Originally Posted by
Road Fan
Your inductance and ohmic resistance sounds to be in the right ballpark, based on small motors and actuators I've been involved with in the automotive sector. Effective resistance (your model value that replicates the observations) nearly 3 ohms sounds high. But the difference could be based on core loss/magnetization effects that cause heating, which dissipate energy and hence detract from power delivered. Whether 0.8 ohms is plausible from a physical point of view, I can't say. But from a designer's point of view, if the core loss is 30 to 40% of the resistive loss, I'd say that was a good day's work! It's probably a plain iron laminated core, rather than any more efficient steel, not being a high-speed machine. I think I'd expect a Velological, with it's much higher shaft speed, to perhaps have thinner lams and maybe some silicon in the steel.
The model has 3 parameters that can be tweaked... the scaling factor for the open circuit voltage, the inductance, and the series resistance. The values that I ended up with for each parameter were adjusted to provide a reasonably good match between the V-I (voltage-current) curves for the model and the test data for the range of speeds and loads that were of interest to me.
The model does have a value of 2.8 ohms for the series resistance, while the winding itself measures 2 ohms (IIRC). Close enough for gov't work?
The fact that the model and the data match pretty well shows that the resistive portion of the dynamo impedance is much smaller than the inductive portion over typical operating speeds.
As far as design choices for a dynamo... that's beyond my expertise! I had a nice class on electric machinery back in my undergrad days, and it was pretty interesting. Never had a chance or reason to get any further experience, beyond some details on sources of losses in the magnetics in a switching power supply. Well, I've spent some time on EMC, where the goal was to add losses at specific frequency bands through the use of ferrites. That doesn't seem to be too relevant to bike dynamos, though.
Originally Posted by
Road Fan
For that matter, I think the oddly peaky curve in the first scope photo is just showing the effect of magnetization, the B-H curve. I think you said it the generator was not loaded.
correct.. that was for the unloaded dynamo.
I think I've heard a similar comment regarding the reason for the shape of the waveform, but that's beyond my knowledge of magnetics.
It does serve as a cautionary tale to people who want to design circuits for dynamo lights and assume that the voltage will always be a sine wave.
Originally Posted by
Road Fan
It also looks (for Duppie's question) like the SON can deliver 7 watts with a load larger than the nominal 12 ohms (your test point was 20 ohms, I think?) so maybe a series connection is a better choice than a parallel, assuming the headlight actually is around 3 watts.
my guess is that series will probably yield better results, but that does depend on the design of the light. Not sure that I can imagine a light that would actually produce more light when wired in parallel, but I'm not ruling out the possibility.
Steve in Peoria