated
output voltage from a variable input voltage. If you have this, there is no
longer the requirement to have so much leakage inductance to regulate the
voltage. The output of the generator also doesn’t have to be 6 V AC, and
if we look at modern LED lights and lithium ion batteries, 6V doesn’t make
sense anymore. 5V actually makes more sense in terms of LED voltage drop,
lithium ion battery voltage and USB powered accessories.

What would make sense to me would be a generator based on modern brushless
DC e-bike motor design, with a maximum output somewhere in the 25 W range,
but normally limited to around half of that (because how much drag do you
want from your dyno hub?). Generate at a slightly higher voltage (for more
efficient rectification, but not so high that the wire in the rotor is too
fragile), then regulate it down to 5V DC for distribution. Add a
controller that figures out how much to ask the generator to produce and
intelligently divides up that power between the battery and the connected
loads and you have a nifty scheme. Unfortunately, the market is to small to
repay the poor shmuck who ends up re-engineering the entire ecosystem.
Maybe somebody with deep pockets (Shimano?) can lay out the standard, then
others can join in.

On 4/2/2021 8:51 PM, John B. wrote:
On Fri, 2 Apr 2021 16:45:38 -0700, "Mark J."
wrote:

On 4/2/2021 4:13 PM, John B. wrote:
On Fri, 2 Apr 2021 12:32:51 -0700, "Mark J."
wrote:

On 4/2/2021 11:59 AM, Ralph Barone wrote:
sms wrote:
On 4/2/2021 9:00 AM, jbeattie wrote:

snip

I don't love it. I would love a dyno light with a solid 800 lumen
output, a little more upward spew and a stand light that was stronger
than the light on my give-away key chain from Wells Fargo -- and one
with a battery so I could use the light off the bike. We
transportational cyclists often need a light for use off the bike. A
flasher would be nice for dusk and dawn, but not required. And while
we're wishing, how about something lighter and more efficient than a
bunch of magnets whirling around. There must be some other way of harvesting electrons.

Such a light would be wonderful but it would be a stretch with a 6V/3W
dynamo, even at higher speeds where you can get more than 3 watts out of
it. Some LED makers are claiming 300 lumens per watt, at least in the
lab, but 200-250 lumens per watt are what is available commercially at
this time.

A 12V/6W hub dynamo (or even a 9V/4.5W hub dynamo) would make dynamo
lights with sufficient intensity more practical, including a beam
pattern where some upward spew would be possible. DRL flash capability
is trivial to add, as are internal batteries to be able to use it off
the bicycle. But there is just not much of a market for any of this.


You seem to have it stuck in your head that the internal impedance of a hub
generator is some immutable quantity and not a design parameter. Why not a
6V/12W hub dynamo? Hell, if you were willing to do frequency dependent
series capacitor switching, you can get a lot more than 3W out of a 3W
labelled hub.

And yet virtually all the commonly available bike dynamos come out with
a half amp nominal design. (So that wattage is half of voltage). I'm
told their coils "saturate" (or something like that) at a half amp.

I've only been using dynamos over a span of 50 years. I've owned 6V/3W
and 12V/6W generators, and once I saw an 8V/4W claim. If there was a
big improvement to be had with some other amperage design, I would think
someone would have tried to market it by now. Dynamo design might have
some complications, but surely they are well understood at this date.

Mark J.

It is because cyclists are such puny power supplies. It would be no
problem to design a more powerful generator into the hub of a bicycle
wheel if the power supply were great enough to power it.

No doubt this is why the *wattages* of all these generators are pretty
low. We had a 12V6W bottle generator on our tandem for years. It was
not an efficient model, but still. Even with double the cyclist power,
you could really feel the drag with that thing.

That doesn't explain why the *amperage* is consistently 0.5A on
virtually all the bike generators of the last ~30 years. I'm just
saying this consistency probably has a reason behind it.

Mark J.

Well, how much amperage does one need to power a head and tail light?
Or perhaps more accurately how many watts?


Industry standard head lamps are 6v 2.4w, tail 6v 0.5w,
nominally 6v 3w system.

