To make evaluations on how good an AC/DC converter is, I personally do not even need very sophisticated laboratory instruments since I make all relevant measurements directly on DC side. To do that I use a real generator, a real AC/DC converter (the one under test) and an adjustable ohmic load made-up of calibrated ceramic resistors.
Some people make use of very expansive electronic loads, but that's a different story ...
In my case the wheel/generator speed is controlled and finely tuned via an asynchronous motor powered by a 3-phase inverter. At this point the test method is extremely simple: I adjust (sink) the load till to get the maximum transferred DC power to the load taking care of course that the voltage doesn’t go below a certain “low limit”. For instance for usb 5V voltage I’d set this limit to 4.75V. The results seem to be pretty reliable so far. The basic estimate criteria would be that an AC/DC converter is as better as bigger is the amount of ACTIVE POWER that it will let to flow from the generator to the load. The test system I use is THIS ONE, home made but pretty well performing. Here below a couple of sample plots based on a Shimano DH-3D32-QR hub dynamo and an e-werk respectively set at 4.9V (1st plot) and 5.6V (2nd plot). As you can see the e-werk at 20 km/h doesn’t seem to be able to transfer more that 2.75W.
The third plot, worked-out by an indipendent lab, http://fahrradzukunft.de/, well confirms above data.
But what's most important, is that it shows that at 20 km/h some alternative devices, such as Forumslader-V5 or Dynamo Harvester Plus, can definitely do much better arriving to deliver more than 5 W !!
|Courtesy of http://fahrradzukunft.de/|