Originally Posted by
Tourist in MSN
While the electronic devices to convert power to USB direct current at 5v are quite energy efficient, the dynohubs are much less so. I would not be surprised if when the light unit draws 12 watts, that the hub added 20 or more watts of drag. But, to put out 12 watts, I suspect that the speed is downhill speed, not average rider speed. So, that is not a realistic comparison.
...
I haven't seen any test data on how losses change when extracting extra power, but...
A handful of years ago, I rigged up a switchable resistance to load my 1st gen SON while riding. I attached a multi-meter to the handlebars, and went out to take data at different speeds and different loads.
This was mostly to get accurate data, since I'd seen data from someone else and pretty much knew what to expect. My results were largely what I expected, and allowed me to put together a suitable electrical model for my SON.
One of the lessons from this is that you can only get more power from a SON by adjusting the load.
Well, you can also add some capacitance in series with the load, and that will help draw more current from the SON, but it's much easier to adjust the load resistance by means of a small switching regulator that you need to have anyway. Up to a point, increasing the load resistance will let you get more voltage and power from the SON while slightly reducing the current.
In regards to drag, some is due to the resistive losses in the SON's coil and the wires between the dynamo and light. If you draw more power and less current, then these resistive losses will go down.
There is also energy loss due to the changing direction of the magnetic field in the steel core of the dynamo. I don't think this changes much due to changes in the dynamo current, but would be happy to learn about data that sheds some light on this.
Based on this, I'm not sure that drawing more power from the dynamo will increase losses in the dynamo. It will certainly increase the power required to rotate the wheel, but only by the amount equal to the extra electrical power extracted (... I think...).
The one thing that confuses me is the mention of 12 watts. I'm not sure that Schmidt is saying that it can get 12 watts out of the dynamo. Based on my data with my SON, at 20 mph, the most I can get out of it (with a resistive load) is 7.7 watts. Maybe Schmidt is just saying that 12 watts will occur at some higher speed, such as 30mph?
Beats me, but I'm curious to see what develops.
Steve in Peoria