On Wednesday, March 15, 2017 at 3:07:41 AM UTC-4, Jeff Liebermann wrote:
On Wed, 15 Mar 2017 16:45:57 +1100, James
wrote:

On 15/03/17 16:15, Jeff Liebermann wrote:
On Wed, 15 Mar 2017 15:14:56 +1100, James
wrote:

On 15/03/17 13:43, Jeff Liebermann wrote:
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.

Do you mean eddy currents in the core?

Nope. I meant hysteresis losses. Eddy currents do contribute to
losses by "bucking" the build up and collapse of the magnetic field,
but most of the heat is produced by hysteresis losses:
https://www.quora.com/What-is-hysteresis-loss-Where-does-the-loss-actually-occur
See an induction heater or stove for an extreme case of heat being
generated by eddy and hysteresis currents:
https://en.wikipedia.org/wiki/Induction_heating

Hysteresis losses are different from saturation. Saturation shouldn't
occur if the core has been adequately designed to accommodate all the
permanent magnet flux & MMF.

Most (not all) dynamos are designed to provide some form of
self-regulation. This made sense back in the days when the load was
just a simple 6v incandescent light bulb. It wouldn't do to have
Kamakazi downhill speeders producing enough revolutions to have the
dynamo belch 12VAC and burn out the bulb.

Fast forward to today, and we no longer use incandescent lights on
dynamos. Most (not all) LED lamps have built in regulators and really
don't need to have the dynamo perform any additional regulation.
However, the industry is conservative, and things change very slowly.
Kinda like the automobile industry requiring 25 years to get rid of
the buggy whip socket. So, we still have dynamos that intentionally
designed to NOT produce a linear increase in output for high RPMs.

Fortunately, the problem is not universal.
http://www.pilom.com/BicycleElectronics/HubDynamo.htm
Notice the 2nd graph of Hub vs Bottle Power. The Shimano DH-3D71 hub
dynamo produces a linear increase in output power with no saturation
visible. (However, at my cruising speed of 15 km/hr, it only produces
4 watts and is therefore only a slight improvement over the common
bottle dynamo).

Snipped

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

I remember burning out bulbs with my bottle dynamo if I rode really fast. I also remember the slipping problems when the wheels were wet even when I used the rubber boot sold to go on the bottle dynamo roller. Hub dynamos today really seem to fall down at slow speeds.

Cheers