On Tue, 14 Mar 2017 07:54:26 -0700, Joerg
wrote:

On 2017-03-13 20:00, Jeff Liebermann wrote:
On Mon, 13 Mar 2017 12:38:07 -0700, Joerg
wrote:
55V at 500mA. This is encouraging.

That's 27.5 watts out of a 3 watt dynamo. I was impressed, until I
converted 136 km/hr and found that it was 84.5 mph. With a rocket
assisted bicycle, I might be able to do that.

Well, yeah, they just wanted to see where the limit is. I guess the
enameled copper wire inside would smoke out if you kept that speed for long.

Only the resistive part dissipates power in the wi
P = I^2 * R = 0.5^2 * 2 = 0.5 watts
So, it won't be the wire that gets hot. However, the cores in
saturation are going to get warm. Offhand, I don't know how to
calculate how hot.

However, this means I should be comfortably able to milk 10W or more out
of such a dynamo on a long downhill stretch and maybe north of 5W during
regular rides on flat terrain. IOW there seems to be nothing that
inherently limits things to the usual measly 3-4W.

Maybe. Let's pretend that the test demonstrates that the dynamo can
deliver 28 watts for perhaps 60 minutes before blowing up. Assuming
constant energy (Joules or watt-seconds) at any speed. That would be
10 watts for 168 minutes. Assuming 10 watts output is a comfortable
ride for you, you would have about 3 hrs of bright light before the
dynamo self-destructed. Actually, it would probably be more like 4 or
5 hrs since the system is not adiabatic and the heat radiation
efficiency of the dynamo case is time dependent.

The author is mostly correct about hubs not going into saturation as
easily and bottle dynamos. However, they do go into staturation, just
at a higher RPM.

If that happens above 84mph I shall be happy :-)

Dream on. If you look at the dynamo speed vs power curves at:
http://www.myra-simon.com/bike/dynotest.html
They all begin to go into saturation at 15 to 20 km/hr. At 136 km/hr,
the dynamo will produce 27.5 watts, but the rest of the input energy
will be wasted in core saturation heating. If you paint flames on the
fenders, maybe other riders will think the burning dynamo is normal.

http://www.ebay.com/itm/122373782338
(Yet another project that I'll never finish).

That is a neat little instrument.

Yep. I don't have one yet, but it's a real temptation to install one
on all my various unmetered power supplies and battery packs. However,
there's a catch. The common ground is positive (+), not negative.
You can see that in the schematic:
https://img.alicdn.com/imgextra/i3/121163002/TB2NoBogpXXXXahXpXXXXXXXXXX_!!121163002.jpg
where the + leads of both the "DC in" and the load are connected
together. The only ways I could make it work in a negative ground
system was either an isolated power source, isolated load, separate
isolated power supply to run the meter, or a DC-DC inverter.

Anyhow, I don't
think a dynamo can easily be simulated until you start with a mechanical
model of it and that gets into COMSOL and other really expensive
modeling software. Maybe possible with MathCad which I have but rarely
use so I became rusty there.

Yep. That was pretty much my conclusion. Jim Thomson posted this
Pspice component model:
https://groups.google.com/forum/#!topic/sci.electronics.design/yPVvclIIirQ
which I have been unable to convert to LTSpice. He seems to just put
a sharp knee where the dynamo starts to go into saturation. That's
probably good enough to get started. The rest is mechanical and
rather messy if I add nonlinearities, friction, and air resistance. I
do have one advantage with a hub dynamo. I can characterize it as a
motor on the bench and then simply invert the derived functions to
produce a generator. That might be a worthy shortcut.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558