Termyn8or
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Joined: 11/12/2005
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FR
A couple of posters have touched on this conversion issue. The statement that AC is better for distribution is absolutley true, or was up until certain technologies got better, more on that later.
Now these inverters are not your run of the mill buzzboxes you hook up in an RV or camper, not at all. This type of inverter must operate in a phase locked loop with the 60 Hz already on the grid. This requires a pretty nifty current detection system and very accurate frequency and phase control. Any error costs money in terms of efficiency. A 90 degree phase shift nullifies EVERYTHING. Though the scale is logarythmic and one degree is not very significant, the efficiency decreases to zero in only one quarter cycle. There does have to be a loss across some ballast resistors as this type of current detection simply cannot be done totally inductively. (or it's more trouble than it's worth)
There are however some things in the house that absolutely need AC power, but their dominance is diminshing. Most things with big motors need AC. Your furnace/AC blowers and AC compressors are usually AC only. Anything that uses a conventional power transformer as well. Many many other things do not need AC, in fact the first thing that happens to the AC inside is that it is converted to DC. Stereo equipment usually needs AC for a specific engineering reason, also some things that have a time clock use the AC pulse for synch. The former is something that makes the music - maybe - sound a bit better, the latter can easily be replaced with a quartz crystal oscillator, such as in a battery operated clock. The only reason the AC pulse is used in alot of internal clocks is that even if the grid's 60 Hz leads or lags, it is corrected to NIS standards whenever possible, almost conintually in fact. And don't think you can pick it up on the radio or something like WWV, it doesn't work that way. For example in Cleveland in certain areas you have a choice between CPP and CEI for an electric company. Years ago CPP used to be significantly cheaper and many people used it when they could, but even moreso industry did. But I owned a business where there was a factory in the back of the building that used CPP, while my shop used CEI and I know for absolute certainty that they were not in sync. It apparently depends on load. As well regulated as they are, they can fall behind.
The two grids are different geographically, CPP is synched with the PASNE grid and CEI is synched with the TVA grid. Now back to distribution. In days past AC was the only logical choice for distribution because transformers would step the voltage up, which of course lowers resistive (as well as inductive) losses in long runs. (included just for those unfamiliar with electricity)
However now that there are extremely efficient semiconductor (transistor) devices things have changed. That is why there is no conventional transformer in your TV set for example, nor in your monitor or computer. They immediately rectify it to DC and use said transistor(s) [type device, there are many clases], to convert to high frequency AC.
It's called switched mode. Now with a single coil, not a trnsformer, using a transistor you can step DC up and down quite efficiently. A single ended stage can be, and frequently is more efficient than a conventional 60 Hz transformer. Imagine now a circuit that literally, without a transformer, puts out more current than it draws. And it is technically a series cuircuit so the old laws of electronics say it is impossible, but it is possible now.
The one advantage to DC distribution would be the elimination of inductive losses. Right now it's not worth the trouble, but the trend is there. Even though our electronic devices do have the tendency to be of more efficient designs as the years pass, we seem to get more and more of them. Plus no matter what people say about birth rates it is only logical to assume that population will increase. The final thing is that the cost of these semiconductor based convertors is bound to come down. With these three factors we may one day see a DC transmission system.
One serious ramification of that would be that it would be a hell of alot easier to use batteries for storage and sell power to the grid, as there would be no need to sycnchronize. The amp - hour meter just reads the other way and that is that. Convert your DC to higher voltage and it's a done deal. This makes batteries very attractive, and that technology is also coming along. We could concievably live to see a world with no power outages.
But the fact is DC conversion has a bit of a way to go. Conventional transformers are used in microwave ovens by necessity, devices to do it the other way would just be too expensive. Motors that are AC only are another problem. You would need an invertor for your refrigerator, AC/furnace and a few other things. But it could be done.
In fact it is. In a large building's HVAC system, if it's modern it might use VF motors for blowers and pumps. In these the AC is converted to DC, and then back to AC, but at the frequency they want. They efficiently control the speed of the motor with the frequency. A synchronous motor is pretty effecient and easy to build without brushes. On 60 Hz you got your choice of submultiples of 60 RPS, which is 3,600 RPM. It's usually 1,800 but with some inefficiency comes out 1,750. More poles, less RPM, but always a submultiple of the input power's frequency.
If they find this more efficient in large buildings, how long will it take until this technology trickles down into the home of the average Joe sixpack ? I give it about a decade.
T^T
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