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971

On Sat, 3 Apr 2021 02:14:42 +0000 (UTC), Ralph Barone
wrote:

Since light bulbs don’t put out much light at low voltage and tend
to die at high voltage (I seem to recall bulb life being inversely
proportional to the fourth power of voltage), it was important to try and
find a way to produce a constant voltage from a variable speed generator.

I beg to differ. As I understand it, the bulb life is thermal
problem, not a voltage problem. For example, a halogen light needs to
be run a rather high temperature so that the tungsten particles
evaporated off the filament are re-deposited back onto the filament,
thus giving the halogen bulb a longer life. This doesn't work unless
the filament is really hot. Since the temperature of the filament is
mostly determined by the power (watts) dissipated in the filament,
lamps of equal operating wattage will last about the same number of
hours no matter what the voltage. Or more crudely, a 6v 1A lamp
running at rated voltage, will last approximately the same number of
hours as a 12v 0.5A bulb running at rated voltage, because they both
burn the same number of watts and operate at the same temperature.

Notice I wrote "approximately". The difference is the surface area of
the helical filament. A low voltage filament uses heavier wire than a
high voltage filament. Since brightness and tungsten evaporation
amount are partially determined by the surface area (for the same
wattage), things go non-linear in a hurry. We would probably be using
heavy gauge filament wire for maximum surface area if tungsten wasn't
so expensive.

Where the 4th power rule comes in is where the operating voltage is
lowered in order to reduce the current, which reduces the dissipated
wattage, which dramatically increases the bulb life. It's not by the
4th power but rather by the 3.5th power:
https://www.edn.com/incandescent-lamps-and-service-life/
"Candlepower = (Applied voltage / Rated voltage)3.5 × MSCP
or the mean spherical candlepower at the design voltage.
From there, Lumens = Candlepower × 4 pi "

Reality tends to be more complicated. See the section on Pg 7 "Lamp
Life" in:
https://www.alliedelec.com/m/d/04fe4bc2ac7e247d70e8e7d88fe48b19.pdf


--
Jeff Liebermann
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558

On Fri, 2 Apr 2021 20:47:00 -0400, Frank Krygowski
wrote:

The efficiency of bike dynos varies pretty greatly. I could check
numbers, but IIRC top quality hub dynos can hit 65% efficiency. Cheap
bottles can be as low as 40%.

http://www.myra-simon.com/bike/dynotest.html
See "Normal Speed Efficiency" bar graph. Looks like efficiency varies
from 22% to 61%.

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.
Actually, that's not quite correct. LED's are current driven devices,
so replace it with a current regulator and take whatever voltage is
available.

If there was some way to get the dynamo RPM up high enough, an
alternator would be a further efficiency improvement. Extra points
for a fixed RPM mechanism, where the efficiency is optimized for a
narrow range of RPM's.

The average cyclist has to deliver about 200 watts to sustain 15 mph
on a level roadway. A 3 watt lighting system with 50% dynamo
efficiency will require 6 watts from the cyclist. That's only 3% more
power needed to maintain 15 mph with the lights on.

However, if you want 800 lumens output using 125 lumens/watt LED's
(including optical losses), it will take:
800lum / 125lum/watt = 6.4 watts
to power the light. If someone produced an oversized 50% efficient
bottle dynamo that delivered 6.4 watts, it would require 12.8 watts
from the cyclist or
12.8 / 200 = 6.4%
additional cyclist power to run the lamps. Try that on an exercise
machine or with a cycling computer on a bicycle. It's possible to do
it for short intervals, but not continuously.

What's needed is a dramatic increase in efficiency. If the dynamo
were 100% efficient, then an 800 lumen light would require only a
little more power from the cyclist than the original bottle generator.
100% efficiency is impossible, but there's plenty of room for
improvement between 50% and 100% efficiency.


--
Jeff Liebermann
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558

On Friday, April 2, 2021 at 9:41:20 AM UTC-5, Frank Krygowski wrote:
On 4/2/2021 12:11 AM, jbeattie wrote:
I rode across the US from east to west and north to south with a battery light. I did have a roller dynamo while commuting in San Jose in the 70s.
I did east to west with a roller dyno and a halogen headlight. I'm
betting your battery light was nowhere near as effective as my various
B&M lamps, yet we both survived. Yet you seem to now be claiming my
lights are ineffective.

