Re: Sheldon's Latest Madness...

On 25 Aug 2005 20:19:30 -0700, frkrygow@xxxxxxxxx wrote:
>Jasper Janssen wrote:
>>
>> Regular bottle dynamos also have a 'regulator' -- after you hit
>> the speed where "6V/3W" comes out, it keeps on taking more energy on board
>> from the wheel, but that energy is mostly directly poured into heating up
>> the dynamo, rather than out of the electric contacts. Something about the
>> weak-iron core saturating or sumfin.
>
>Hmmm. That's not as I understand it.
>
>In a bike generator (which, BTW, puts out AC) the frequency of the
>voltage is proportional to road speed. And the frequencies are
>extremely high.
>
>Here's how the power gets regulated. If not for the inductance of the
>generator's coils, output voltage would be approximately proportional
>to road speed. But the higher frequencies cause more inductive
>reactance in those coils. This inductive reactance fights directly
>against the generated voltage. It's not a "lossy" regulation - that
>is, it's not a case where the energy is wasted as heat. As I
>understand it, the energy just isn't created in the first place.

>There are other effects that do create energy losses, of course. For
>example, those coils of wire in the generator have resistance, which
>causes losses. Also, on most generators, there's friction at the
>tire-roller interface, and that creates a lot of the loss. You've also

That exists in the LightSpin as well -- they say the generator part is
over 90% effective, but overall efficiency is in the 60-70 percent range,
which means you lose about a third of the energy in the tyre/rotor
interface.

>got some windage losses inside the high-rotational-speed generator.
>Finally, there are always some eddy current losses in the metal
>associated with a generator. All of these convert otherwise useful
>energy to waste heat. But the inductive reactance doesn't, as I
>understand it, and that's the biggest part of the regulation scheme.
>
>Hub generators are more efficient because they remove almost all the
>windage losses and the tire interface losses.

Hmm. I'm pretty sure that I heard the "core saturates, excess energy
dumped into core" story several times. Whether that means it's accurate, I
don't know. Since saturation is affected by frequency, and as you say,
frequency rises directly proportional to road speed, though, that does
seem unlikely.

>> If the lightspin has enough electronics onboard, it could have a generator
>> bit that didn't have the saturation thing (or at least not until past
>> <ridiculous> speed, and a regulator after that. Possibly it could even do
>> an actual alternator, car-style. Probably not, but an alternator would be
>> the best way of getting just the amount of current necessary out of a
>> generator. That's why all cars use 'em, after all. Basic principle of an
>> alternator is that instead of spinning a permanent magnet by a set of
>> coils, you spin a rotor with some coils. The regulator, when it notices
>> output strength dropping, ups the amount of current going through the
>> rotor, thus drawing off more power from the engine and getting output up
>> (and vice versa).
>
>I read once, long ago, that car alternators are about 65% efficient.
>These days, they may be better. But the best hub generators for bikes
>(that is, the Schmidt ones) get about 60% efficiency, IIRC. Too bad
>they're so pricey!

The main thing about alternators is that you can design them to be
suitable for wildly varying loads and speeds, limited only at the top by
DC resistance of the various coils plus mechanical strength, and at the
bottom by the fact that it takes a bit of energy to generate the rotor
field, which makes it less and less efficient at lower loads. Dynamo
circuits, as opposed to the ubiquitous bottle generators and gearhubs,
which pretty much have to be designed for a fixed load, and are very hard
to make efficient across a wide range of speeds. The main losses in car
alternators would probably be due to the rotor mass (which slows down and
speeds up with the engine, which costs energy) and bearing losses, not so
much the electrical part.

The Lightspin has basically refined the dynamo technology a lot -- it's
still got spinning magnet, stationary coil, but it has a much more
expensive and efficient magnet formation (involving multiple separate
highstrength magnets and weak-iron to direct the field lines), and it has
magnets on the outside of the coil as well as on the inside. This compared
to the one block of fairly weak magnet that the average $5 dynamo has.
They also claim that their coils are particularly good. Then, after making
the electricity in a much more efficient way in the first place, they
condition the power using a bunch of electronics, which ups efficiency and
reliability.

>> Whatever the exact reason for the high efficiency, though, I would imagine
>> that if you incorporate the same generator type and electronics in a
>> dynohub, you should be able to beat the lightspin itself silly. When the
>> patent runs out, as it eventually will, this will no doubt happen.
>
>That is an interesting idea! But I think a more likely improvement
>will be better efficiency from LED headlamps. That's a number that's
>climbing all the time, to satisfy markets a lot bigger than the
>bicycling market. If we get enough lumens per watt from the LEDs, we
>can design a system that requires a lot less watts to adequately light
>the road. Then efficiency will matter even less than it does now.

Well, bottle dynamo efficiency will inevitably go down with less power
draw, since a large part of the mechanical losses involved in the tyre
interface are fixed rather than relative. Plus, you can't use regular
dynamos with more efficient lights, since they don't really respond well
to low loads. Not without the sort of power conditioning circuits the
lightspin does have. You'd have to sell dynamo/light combo kits, which
also means one company has to either develop both or go into some sort of
partnership.

There was one post on here a while ago from people that were marketing
just such a kit: a high-strength, fairly lightweight rare earth magnet to
clamp on to one of the rear wheel spokes, a coil for one of the stays
(basically an open-frame dynamo, IOW), and sets of led lights for rear and
front. Now, that kit is flawed, but it's also pretty damn cheap. Flaw one:
regular high-power white leds for the front light, without reflector or
anything much. Very much a be-seen front light, not a see-stuff light.
Flaw two: They don't have much power conditioning circuitry, and so the
LEDs get power once every wheel revolution. Which means they flash. In a
rhythm with your speed, even. Flashing bike lights are the bane of my
existence. Still, for under $15, it's good value.


>Not that there's much problem with current generator technology. It's
>perfectly fine for most people. Simple, reliable, always ready to go.
>
>There's lots of good generator data at
>http://www.myra-simon.com/bike/dynotest.html

Just no Lightspin, and I'm told the newer Shimano hubs are better than the
old one as well. It'd be nice if they could update the tests.

Jasper
.

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