Matching to the impedance of an alternator.

The alternator in question is a 6V 1.8W (0.3A) Sturmey Archer (Normally referred to as a "dynamo" in bicycle terminology).

Please can anyone direct me to any sources of info about matching loads to the impedance of an alternator?

Apparently the original equipment uses the precarious arrangement of using the front & rear bulbs to load the armature output to the specified voltage.

What I am intending to do is charge a 1.2Ah SLA battery, I'd like to know whether I can get more power from the alternator by allowing the output voltage to rise above the rated 6V by using a step down converter to reduce the current draw.

Any info on alternator characteristics & behaviour, or how to impedance match them might be useful.

Thanks.

Reply to
Ian Field
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Cool, they burn out together.

That will probably work, but you should test the actual alternator at speed and measure its voltage and impedance.

John

Reply to
John Larkin

ATM my workshop is dismantled for refurb - even if it wasn't, I don't really have the facilities to set up and drive the alternator under test bench conditions.

It crossed my mind to maybe buffer the O/P of my signal generator and plot f-Xl curves (whatever that might enable me to calculate).

It'd be handy if I could find an: alternators -> beginners start here, webpage.

Reply to
Ian Field

Crikey! Ian returns with his old post, "Very low power dynamo (alternator actually)", renamed :-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

That won't tell you anything about the alternator.

Can't you even carry a DVM and pedal? Open-circuit voltage and short-circuit current would be a good start.

John

Reply to
John Larkin

In my cycling days (before high efficiency LED lamps and modern batteries that imho obsoleted bicycle generators) it was true that if your headlamp opened up (it was the major load) and you were going fast, you WOULD blow the tail light. Would seem that the generator was designed for a maximum current and output winding would saturate at rated load. This was in effect a regulation but didn't help if you lost part of the load. I rarely lost a lamp but did try using a shunt regulator just as a protective device.

Reply to
Oppie

Sturmey Archer????!! I had one of those when I was a kid, so that is like 50+ years ago. Do they actually still make these? It was a real piece of junk then, and I even knew it as a kid.

Jon

Reply to
Jon Elson

You need to characterize the thing.

Not sure if they do this on bicycles, but in the old days, I had a motorcycle that used the leakage inductance of the alternator to limit the current. As the RPM went up, the frequency of the ac generated went up. The leakage inductance impedance is higher at the higher frequency counteracting the increased voltage. Worked very well for a carefully designed system.

Does your alternator put out DC? The ones I've seen have been AC, the bulb doesn't care. So you may have to rectify it and incur those losses as well.

Why SLA? In the general case, there are many advantages to 4-AA NiMH.

Reply to
mike

They still make them, much better now-a-days. Ian is looking for his typical free ride... literally ;-)

But his time would be better spent buying a new hub.

Besides he's not bright enough to take enough diagnostics to see if he can boost the output :-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Is Ian strong enough to pedal against a short-circuit ?:-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

"Ian Field"

** Do any Goolge searching at all ?

formatting link

" A dynamo is a current source"

... Phil

Reply to
Phil Allison

1.8W/746W/hp = .0024hp

Probably.

John S

Reply to
John S

Matching a load impedance to a source impedance is what you do when you want maximal power transfer and don't mind about poor efficiency. Alternators (generators, dynamos, whatever) are generally made to be far more efficient than that, so matching your load impedance at all bicycle speeds may well overstress the alternator and make it go Pfffft!

You want to do several things, all at once:

  • Pull the maximum power you can out of the alternator when it's going slow (I assume -- do you want it to bog you down going up hills?)

  • Limit the current draw from the alternator at higher voltages so it won't go up in smoke.

  • Charge the battery appropriately (i.e. limit both current and voltage; that's how SLA's like it).

You probably also want to do this in such a way that all of your precious muscle energy is going to light production, rather than getting burnt up in the electronics.

If this is so, you want something like a switching supply, and (I hate to say it) you probably want it managed by a microprocessor. Since you'll never know if the source is higher or lower than the load, you probably want a SEPIC converter.

So, you just need a microprocessor with a PWM output, a gate driver, a MOSFET, some caps, two coils, and a zillion lines of code.

It should be a snap.

Have fun!

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Reply to
Tim Wescott

Actually, the maximun power transfer happens with a conjugate impedance match.

and don't mind about poor efficiency.

Probably not. Alternators like this are designed to be current sources, and operate with nearly-shorted loads. Current is what will fry an alternator, so if you run it into a higher-impedance load than normal, you can get more power out of it but run it at less current, which will run cooler.

It probably does that already. Higher speed makes higher frequency, which makes higher open-circuit voltage but roughly constant loaded current. That's why a little bicycle alternator can be connected directly to incandescent bulbs.

John

Reply to
John Larkin

"Tim Wescott"

** A bicycle dynamo is lucky to generate 5 watts and is a varying frequency, highly inductive source.

Do stick to the context in hand.

Arguing from the general to a special case like this one is idiotic.

... Phil

Reply to
Phil Allison

The average relatively in shape human can produce 100 W on a bike for a reasonable amount of time. On a WattBike, I managed to get almost up to 746 W for a few seconds. Pro cyclists can maintain 400 W for over an hour.

Reply to
Ralph Barone

voltage.

well.

Yup, that's the design. Permanent magnet alternator, so frequency and voltage are proportional to speed. Lots of leakage inductance which adds impedance proportional to frequency. For the right size load, over a reasonable range of speeds, it's a constant current AC source. An open circuit and short circuit test at a few different speeds should give enough information to produce a model.

PS: I thought I had replied earlier giving my thoughts on how one could suck the last vestige of power out of a dynamo. Perhaps that reply got lost in the pipes.

Reply to
Ralph Barone

As a kid (much fitter and faster) I got seriously pissed off about constantly paying to replace blown bulbs.

Early attempts with inverse series zeners gave dissapointing results. I the end I fed a moped shunt regulator via a bridge rectifier, the moped zener +

2xVf lit the lights a little too bright but saved an awful lot of bulbs.

My homebrew LED lighting set uses significantly less than the generator can put back in the SLA battery, but I'd like to add an occasional use 3W Cree as a "main beam" for unlit cycleways - its nearly twice the generator output (+ the existing lights) so can't run all the time, but I'd like to make the option as long as I can get away with.

Reply to
Ian Field

Not sure - the one I have is stamped 1974, there may be an older one in the shed.

It was a real

They're mainly crap because the magnet is crap - if you remove the armatire from the magnet without sliding a keeper in behind it the magnet demagnetises instantly.

There's a webpage somewhere describing a homemade upgrade with neodymium magnets bonded to an iron ring that fits where the original magnet did.

The Shimano generator hub is nearly twice the output and has the right number of spokes - as I need to get a wheel rebuilt, I've half an eye open for a cheap S/H Shimano hub.

Reply to
Ian Field

Its AC, but the bridge rectifier has one advantage - I can add 2 capacitors to make it a voltage doubling bridge, in practice the SLA battery will prevent it doubling but it does increase the current.

NiCd cells don't take kindly to freezing temperatures - I'd assumed the same limitation applied to NiMh.

I'd be delighted to be told I'm wrong on that score - I have a big bag of assorted NiMh and last time I checked Maplin, the price of the 1.2Ah SLA had more than doubled!

Reply to
Ian Field

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