I did not ride across the USA. But I did ride 4000 miles around Europe in the summer of 1992. I did not have a front light on my loaded touring bike.. I did have a red blinky on the back. I never had a reason for a front light. I was on vacation. I had no schedule to keep. I only rode after the sun came up and stopped before the sun went down. My trip was about Memorial Day to Labor Day. So 16+ hours of daylight every day. I stayed at hostels, pensiones, inns, etc. They provided some kind of breakfast in the morning. So I always ate it. And they did not get up at 3AM to get it for a crazy cyclist who wanted to start riding 3 hours before the sun came up. I rode 50-80 miles a day. Which was easy to accomplish while the sun was up. No reason for a front light. The few times I got rained on, I did turn on the rear red blinky to help visibility.

On Friday, April 2, 2021 at 11:40:08 AM UTC-5, Mark J. wrote:
On 4/2/2021 3:09 AM, wrote:
As for dynamo lights. For randonneuring, they are almost required because you are riding so long in darkness at night. Batteries would not work. But for riding around town, I have been more of a battery person than dynamo person. I have both types of lights. Battery lights are much more powerful and provide a couple hours of light. More than enough for evening recreational rides or commuting. One two hour evening rides a couple times a week or commuting morning or evening are the predominant uses of lights. Batteries work very well for these kinds of rides. Its very uncommon for someone to ride all night long and require a dynamo.

While I like my dyno hub and Luxos headlamp for randonneuring, I did two
of my super randonneur series with batteries, and liked 'em just fine.
And the first one was with lead-acid (heavy!) batteries. It was the
superior beam pattern on the Luxos (bought for my commuter) that
persuaded me to convert the rando bike to dyno.

Frankly, prior to well-engineered LED headlights, dynos just didn't
provide enough light for my tastes. Riding all night is much more
stressful when you have to struggle to see road hazards.

Mark J.

Yes I can see that. Depending on how you ride randonneur rides. 200k, no light needed, 13.5 hour time limit. 300k, 20 hour limit, only need light 1/2 hour in the morning and 2-3 hours maybe in the evening. 400k, 27 hour limit, 1 hour morning, 2 hours evening, stop for sleep, 2 hours next morning.. On low power batteries can give 4-5 hours of light. 600k, 40 hour limit, 1 hour morning, 2 hours evening, stop for sleep, 1 hour morning, ride all day and get in before need lights in evening. You may be cutting it a bit close if you do not finish quick enough on the second day of riding. No margin for error. But doable. And for the 1000k at 75 hours and the 1200k at 90 hours, they will also work with batteries but will require a new battery/recharge for the second half of the rides. Again doable with the correct planning and no unexpected emergencies arise.

But dynamo on brevets eliminate most of the worry from the unexpected emergencies and you don't have to do any planning at all for night riding. I'd definitely recommend a dynamo for randonneur brevet riding. On my PBP I had a support car so I could have maybe planned to meet my support at approved stops at the right times before dark and gotten fresh batteries. That would have taken a whole lot of planning and luck to work. Not something I want to count on during a timed 1200k. And of course with the much better LED lights for dynamos now, you can get good lighting out of a dynamo. If you are willing to pay the upfront cost and your usage, riding patterns warrant a dynamo.

I used the old filament lights on my dynamo in 2007 and the prior year too when I did an extra 1200k to get ready for PBP. Busch Mueller 6V lights. Two of them for my Shimano dynamo. One each side of the front hub. They provided the minimum amount of light. Minimum. You always wanted more light. I have since upgraded to some B&M LED lights. Two. Much, much better. When I rode brevets, I sometimes rode through the night on 400, 600, 600, 1000, 1200 rides. No sleep stops. So I needed longer run times than batteries could provide.

