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Re: DC-> AC Power Switching
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- Subject: Re: DC-> AC Power Switching
- From: "Tesla list" <tesla@xxxxxxxxxx>
- Date: Tue, 29 Mar 2005 20:38:12 -0700
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Original poster: "Dr. Resonance" <resonance@xxxxxxxxxx>
There is a cryogenic company in Middleton, WI, that uses superconductivity
to store large amounts of power for commercial applications. They might
have a lot of data on this subject but I will have to try to find the name
of the company for you. Contact me in 3-4 days.
> There's been a lot of discussion of some heavy duty power switching
> here on the list, particularly between AC and DC supplies, and, well, I
> just out of my depth on a science project my son has suggested. Can I
> present it and just ask for comments? It's quite obvious that the EE stuff
> I learned in 1976 has either faded from my brain or been overtaken ... I'm
> quite aware that this may be a stupid question, and am willing to take the
> heat for that, but it might also prove to be a fun question. If it can't
> work, I'd love to know *why* for the knowledge of it. I am in the
> position of being The Dad Who Does Not Know The Answer, and since I have a
> scope for my computer stuff and build computers, he thinks I know All
> electricity, which I just plain don't.
> I'm going to ask (beg, bribe, plead) the moderator to allow this note
> on the TC list in the hopes of finding someone who can tell me if his idea
> might work.
> His idea, which I find interesting, is to store power. Charge the
> batteries during the night, when the electrical networks around the
> are pretty unloaded, then release it during the day, especially at peak (4
> PM, according to the California ISO people anyway). He's done some pretty
> interesting research that shows that areas like California have the
> generating capacity but their timing is wrong; if they could store some
> power at night, they'd do okay during the day. (Yes, I'm aware this is in
> thousands of megawatts.)
> He also found that in Colorado, we have done that, by pumping water
> from one lake upstream to another at night, and letting it flow down
> the day. This explains a lot of what I've seen on Guanella Pass.
> One reason TO do this thing is the politics at his school. They're
> having a "green" year and the science projects are supposed to be about,
> well, ecological stuff, solar power, whatever. Personally I'd prefer to
> blow something up or bounce a laser off the Moon or something fun, but you
> know how it goes.
> He proposes using "cheap tech" to solve this -- lead acid batteries.
> Are lead-acid batteries efficient enough to even bother? This is an answer
> I am having a curiously hard time finding an answer to ... what percentage
> of energy going in comes back out? I don't propose to abuse the batteries
> (deep drain them or chill them).
> Here's the idea. Take 10 lead acid batteries (basically car
> batteries). Connect them through a relay network (note, relays just for
> safety, semiconductors don't bother me) so they can be in one of two
> Either being charged at 12V each with a trickle charger, or connected in
> series to form (up to) 120 VDC. [Guess I could charge the group at 120VDC
> but I've been told that is bad.]
> (Yes, ideally, the chargers should be set to a precise voltage in the
> 12.x region and so forth, taper the charging, and so forth. That'll be for
> the government version).
> Inverters are expensive and the transformers and sheer iron are
> terribly expensive. Again, "cheap tech". He came up with the idea of
> patterning a sine wave, from 0 to 120 volts and on down to 0, by
> an increasing number of batteries in series with medium speed, computer
> driven, semiconductor switches (probably transistors). I mean, at 60 times
> per sec, I don't think relays are going to keep up, but probably someone
> will mention how wrong I am (grin).
> Thus at (+)0, nothing is connected; at the first point in the sine
> wave, as it approaches +12, the first battery connects, as it approaches
> +24, the second battery connects, and so forth, up to 120V, then back
> down. Yes, I know a true AC sine wave goes up past 120V, but this is more
> of a "does this approach stand a chance" versus "nailing it on the dot"
> question. We'll definitely be outputting sinewave data much faster than
> Nyquist requires; 60 Hz is not a big challenge to even a slow 8 bit
> This will give us modulated 0 to plus 120 VDC. You can see where I'm
> going, right? Now one idea is to simply switch poles so we're now working
> with (-120) to 0 VDC. If that proves impossible, maybe going for another
> batteries, to form a negative 120 rail. Note: I don't propose to buy 20
> car batteries for a science project no matter how nice my son is about it.
> We'll use small 12V lead-acid ones.
> So, we're at an idea of running a -120 volt rail and a +120 volt
> and using semiconductors to switch the voltage to match a normal AC sine
> wave. (As you can see, this is a clumsy way of doing modulation with an
> additive voltage ladder). Without an inductor or a capacitor to smooth
> things down, it's going to stair-step. Adding those is possible. Adding a
> 6V or even 3V battery to get very smooth is also possible.
> I do see that going from 120V sources and doing PWM would also work,
> but I have deep concerns about matching the current and voltage needs,
> especially as they change with a load, "on the fly" with PWM.
> I believe I can handle the DAC stuff to measure the current and match
> phase to an outside AC source (although of course I'd have to chat with
> power company about that; probably just showing it would work would be
> Now I could probably model this system with relays and run it at 10
> hz. But I am not comfortable using transistors at this voltage and power
> level [not to mention buying all this stuff!] without asking someone if
> this has a prayer of working. Believe me, I have made my share of
> expensive smoke clouds just with computer stuff! I did a search on the
> for this sort of stuff and found a grad student over in Taiwan made AC
> PWM, and it sounds like he worked his tail off. I found nothing else!
> I don't know much past what'll happen when the 60 hz cycle is working
> and the 'scope says it matches. Bring it out to an outlet? Isolation
> transformer? To be a good project it should generate a good current. "The
> Battery Page" has taught us a lot. I'd like to be able to steadily
> AC at 20A and peak to 40A or so, because there are going to be spikes (see
> also: inductive loads, turning something on, etc). The spikes worry me a
> lot, because I don't want evils like ringing and undershoot to hit the
> transistors and kill them. And I am totally out of my depth answering a
> question like, "What impedance do I match to", because matching that is
> what prevents said ringing. I clearly need to talk to someone who knows
> power engineering; hence this note.
> For example, if an inductive load like a vacuum cleaner powers up,
> there is going to be an initial heavy draw (if I remember EE correctly,
> theoretically infinite at start). I have dreams of expensive power
> transistors letting the smoke that makes them work out at that point. When
> it powers off, I believe there's going to be a voltage spike as an
> turns off. (See also: smoke coming out). Things like this worry me. An
> isolation transformer is looking really much better.
> The only thing I've seen online that can handle around 40A are some
> power transistors. I haven't seen anything in SCR land that can. The
> thing about SCR's that bothers me is the heat they generate handling even
> small loads.
> Anyway, there you go. With the "green" teachings his school has
> , if this project can be made to work, he'd probably do pretty well on the
> science fair side. If it's a nitwit idea, mea culpa -- he is 16. If it
> needs tweaking but is possible, that would be nice too. I wish I knew more
> on this but I just plain don't, so I'm asking some people who would know.
> Many thanks,
> Dave Small
> p.s. Feel free to email me direct at
> <mailto:davetracer@xxxxxxx>davetracer@xxxxxxx to avoid cluttering up the
> mailing list, probably unless you think your answer is also relevant to
> something like running a DC powered Tesla Coil ...
> p.p.s. I think trying to do 3-phase is probably not a good idea. *grin*
> p.p.p.s. "1.21 Gigawatts! Tom, what am I going to do!!??" -- Back To The