On 3/8/2017 9:22 AM, jbeattie wrote:
On Tuesday, March 7, 2017 at 6:15:05 PM UTC-8, Frank Krygowski wrote:
On 3/6/2017 2:12 AM, DATAKOLL MARINE RESEARCH wrote:
Assumptions or measured ?

I forgot to respond to that question.

I've measured dynamo torque vs. speed. As James says, the resistance
torque of a dyno decreases as speed increases. It doesn't decrease as
much as speed increases, however, so the power required still increases
with speed.

Example: Soubitez roller dynamo

Speed (mph) 6 12 18 24
Torque (N*mm) 26.4 22.6 18.1 14.3 with 12 Ohm load
Torque (N*mm) 22.6 23.4 19.6 15.1 with 24 Ohm load

I found similar behavior with other dynamos.

With an open circuit, drive torque did increase with road speed.

Why would torque increase with speed with an open circuit?

I think because losses are inevitable. In addition to bearing drag,
there's a certain amount of loss due to eddy currents in the dynamo's
components, and there's windage loss churning the air around inside the
thing. Bottle dynamos spin at surprisingly high speed, BTW. 5000 rpm
would not be unusual.

--
- Frank Krygowski