On 4/3/2021 1:09 AM, Jeff Liebermann wrote:
On Sat, 3 Apr 2021 02:14:42 +0000 (UTC), Ralph Barone
wrote:

Since light bulbs don’t put out much light at low voltage and tend
to die at high voltage (I seem to recall bulb life being inversely
proportional to the fourth power of voltage), it was important to try and
find a way to produce a constant voltage from a variable speed generator.

I beg to differ. As I understand it, the bulb life is thermal
problem, not a voltage problem. For example, a halogen light needs to
be run a rather high temperature so that the tungsten particles
evaporated off the filament are re-deposited back onto the filament,
thus giving the halogen bulb a longer life. This doesn't work unless
the filament is really hot. Since the temperature of the filament is
mostly determined by the power (watts) dissipated in the filament,
lamps of equal operating wattage will last about the same number of
hours no matter what the voltage. Or more crudely, a 6v 1A lamp
running at rated voltage, will last approximately the same number of
hours as a 12v 0.5A bulb running at rated voltage, because they both
burn the same number of watts and operate at the same temperature.

Notice I wrote "approximately". The difference is the surface area of
the helical filament. A low voltage filament uses heavier wire than a
high voltage filament. Since brightness and tungsten evaporation
amount are partially determined by the surface area (for the same
wattage), things go non-linear in a hurry. We would probably be using
heavy gauge filament wire for maximum surface area if tungsten wasn't
so expensive.

Where the 4th power rule comes in is where the operating voltage is
lowered in order to reduce the current, which reduces the dissipated
wattage, which dramatically increases the bulb life. It's not by the
4th power but rather by the 3.5th power:
https://www.edn.com/incandescent-lamps-and-service-life/
"Candlepower = (Applied voltage / Rated voltage)3.5 × MSCP
or the mean spherical candlepower at the design voltage.
From there, Lumens = Candlepower × 4 pi "

Reality tends to be more complicated. See the section on Pg 7 "Lamp
Life" in:
https://www.alliedelec.com/m/d/04fe4bc2ac7e247d70e8e7d88fe48b19.pdf

That all sounds correct, but largely moot for our purposes. I doubt
anyone is doing original engineering with the intent of using halogen
bulbs in bike headlights.

In fact, this company http://www.reflectalite.com/ sells LED
replacements for many halogen bulbs.

IME the resultant beam focus is not quite as good, but the light output
is better. I gave one to a friend who still uses a low power battery
light and he says he likes it.


--
- Frank Krygowski

On 4/2/2021 10:14 PM, Ralph Barone wrote:

Pretty much all dynamo design dates back to a time when electronics didn’t
exist. Since light bulbs don’t put out much light at low voltage and tend
to die at high voltage (I seem to recall bulb life being inversely
proportional to the fourth power of voltage), it was important to try and
find a way to produce a constant voltage from a variable speed generator.
The solution that was found was to build the generator with a large leakage
inductance. This inductance provides more and more resistance to current
flow as the frequency (ie: speed) goes up. Since the voltage also goes up
with speed, the two effects cancel each other and you get a reasonably
constant output into a constant resistance load. This became its own
standard (with a 6V output because that was the voltage of three lead acid
cells or four carbon cells).

It is the adherence to this 6 V 500 mA standard which is inhibiting
progress. With modern electronics, it is possible to produce a regulated
output voltage from a variable input voltage. If you have this, there is no
longer the requirement to have so much leakage inductance to regulate the
voltage. The output of the generator also doesn’t have to be 6 V AC, and
if we look at modern LED lights and lithium ion batteries, 6V doesn’t make
sense anymore. 5V actually makes more sense in terms of LED voltage drop,
lithium ion battery voltage and USB powered accessories.

What would make sense to me would be a generator based on modern brushless
DC e-bike motor design, with a maximum output somewhere in the 25 W range,
but normally limited to around half of that (because how much drag do you
want from your dyno hub?). Generate at a slightly higher voltage (for more
efficient rectification, but not so high that the wire in the rotor is too
fragile), then regulate it down to 5V DC for distribution. Add a
controller that figures out how much to ask the generator to produce and
intelligently divides up that power between the battery and the connected
loads and you have a nifty scheme. Unfortunately, the market is to small to
repay the poor shmuck who ends up re-engineering the entire ecosystem.
Maybe somebody with deep pockets (Shimano?) can lay out the standard, then
others can join in.

