3 to 9W VPG … variable power generator

[ConRAD]

.. that maximum is arguably not needed any more...

.. indeed it isn't, I guess !!

... yet the point for me remains unanswered: why 3W ? who decided that nice round number !! ... this thing reminds me the old story of the egg and the chicken !! Were dynamos, so far, designed to properly match filament bulbs or, conversely, bulbs designed in such a way not to blow when properly matched to a standard 3W dynamo !!

So, why "3" ?
- perhaps dynamo maximum permissible dragging ?
- filament bulb minimum required "lumen" production ?
- what else ?

[andrew_s]

The German are now allowing a 1.5W dynamo, if used with LED lighting.

The 3W requirement would have been to get enough light with old non-halogen bulbs. The spec was to match the available bulb technology. You need 2 watts or so in a non-halogen bulb to be adequate to see by, add on the rear light, and a bit for luck/round numbers, and there's your 3W.
The other limitation on dynamo power is the traction between the roller and the side of the tyre - try to get very much more than the 3W, and the roller slips every time it rains. A stronger spring for less slip, and you've then got extra drag and tyre damage to worry about.

[Brucey]

well in the UK we decided that a front battery light could be 1.5W (or less) and that would be fine, and dynamo rear lights everywhere were no more than 0.6W and some were OK at about half that, all with boring old tungsten bulbs. [Maybe we eat more carrots here, eh... ]

The side effect of the 3W limit was that it effectively stifled generator development for years. In the UK we had efficient 2W hub generators about 70 years ago, but these were never legal for sale in Germany (or Holland come to that where (just to be different) they stipulated 2.4W... ).

When our government started making noises about harmonising lighting regs with the EU, the unintended consequence was that instead of being able to pick between a 2W hub generator or 'something else' we suddenly only had the latter choice; they simply stopped making the most efficient and reliable generator then available.

Now it has come full circle. A while back I sold a sturmey archer FG hub ( first made in 1947) to a German chappie, for conversion to five-speed (which is easy). Finally the generator is (sort of) legal for use in Germany. It is a heavy hub, the FG, but it is very reliable and (together with a normal front hub) is barely any heavier than the lightest modern 5s IGH and a typical front hub generator. So much for progress....

cheers

[NATURAL ANKLING]

Hi,
The last cycle I sold long time ago, I left the bottle dynamo on it with working lights.
A very young child bought it accompanied with his parents.
I thought that at the time it was a good idea, though I probably wouldn't bother to be so thoughtful tomorrow.

Warming to one.
I could easily whip this into my many MTB wheels, what do think £14.49
http://www.ebay.co.uk/itm/Shimano-Dynam ... 43d63731f3

Edited - I need spokes
Or second hand bike for less than £ 10 but probably 27 " wheel. So long is that 27.5"

[ConRAD]

I'm not sure: is it correct to test and state that a 3W dynamo is working properly (i.e. within specs) if loading its output on a 12 Ohm resistor and making a 28" wheel to spin at an equivalent of 20 km/h ... as a result I get 6V on top of the resistor itself?

[Brucey]

Yes, provided it is either 6V DC or 6V RMS.

cheers

[ConRAD]

My understanding is that a common 3W/6V dynohub is generally conceived to be normally operated at its nominal electrical parameters, i.e. roughly 6V-500mA.
Now it’s clear enough that from a 3W/6V dynohub is nevertheless reasonably easy to draw up 5-6 W and so I’m just wondering which might be its possible collateral drawbacks, such as overheating, overwearing, etc., during a prolonged overload of the type 12.75V-0.43A-5.48W.
IMO, in such a specific case, losses should be even less than the nominal ones … aren’t they ? (lower copper losses associated with a lower current, iron losses increase due to higher voltage shouldn’t increase significantly, mechanical losses should basically remain the same, …. what else ?).

[Brucey]

I think that there shouldn't be too much trouble for the reasons you describe. Plenty of people run 6V lights in series at speed, drawing that amount of power, more or less, with no worries.

