Thanks for the info. You did a good job. I have done a few rough
calculations as below
The electrical efficiency is about 60% in either case (theoretically the
same) and any change in overall efficiency is in the mechanical side.
For the Bottom bracket machine which appears to be the one that the test
results were for:
No capacitor: internal impedance at 190Hz =3.8+j8.07 ohms and total
impedance magnitude is 17.74 ohms so the internal voltage is about 9.5 V
With capacitor the total impedance drops to about 15.8 ohms and the internal
voltage generated is still 9.5V. Electrical efficiency in both cases is
about 76%
For the Union machine:
Zinternal =7.9+j7.4 ohms (190Hz)so total circuit impedance is about 21.3
ohms without the capacitor so, using 0.53A or 6.4V the internal generated
voltage is about 11.3V
With capacitor the total impedance drops to 19.9 +j1 ohms or a magnitude of
19.93 ohms and the internal voltage is about 12V.
The difference may be some drop in demagnetising effect or a slight change
in speed. Electrical efficiency about 60%
I used the higher inductance as this is the one that actually counts (direct
axis vs quadrature axis inductance).
Actually, your bike generators are, electrically, rather conventional
synchronous machines with permanent magnet fields and the internal voltage
is speed dependent, with the current depending on the load and the internal
impedance. It is simply a lousy "voltage source " which is identical to a
lousy "current source".
However in the case of the bottom bracket machine, the inductance is
dominant and rises with speed as well so the total circuit impedance rises,
tending to limit the current somewhat. Note that at twice speed and a 12 ohm
load the current will be about 0.85A and the power about 8.6 watts. Not
really constant current. However there may be other factors involved such as
the demagnetising effect of the current actually reducing the voltage a bit.
That depends on the actual characteristics of the field magnets.
If you put a 24 ohm load on, at the original speed, then you'll get about
7.9V,0.33A and 2.6 watts. Lower current and power. At twice the speed the
internal voltage will be doubled as will the inductance so the output would
be 14.2V (probably a bit less) and current 0.6A for 8.4watts (actually less
current in practice as bulb is hotter). The Union machine will likely have a
wider current range due to its lower inductance.
These figures are based on the data given and other factors come into play.
Some other experiments for you to while away spare time that you could spend
riding!.
In any case, the addition of a capacitor isn't worth the effort and may
actually reduce performance at higher or lower speeds.
--
Don Kelly
[email protected]
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>
> Don Kelly wrote:
>> Thanks- that is enough info to determine the inductance but not the
>> resistance of the generator.
>
> We measured the resistances and inductances directly. We had the
> instruments.
>
> Union bottle generator: 7.9 Ohms, 5.45 mH (up to 6.2 mH)
>
> Soubitez bottom bracket generator: 3.8 Ohms, 6.76 mH (down to 4.9 mH)
>
> Inductance measurements were done using an impedance bridge. They
> varied with angular position of the generator shaft... whether the
> generator was in one of it's "notches" or held in a different position.
> Ultimately, we decided it didn't matter much which value we used; we
> were measuring just to calculate a roughly appropriate capacitor size,
> and if it was off 30%, it didn't matter much for our purposes.
>
>> I should also have asked for the open circuit
>> voltage and the DC resistance of the generator to get a better handle on
>> the
>> model.
>
> Well, I think I've got those open circuit voltages somewhere...
>
>> Also I have been assuming the capacitor is in parallel with the load
>> resistance. Is it?
>
> Nope. In series.
>
>>
>> Out of curiosity - why 12 ohms?
>
> Briefly, a standard 3 Watt generator bulb (assuming only headlight, no
> taillight) is 12 Ohms. A 2.4 Watt bulb used with a 0.6 Watt taillight
> in parallel has a combined R of 12 Ohms.
>
> Bike generators are, roughly speaking, constant current devices. Open
> circuit, their output current is zero, and their output voltage is
> roughly proportional to their rpm (up to a certain limit).
>
> When given a resistive load, they will do their darndest to put out
> their rated current. Most bike generators are designed to produce 0.5
> Amp. But their rating is invariably stated as 6 Volt, 3 Watt. That
> rating depends on having a 12 Ohm resistance in the load.
>
> It's interesting that you can get more power out of a generator by
> giving it more resistance. For example, seeing a 24 Ohms load, the
> generator will try its darndest to put out 0.5 Amp. To do that, it
> will generate 12 volts, and produce 6 watts. Same generator, twice the
> power. (This only works if the speed is high enough.)
>
> Problem is, most generators won't succeed at that job, because their
> drive wheels need about twice the torque as usual. They'll slip. One
> reason I like the Soubitez bottom bracket generator is that it can pull
> this off without slipping. So, of course, can the hub generators like
> the SON (or Schmidt). Bottle generators usually can't do it.
>
>
>>
>> As for fluctuations in torque due to the capacitor- there shouldn't be
>> any
>> other than double frequency components which would have an average of 0.
>> Fluctuations of this nature will also be present without the capacitor.
>> As
>> for demagnetisation, that is unlikely as the generator probably can
>> handle
>> heavier loads and also a more leading pf will reduce demagnetisation.
>
> The fluctuations we saw weren't at anything like double the frequency.
> They had a period measured in (by memory) a second or so - i.e.
> frequency of 1 Hz or less.
>
> To make this clear: We measured reaction torque by having the
> generator mounted in a sort of gimbal arrangement, with a long torque
> arm pressing on a digital scale. (We used a balloon between the arm
> and scale to absorb vibrations.) Anyway, the measuring system worked
> well in "normal" mode, but when we used it with the capacitor in the
> circuit, scale readings (i.e. torque readings) varied quite a bit.
>
> Again, as with the inductance, we were just checking to see if adding
> capacitance was possibly worthwhile. Even without precise results, we
> learned enough to say capacitance was not worthwhile.
>
> - Frank Krygowski
>