As an ME, not an EE, your ideas seem technically possible to me, but are
they really practical? ISTM if you view a dyno light system overall as
something that converts rider wattage to road illumination, there's not
much opportunity for big improvements.

Roughly speaking, dyno efficiencies are high enough that you lose maybe
one or two Watts of rider power there. The other inefficiency is
whatever is wasted as heat instead of light in the LED and its
circuitry. I don't know that efficiency as a percent, but it has to be
minor. I know some LEDs are now pushing 200+ lumens per watt. Properly
focused, that should allow 35 mph descents, and almost nobody wants to
go that fast on a bike at night. Headlights using those LEDs should be
enough to satisfy anyone this side of "more lumens are always necessary"
craziness.

I'm aware of the reasons for 5V electronics, but given modern electronic
expertise, I'd think it wouldn't be difficult to inexpensively mass
produce an efficient 6VAC to 5VDC transformer.

Disclaimer: Again, I'm a Mechanical Engineer, not an Electrical Engineer.

--
- Frank Krygowski

sms wrote:
On 4/2/2021 11:59 AM, Ralph Barone wrote:
sms wrote:
On 4/2/2021 9:00 AM, jbeattie wrote:

snip

I don't love it. I would love a dyno light with a solid 800 lumen
output, a little more upward spew and a stand light that was stronger
than the light on my give-away key chain from Wells Fargo -- and one
with a battery so I could use the light off the bike. We
transportational cyclists often need a light for use off the bike. A
flasher would be nice for dusk and dawn, but not required. And while
we're wishing, how about something lighter and more efficient than a
bunch of magnets whirling around. There must be some other way of harvesting electrons.

Such a light would be wonderful but it would be a stretch with a 6V/3W
dynamo, even at higher speeds where you can get more than 3 watts out of
it. Some LED makers are claiming 300 lumens per watt, at least in the
lab, but 200-250 lumens per watt are what is available commercially at
this time.

A 12V/6W hub dynamo (or even a 9V/4.5W hub dynamo) would make dynamo
lights with sufficient intensity more practical, including a beam
pattern where some upward spew would be possible. DRL flash capability
is trivial to add, as are internal batteries to be able to use it off
the bicycle. But there is just not much of a market for any of this.


You seem to have it stuck in your head that the internal impedance of a hub
generator is some immutable quantity and not a design parameter. Why not a
6V/12W hub dynamo? Hell, if you were willing to do frequency dependent
series capacitor switching, you can get a lot more than 3W out of a 3W
labelled hub.

Not stuck at all. The problem with that approach, and it's already been
done via lowering the impedance by putting two bulbs (or LEDs) in series
is that you don't reach sufficient power at lower speeds.

OK. First off, putting two LEDs in series raises the impedance, and it
works by allowing the voltage from the generator to go up. If you had a bit
of intelligent electronics (a frequency sensor with some hysteresis), you
could short out the second LED at low speeds where there isn’t enough
voltage to drive two in series.

One thing that some people have done (though it may be illegal in some
countries) is to use a dyno hub designed for a larger wheel on a smaller
wheel. A 20" wheel is around 1010 revolutions per mile, a 28" wheel is
about 720 revolutions per mile. This would get you more output at lower
speeds, but the current would still be 500mA, just at a higher voltage.
And of course it assumes that you want to ride a bicycle with small
wheels! And of course the drag would be greater than a dyno properly
sized to the wheel.

Practically speaking, the advantage of higher output dynamo would be
used with a bridge rectifier and a DC-DC buck converter to charge a
high-power battery powered headlight, or to operate it continuously at
lower than maximum power.

You’re almost there. The other advantage of a DC-DC converter (don’t get
stuck on buck - it could be that a boost or buck-boost or Cuk is the way to
go) is that it acts as a DC transformer allowing the system (through a
change in duty cycle) to change the effective impedance of the LED to a
value that better matches the source impedance of the dynamo hub at that
particular speed, thereby maximizing power output at all speeds.