Another cheap Chinese rechargable light $6

On 07/01/16 09:17, Phil W Lee wrote:
James considered Wed, 6 Jan 2016 10:56:25
+1000 the perfect time to write:

On 05/01/16 16:01, Frank Krygowski wrote:
On 1/4/2016 9:04 PM, James wrote:
On 04/01/16 10:20, Frank Krygowski wrote:


Why not contrive a way to actually measure the torque? That's what we
did.


Someone with a laboratory (of sorts) has done the work for me.

Look at the results obtained by our old mate Andreas Oehler here;

http://fahrradzukunft.de/14/neue-nabendynamos-im-test/

Use google translate if you need.

Compare the results of "Figure 16: Motor power when operating at 12 ?"
and "Figure 18: Motor power when operating at 24 ?".

In particular I looked at the results at 10km/h for the SP PV8-1 and
read as accurately as I could from the graph.

12ohm load @ 10km/h, motor power = 3.2W
24ohm load @ 10km/h, motor power = 2.75W

If you read off the data at other speeds for that dynamo and plot the
torque versus speed you will find that the shape bears uncanny
resemblance to those I produced. The difference being fewer data points
of course, and that my data starts from much lower speed.

The 24ohm motor torque curve certainly starts lower, crosses the 12ohm
load curve, reaches a peak later, and then remains above the 12ohm curve.

Do you dispute Andreas' work and/or my interpretation?

I guess I'm disputing your interpretation.

To remind anyone still reading what this is all about:

On 12/21/15 you (James) said: "Again, two lamps in series should
present a higher resistance and consequently lower dynamo torque, just
as it does for incandescent lights as I explained above."

I replied "I don't think that two lamps in series will yield lower
dynamo torque." And I think that's what we're still discussing.

Looking at Andreas's work, you're certainly correct that at 10 km/hr
(i.e. 6 mph) for those hub dynamos, he measured lower drive torque for
24 ? than for 12 ?.


A good start.

However, at 20, 30 and 50 km/hr (i.e. 12, 18 and 31 mph) he measured
higher drive torques with the higher load resistance, as I said, and as
I measured. (I printed out larger images of his bar graphs and
laboriously estimated all the power readings.) For example, the SP
PV8-1 you mentioned gave 12 ? drive power of 5.8W, 7.7W & 11.3W
vs. 24 ? drive power of 6.3W, 9.7W & 14.5W
at speeds of 20, 30 & 50 kph.
Other hub dynamos showed the same general tendencies.

However the _maximum_ torque attained with either load is not very
different.

I'll grant you this though, because the maximum torque value for 24ohms
occurs at a higher speed, the additional bearing friction torque and
magnetising losses will also be higher. These factors are not taken
into account by the simple electrical model I used, but of course are
included incidentally in the measurements you and Andreas made. In a
poor quality dyanmo with low efficiency, these may be more significant.


So you're correct in that fitting two headlights in series (in hopes of
getting more light output) would result in less load when riding about
as slow as most people can balance, if they're using a hub dynamo.

I'm certain "most people" can ride slower than 10km/h and not fall off.

Regardless of where the dynamo is located (hub, roller, sidewall), the
principle of operation is the same. Some turn faster and have fewer
magnetic poles, and others turn slower and have more magnetic poles.
The circuit model is the same in either case. The bearing losses may be
different.

Well, a hub dynamo has the same bearings, and therefore losses, as the
hub would even without a dynamo, whereas a roller dynamo has it's own
set of bearings, and therefore additional losses. So a hub dynamo
should always be capable of being more efficient.

As part of the complete bicycle, yes.



At any speed where one would actually want the output of two lights, and
where the lights would work reasonably well, it looks like I'm correct.

Sure, but from the point of view of whether slippage will occur, that
should happen close to the point of maximal torque - which is near on
the same for either load.

So if you can drive one light without slippage, you can almost
certainly drive two.

