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Re: DC-> AC Power Switching

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.

Dr. Resonance

> > ------------------------- > 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 am > 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 oh-so-fun > 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 about > 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 country > 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 during > 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 modes: > 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 connecting > 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 Mr. > Nyquist requires; 60 Hz is not a big challenge to even a slow 8 bit > microprocessor. > > 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 10 > 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 rail, > 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 the > power company about that; probably just showing it would work would be > sufficient). > > 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 Net > for this sort of stuff and found a grad student over in Taiwan made AC with > 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 generate > 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 inductor > 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 other > 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 [sigh] > , 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 Future > > > > > >