Matching to the impedance of an alternator.

You have said precisely zero about this topic. Because you can't.

Silly old hen.

John

Well I suppose if its a BLDC motor you could lock it by dumping DC into the windings, as you can with a stepper motor - if its got brushes; DC current would drive it round.

Ignoramus. ...Jim Thompson

I you disgree with me, say why.

You don't know anything about this stuff, do you?

John

Sure, if you ramp up the drive frequency from zero, like with a PWM vector drive, the poles will follow it up. If you connect a synchrmnous motor right across the AC line, without ramping up either the shaft speed or the frequency, the torque is oscillatory. Some low-inertia many-pole motors might start this way, get close to synchronous speed in a single cycle, like a stepper for example, but most power-scale motors have enough rotational inertia that they'll just hum.

If you have control over that, sure.

Yup, steppers with 100 poles maybe. They will pull in at a hundred Hz step rate or so with no rampup, but are usually started at controlled acceleration.

If you play with a big bus-connected alternator, you can control current independent of torque. Rotating power factor correctors behave like adjustable capacitors. They pull lots of amps at zero shaft torque. AC isn't DC.

"Lots of amps at zero shaft torque" is almost exactly what we're talking about here, with a shorted alternator. If the current in the windings is quadrature, which is mostly what you get from a shorted winding, the shaft torque averages to zero. Little work is being done, so a lot of torque would violate conservation of energy.

John

Here's some detail:

High current at zero shaft torque.

John

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That follows from what I said.

Not so. The power is lost in the winding resistance.

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Only a little. There's a little bearing, windage, and eddy-current loss, too. In principle, you could have very low winding resistance (superconductive windings?) and still have lots of short-circuit current but zero shaft torque.

If the reactance of the windings is much greater than the copper ohms, the short-circuit current will be almost 90 degrees out of phase with the induced voltage. Lots of current but little power dissipated. It's like a spring pulling sideways on a crankshaft... no net torque.

Again, we're talking about a bicycle PM or similar alternator that's running in its constant-current regime, namely where the inductive reactance is much larger than the coil resistance.

John

You can start a three phase synchronous motor from a dead stop IF you can start at a low enough frequency and the stiction in the motor mechanical circuit can be reached at the low frequency you're applying to the stator. So practically, I agree with you, but theoretically, I disagree.

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Use of series capacitor compensation is a widely known tweak in dynamo lighting systems. Trying to achieve the same result electronically could be fun.

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I think we get back to the idea of a bridge rectifier, filter cap, and a buck switching regulator.

John

No argument, if you have to option to ramp the frequency up.

John

Behavioral Buck Switcher... run time, one blink:

Well, plot voltage vs speed into say 20R, 50R, 100R, 200R; get an idea if more power is available from the thing at higher load resistance. Measure, don't guesstimate ;)

Grant.

Rick, you are "Da Man!!!". Thanks for some real world data. Now, if I recall correctly, these "bottle" dynamos have an approx 1" diam drive wheel and couple to the tire at say 90% of the tire radius. Since we're discussing a Sturmey Archer dynamo, let's assume a 27" wheel and also assume speeds between 5 and 25 mph. So the surface of the dynamo drive wheel is travelling between 4.5 and 22.5 mph. The circumference of the drive wheel is 3.14", so the dynamo is spinning between 1500 and 7500 rpm in normal use. Can you spin that lathe any faster?

Synchronous rectification is probably a good idea when you've only got 6V or so to start with. Now, if you fudge the phasing of your rectification with respect to the actual zero crossings, can you make the downstream circuitry appear capacitive?

. . .

--- Then take Barone's advice and forget the buck regulator.

Since your battery will automagically clamp the output voltage of the dynamo to somewhere in the region of 6 to 8 volts, depending on the battery's state of charge and the drop across the bridge, all you really want to do is pump as many electrons as you can into the battery with as little interference as possible.

I like the idea of the series cap, since that would cancel its own Xc and the winding Xl at some frequency, so what I'd do would be to determine what that frequency would be at some convenient speed, figure out the cap to resonate out the L, and build a micropower detector to light a really low current LED at that frequency.

Either that or just keep track with a mechanical speed-o-meter.

-- JF

Hmm - not that good at insults either.