Yes.


And my measurements say that with a bottle or roller dynamo, even 6
mph (10 kph) causes more drive torque with two lamps. Not that two
lamps are practical at that speed.

I'm at the end of another very long day, so I'll put off any further
digging for now. Perhaps the low speed difference has to do with the
different frequencies of the hub dynamos vs. the roller and bottle units
I measured. Bottles and rollers have much higher AC frequencies, so a
different balance of resistive and inductive effects.


More likely the magnetising current (and losses) is higher in smaller
and low quality devices. These reduce the overall efficiency of the
dynamo, and I think bottle and roller dynamos tend to be less efficient
than hub dynamos. This effect and increased plain bearing drag at
higher speeds would make the peak torque for 24ohm resistive loads
slightly higher.

I would expect anyone seeking the additional light from doubling up
lamps to be prepared to use a reasonable quality dynamo though, so in
practice, this should make little difference.

Sounds fair.


(The peak occurs at near twice the speed, so twice the internal voltage
over a resistive element in parallel with the voltage source would
consume 4 times the power in magnetising losses. V^2 / R.)

Another factor in the original question of does a 24ohm load cause more
tyre slippage for roller and bottle dynamos is that the hump in the
torque vs speed curve occurs over a wider range of speeds (approx twice
as wide and at higher speeds). So if you can get the dynamo spinning
with a 12ohm load, and past the maximal torque point on the curve, it
will be less likely to start slipping at higher speeds I guess.

In any case, the differences in peak torque are not huge, and as you've
found, the addition of a rubber O-ring or similar on a bottle dynamo
makes a huge difference, as I suspect a rubber surface on a roller
dynamo would.

If you're running it on the tyre, I'd expect a rough and hard surface
to be more effective than a rubber one (many years ago, I had such a
setup which slipped on a bottle dynamo, and cured it by putting epoxy
on the roller with sand embedded in it, which was very effective).
The O-ring trick is for driving the dynamo from the braking surface of
the rim, where using rubber is a benefit. It's also quieter!

Though an abrasive surface on a roller dynamo might wear the tyre
faster, especially if it is not perfectly aligned. I would want to
increase the roller to tyre contact patch size. A concave roller would
do better I think, instead of a cylindrical one.


I *never* had my sanyo roller dynamo slip while it was mounted on the
rear brake bridge, where gravity helps to keep it pressed against the
tyre. (And by never I mean with incandescent globes or my home brew LED
light that limits the peak voltage to about 7V.)

--
JS

On 1/6/2016 7:41 PM, James wrote:
On 07/01/16 09:17, Phil W Lee wrote:

If you're running it on the tyre, I'd expect a rough and hard surface
to be more effective than a rubber one (many years ago, I had such a
setup which slipped on a bottle dynamo, and cured it by putting epoxy
on the roller with sand embedded in it, which was very effective).
The O-ring trick is for driving the dynamo from the braking surface of
the rim, where using rubber is a benefit. It's also quieter!

Though an abrasive surface on a roller dynamo might wear the tyre
faster, especially if it is not perfectly aligned. I would want to
increase the roller to tyre contact patch size. A concave roller would
do better I think, instead of a cylindrical one.

I gave some thought to this long ago, and came to the opposite
conclusion. Here's why: If your dynamo drive wheel were concave, you'd
have something like a line (or thin rectangle) of contact patch along
the tire. But that line or rectangle would be moving at a slightly
different linear velocities at its different points, due to differences
in the two radii (tire-to-hub-axle and roller-to-dynamo-axle). So
scrubbing would be unavoidable.

According to Jim Papadopoulos in personal correspondence, that friction
interface is the source of significant efficiency loss. The scrubbing
of a concave roller would probably worsen that.

That was what motivated me to try the o-ring drive against the rim's
braking surface. The narrow o-ring should cause less scrubbing, and
possibly even cut through a water film. It certainly runs very smoothly.

