Driver as an Alternator? - diyAudio
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Old 29th May 2007, 11:11 PM   #1
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Default Driver as an Alternator?

If I used a pulsating pressure to move a driver cone essentially turning it into a linear alternator, could I expect a reasonable efficiency? Would Fs be a good target for operation?
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Old 29th May 2007, 11:27 PM   #2
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You're essentially talking about using the signal from a dynamic microphone as a power source. Obviously it can be done, but power output is limited.

A high efficiency speaker is rarely more than 1% efficient at converting electricity into sound; I don't really think that you're going to do any better than that in converting from motion to electrical power. If you have a linear motion source, I would expect far greater efficiency from converting linear into rotary motion and then using a standard generator.

If you were just looking to do this "just because" or as a demonstration, I would expect to be able get at least several tens of milliwatts out of it. It's going to have a widely varying output impedance depending on frequency, so I think you'd really need to graph power versus frequency into your desired load in order to find the frequency for maximum power.
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Old 30th May 2007, 12:36 AM   #3
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Actually it's a low temperature stirling engine that I want to try, and really just a proof of concept at first, so I'm going with what I have. I put a couple of identical 12's together face to face and got 4 volts out of one with 11 volts input into the other at 20hz with quite a lot of air leakage, so it's much more efficient than I thought it would be. I don't have a firm grip on impedance. While operating, the output driver showed .7 ohms across the terminals. Is that the impedance and how does that affect me as a power generating unit?
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Old 30th May 2007, 12:48 AM   #4
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You need to know the impedance of the load the device was driving in oder to figure out the output impedance. Was the driver just on an open circuit, or was it shorted across a resistor of some sort?

You could get a reasonable voltage out of it, but if it can't drive any current into a load then it's not going to put out much power.
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Old 30th May 2007, 04:00 AM   #5
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Thanks Joe,

You were right. I put a small resistor accross it and voltage feel to 1V with less than 100mA of current. I figured there must be some hole in the idea, because it was just too easy.

Now the question is how to make a linear alternator that can put out some power. If possible, I'd like to cannibalize a cheapie driver to do so, to keep things simple. I have linear output, so converting that to a rotation requires a piston, rod, linkage, and flywheel all of which will eat up efficiency and add complexity. The best and most compact commercial unit uses a linear alternator to put out up of 5kw. I'm operating at a low temperatur differential and only need a handful of watts.

I haven't found any info online about a homebrew linear alternator. Is it possible to do something like just rewind a VC with a more robust wire to generate some power, say 10 watts or more in the 10-30hz operating range?
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Old 30th May 2007, 04:20 AM   #6
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There's currently some commercial interest in harvesting power from ambient vibrations, thermal changes, light, or whatever else one can find. The applications are self powered sensors, battery charging, and other things. Vibrating piezoelectric cantilevers are one method that's been looked at. As you can imagine, efficiency is high on the list of desirable qualities, as there isn't much energy available to these things. You'll probably find some info by Googling combinations of some of the above terms. As for the speaker, there's no reason you can't build a reciprocating linear generator, rather than a rotating one. My guess, which could be completely wrong, is that the electro-mechanical efficiency of a speaker is far higher than it's acoustic efficiency, so those 1% numbers are probably pessimistic.
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Old 30th May 2007, 05:37 AM   #7
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There are definitely some interesting hits on Google when I look for linear alternators.

First of all, did you verify that the sterling engine would be able to do 10+ watts of work? that could be verified fairly easily with a jig to load the sterling output with a known dry friction and some sort of estimate of the speed over time of the movement. It would be a shame to go through this all only to learn that the sterling engine needs a little more oomph.

Have you seen one of these?

http://www.flashlightreviews.com/rev...mond_shake.htm

It's basically a linear alternator. If you bought several of these lights and cut them apart, I believe you could set the up in series/parallel to get what you want out of them. Here's a source where you can buy them for less than $3 each:

http://www.overstock.com/Bulk-Home-G...5/product.html

Actually, I just had a chuckle while thinking of your sterling engine hooked up and shaking a roomfull of 100 of these things

I don't know if this will get you where you're going, but hopefully it's a nudge in the right direction.
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Old 30th May 2007, 08:02 AM   #8
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Thanks again Joe,

Interesting flashlight. We don't get that kind of stuff down here. I didn't realize cap could discharge slowly. That may be the ultimate path forward to electric vehicles.

It gives me an idea for a more powerful easy to make linear alternator, at least I think so. That is to remove the core and glue a beefy coil inside the VC former. First, I'll test how much power I can get out of one of my drivers with a much higher Le. Worst case I can rip the motor assembly off of a burned out driver and add a neo magnet mounted on a rod to push back and forth through a coil like the flashlight. Cannibalizing one of those flashlights and incorporating a rod to move the magnet might be enough. The LED is probably getting 3.5V and 40mA, but powered for 40 units of time using a cycle rate of maybe 5hz.

20W is probably overly optimistic because that 30% solar efficiency is what the big high temp commercial stirlings are doing. I'd be ecstatic with 5 or 10W out. This is just a proof of concept.
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Old 30th May 2007, 08:22 AM   #9
terry j is offline terry j  Australia
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Hi John

curious about the LTD stirling, what is the upper temp you will be using, ie what is the actual temp diff you're looking at using?
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Old 30th May 2007, 09:57 AM   #10
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Quote:
Originally posted by terry j
Hi John

curious about the LTD stirling, what is the upper temp you will be using, ie what is the actual temp diff you're looking at using?
I'm shooting for a 25C cold side and a 60-70 hot side. I ran the numbers earlier and 20 watts was way too high. At half of the Carnot efficiency that's still more than 5 watts with the 12"dia surface area I'm using. If I can get something running using any speaker parts, I'll post it here.
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