Ever since I learned that V-Stroms use a shunt regulator, which is commonly understood to be an inefficient regulation scheme, I have wondered how bad it really is. Having recently measured the alternator characteristics, I have been able to figure out how bad it is. I thought those results, and the reasons for them, would probably be of interest to some of the technically curious forum members. More interesting, perhaps to a larger group, is how much better the power generation system could work if some engineering know-how was applied to the problem rather than leaving it to Suzuki's cheap-trumps-all-else design philosophy.
I have modeled the 3-phase alternator using the following data, all taken from measurements and observations:
frequency: 3 * engine_RPM / 60 Hertz
peak voltage, open circuit, phase-to-phase: 60 * (engine_RPM/5000) Volts
resistance, phase to Y center: 100 milliOhms
This next datum is a commonly accepted value:
power available to 13VDC bus via bridge rectifier @ 5000 RPM: 380 Watts
The following datum is derived from the above, adjusted to yield available power when the shunt regulator is barely at the non-shunting condition:
inductance, phase to Y center: 1.2 milliHenry
For simplicity, and because I lack better data, I model the alternator's output voltage as sinusoidal. There is nothing in the design of its magetic structure or its windings which tends to generate a sinusoid, but since the winding inductance has such a dominant effect upon currents and would tend to filter out harmonics anyway, the sinusoidal approximation contributes only minor error to the summary results; the error is certainly small compared to the performance gains available with a modified power handling design.
I created a SPICE
circuit simulation of the alternator, bridge, and DC bus using the above known/derived data and set the per-phase inductance to get the stated available bus power.
Running the sim at 2500, 5000, and 10,000 RPM, I see that the available power is remarkably insensitive to engine speed. Likewise for copper heating loss (and rectifier loss). At the lower speed and below, the output does fall off a little, consistent with what those using heated gear report in this forum.
The response to shunt regulation is quite interesting. Shunting half (or all) of the available current to ground barely changes stator heating. Based on what I can see from the simulation, and deduce from electronic principles now that I understand how the system actually works, stator heating is hardly affected by adding more power-consuming loads or eliminating some or all of them (when using the stock, shunt regulator). Torque on the rotor is affected by bus load current, as the phase of winding current aligns more or less with generated voltage. (At normal running RPM, the torque reverses 6 times per rotor rotation at each pole, although the sum of pole torques is much more level and always against rotation.)
Surprisingly, at the winding resistance I have, there is a nearly constant, 180 Watt loss in the stator windings -- relatively unchanged by either RPM or load current at normal running speeds. Because this is so high, I am going to make a 4-wire measurement of hot stator resistance to get a more accurate value. This value has almost no effect on available power because the voltage drop due to resistance is quite small compared to the effect of inductive reactance.
The reason power generation is so consistent across RPM and load change is that the stator winding inductive reactance and induced voltage are quite proportional to each other. As the engine speeds up, the frequency of the generated voltage goes up along with the induced voltage. Since the dominant impediment to current flow is the winding inductance, and since inductors impede current flow in proportion to frequency, available current remains nearly constant.
Once I established how a stock system performs, I began the investigation which motivated this work. I wanted to know what could be accomplished with a proper power converter -- one which can efficiently produce a different voltage and current at its output than what is consumed at its input. The result was astonishing to me, at first. (A rethink in light of what I learned from the simulation cured my astonishment.) It is possible, at 5000 RPM, with a relatively simple, buck topology, switching power converter, to get twice the output power (760 Watts) at about 3/5 the stator heating in comparison to the stock regulator (or the Shindengen series regulator.) Or, to get the "rated" output power, stator heating can be reduced to less than 1/6 what the stock design (or installed series regulator) suffers.
I have yet to work out what the improvement would be at delivered power levels below 380 Watts when the series regulator is installed. It acts somewhat like a switching step-down regulator, so its effect on stator heating for bikes with only stock loads is likely good enough to eliminate stator stress as a concern. But the series regulator cannot increase available power over what the stock setup can deliver, and it cannot reduce peak stator stress for bikes with 380 Watts of load.
The reason a power converter can so improve performance is that, at engine speeds used for normal riding, (not lugging), it can take power at a higher voltage and lower current in comparison to the conventional regulator(s). For any engine speed, there is a rectifier output voltage at which the product of that voltage and the available current (which goes down with increased voltage) reaches a maximum. That maximum occurs at about half of the average rectified, zero current voltage. The stock regulation scheme only operates at that maximum at about 2200 RPM. At all higher speeds, it operates past the optimum, on the high current side of the curve.
After this investigation, I believe it would be possible to build a replacement regulator which would make bikes with heated gear and added lighting work well electrically, with significantly reduced stator stress and significantly fewer problems with batteries being drained. (I think the regulator should have a "heated gear" outlet, which would be temporarily powered down when engine speed remains too low for too long to keep the battery up and heat the gear. This would happen after a few minutes at idle or lugging speed.)
I have well begun detailed electronic design of the power-converting regulator. Comments, questions and suggestions are welcome. I plan to build at least one, with an etched circuit board and commercially common parts. After that, (once it is a proven design), I'll see about offering kits or built and tested regulators, likely through an established, after-market vendor.