I do like roller dynamos better than bottle dynamos, and I wondered
about fitting a large, thick O-ring on a roller dyno - ideally, one with
a cylindrical inside diameter, if such a thing could be found. But I
haven't gotten around to trying it yet. Almost all my night riding is
with a hub dyno these days.


--
- Frank Krygowski

Phil W Lee wrote:
avid Scheidt considered Wed, 23 Dec 2015
:03:48:22 +0000 (UTC) the perfect time to write:

:Phil W Lee wrote:
::Frank Krygowski considered Mon, 21 Dec 2015
::23:23:38 -0500 the perfect time to write:
:
::On 12/21/2015 10:26 PM, Phil W Lee wrote:
:: Frank Krygowski considered Mon, 21 Dec 2015
:: 15:51:27 -0500 the perfect time to write:
::
:: On 12/21/2015 2:50 PM, James wrote:
:: On 21/12/15 14:42, Frank Krygowski wrote:
::
::
:: You say your homebrew is (or was?) clipped at 7V, because each phase of
:: the AC saw two of your LEDs in series. Most modern commercial LED
:: headlamps have just one LED. If they're running at about 3.5V (a
:: reasonable value) then ISTM the actual input power to the LED would be
:: just 3.5V * 0.5A or 1.75W. Consequently, ISTM the input power (or drag)
:: of the dynamo would also be less than for an incandescent bulb.
::
::
:: Probably not. It's likely the electronics convert the AC to DC with a
:: bridge rectifier of sorts, and then a current regulator to set the LED
:: current precisely. BTW, my B&M light is rated 2.4W IIRC.
::
:: Mine are too. I'd like to see some data on actual power input, to see
:: if that's confirmed.
::
:: Don't they have to be, to comply with StVZO?
::
::I don't know the details of that specification - it might depend on the
::wording, and I'm frustratingly monolingual.
::
::For example, it's conceivable that the specifications are written to set
::2.4W as a maximum power requirement, but permit it to do its job while
::consuming less. That would certainly be a logical approach, especially
::since the regulation does specify light output in plenty of detail.
:
::There's a section on the electrical interface between dynamo and light
::though (which is what guarantees interchangeability of lights and
::dynamos).
::It sets strict limits on allowable variation in test voltage, total
:ower, and minimum frequency. Dynamos are tested against dummy loads
:f set resistances (simulating front 2.4w, rear 0.6w, and combined
::3.0w), rather than agaisnt any particular light. They don't say you
::can't use a different resistance to raise the maximum voltage, but do
::set minimum voltage levels, which would imply that the normal 0.5A
:utput would have to be maintained, and at the normal voltage. I
::suppose there's nothing to stop a manufacturer fitting a bypass
::resistor in the light unit to shunt excess power round the light, but
::it would be pretty pointless.
::The only thing that is set in relation to maximum power is what it can
:roduce at the reference resistance - so you can go up but not down
::from there.
:
:The reference resistance is an incandescent lightbulb. (well, an
:idealized lightbulb, but the difference is very small.) An LED is
:approximately as much like an incandescent lightbulb as it is a
:zebra. So, when you hook an LED light up to a generator, you're
:absolutely allowed to take advantage of the particular features
:of the generator you're attached the light to, as long as behaves
:minimally well attached to any conforment generator they test with.
:
::I did have a full translation somewhere, which I'll send to anyone
::that wants it (email address is valid, so just ask), if I can find the
::thing. I can't even remember what format it's in, so I may be a while
::digging it out - it's a while since I was researching all that stuff
::in preparation for submissions to our government on changes to our own
::standards.
::Of course, that also means my recollection may be faulty.
:
:If you find it, let me know.

:I found it.
:It's in .Doc format (I'm not sure which version) but perfectly
:readable to OpenOffice).
:201kB in size so can email to you - is your posting address valid?

yep.

--
sig 126