However as someone once said 'expect the unexpected'.... so in time you may find that there are shortcomings. I can think of one, which is that if you are drawing ~6W from the generator , it'll 'cost you' about 12W to do it, and that is enough that most riders would notice the extra drag for sure.

cheers

[MikeF]

My understanding is that a common 3W/6V dynohub is generally conceived to be normally operated at its nominal electrical parameters, i.e. roughly 6V-500mA.
Now it’s clear enough that from a 3W/6V dynohub is nevertheless reasonably easy to draw up 5-6 W and so I’m just wondering which might be its possible collateral drawbacks, such as overheating, overwearing, etc., during a prolonged overload of the type 12.75V-0.43A-5.48W.
IMO, in such a specific case, losses should be even less than the nominal ones … aren’t they ? (lower copper losses associated with a lower current, iron losses increase due to higher voltage shouldn’t increase significantly, mechanical losses should basically remain the same, …. what else ?).

It's the current that causes the heating IxIxR. So if you have 2 lights in series then the current will be the same. That will be the current flowing through the generator as well. If the lights are in parallel then the current, and hence heating, will increase.

[Brucey]

there is also heating from magnetic hysteresis, there is increased mechanical vibration... there are a number of things. But a hub generator is not normally highly stressed in these regards.

cheers

[RickH]

... they plucked a (nice round) number out of the air ...

... ohh, that's fantastic !!! ... how many other things are like that in the world !!!

63,218!

[SA_SA_SA]

I second andrew_s's earlier answer above :

http://forum.ctc.org.uk/viewtopic.php?f=5&t=83892&start=60#p863394

I thought the UK approach was generally that bike lights were for being seen by:
in the days of vacuum/krypton filament bulbs, I found everything below 2.4W was useless in the absense of streetlamps. Perhaps Brucey-like persons ate more carrots than me .

[ConRAD]

... just dusting (or undusting ?) this thread in the attempt of not forgetting some dated ... yet important conclusions.
A common rated 3W/6V hub dynamo is supposed to have been designed to deliver 3 W on a 12 Ohm resistive load at 20 km/h (or something like that) and it’s normally regarded to act as a basic “current generator” when connected onto a 3W/6V bulb (equivalent this one to a 12 Ohm resistance under incandescent conditions).
As said above this is true until the speed and in turn the inductive component of the internal impedance of the generator prevails on the sum of its internal resistive component (roughly 2.7-3.2 Ohm) and the external resistive load.
But as soon as the total resistive component becomes increasingly significant compared to the inductive reactance, the dynamo output will act as any other generator and will vary significantly with respect to speed too.
Under short-circuit conditions the current will top to its allowable maximum, regardless of speed !!

[Brucey]

nice work; might be worth mentioning what model hub generator you used for the tests.
A couple of things which might be interesting to look at;

1) how does the current vary if the load is reactive (eg if there is a capacitor fitted, eg as per the Pilom circuits?) and

2) there are some generators which are more draggy on an open circuit than when connected to a resistive load. SA SA asked recently if the 'lights off' drag might be lowered by attaching some kind of a dummy load to the generator. I think it might but I don't know what exactly the best load might be.

cheers

[Mick F]

The 3W limit is so that you don't blow the bulb the first time you ride down a hill

All very interesting re-reading this thread.

I bought a Soubitez bottom bracket dynamo yonks ago and used it commuting. Excellent bit of kit, but I regularly blew the front bulb. Xenon thing, if my memory serves me correctly. I seem to remember fitting a halogen bulb in there some time later.
I bought a voltage regulator that I hid inside the rear lamp and ran the feed to it and then in parallel to the front and rear lamps.
From then on, they worked beautifully.
Maybe 1986 to 1995.

From what I remember, the BB dynamo was just that .......... a dynamo ......... and produced DC. I was never able to strip it down to see or maintain the brushes.

The Sanyo version was much inferior to the Soubitez. Sanyo reputedly slipped, but my Soubitez worked brilliantly.

I sold it on here back in 2004 or so.