Is 3phase AC an attempt to mimic High Voltage DC?

Discussion in 'Homework Help' started by pigpen, Jan 26, 2016.

1. pigpen Thread Starter New Member

Jan 26, 2016
23
1
Hi there, my question above isn't to do with homework, just bothering me enough not to be able to concentrate on my readings.
I just want to know if there are instances in which 3phase power is used where High voltage DC would have been preferable? Where the real need would have been a straight line (a large amount of steady current) instead of a sine wave that doesn't drop to zero? Are motors the main and only reason that 3phase AC got invented?

(Sorry this is my first post and it is a desperate post because the question is really distracting me. I can describe my background a bit more later but right now have to finish studying within the hour)

2. Papabravo Expert

Feb 24, 2006
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Using high voltage DC for rotating machinery has certain inherent problems. The most immediate problem is commutation. That can be solved in one or more ways, but they all have drawbacks. In a 3 phase configuration the commutation is done by the waveforms themselves. This is similar to what is done in a BLDC motor which became popular as a result of the microcontroller being able to generate the waveforms and process the feedback.

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3. Kermit2 AAC Fanatic!

Feb 5, 2010
3,854
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The internal structure of the most efficient forms of electric generators and the least wasteful winding schemes led naturally to a multiphase AC power grid.
Function followed form in this case. The early days of power production saw every conceivable form of AC and DC generator. The best got copied and improved in an endless cycle. Like a battle of the fittest, the weak are tossed aside and what we have today represents the state of the art in EFFICIENT power generation. If there was a way that wasted less or cost less it would be in use right now. High voltage DC is currently being looked into because it may be better for very long transmission lines.

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4. hp1729 Well-Known Member

Nov 23, 2015
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Why do you think the system Japan uses is that different from any other country? We all use 3 phase as far as I know.

5. strantor AAC Fanatic!

Oct 3, 2010
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Does this stem from the old Tesla/Edison debate? If so, don't get hung up on it.
We use AC because that's what we standardized on.
We standardized on AC because, at the time we standardized, we didn't have a way to efficiently step up/down DC. It took a motor/generator. Imagine on every street pole where you see a transformer, instead two giant pieces of rotating equipment the each the size of a pickup truck.
Transformers were the most efficient way to step up/down voltage, and they only work with AC. So, AC it was.

Fast forward to today. We have made huge leaps in technology and now with high power switching converters, we can efficiently step up/down DC.
Today, it would make more sense to have DC transmission. Less transmission loss, less conversion loss, less copper waste, less iron waste, altogether better
But all of our devices use AC. They use AC because that's what our distribution system provides, and we've already gone over why it provides AC.

Switching over to DC distribution would be like removing a network of cancerous tumors that are attached to and throughout every organ, every muscle, every bone, and every blood cell in your body.

It just isn't going to happen. AC is what we have and what we will always have, even if it doesn't make sense anymore.
That's the price we pay for being pioneers of technology.
Developing nations may reap the benefits of our pioneering. While it will never happen in the USA, we might see 3rd world countries as they develop, implement DC transmission.
I think India should try it. From what I've seen of their electrical network, the whole country needs to just start over. Get rid of what looks like someone vomited spaghetti all over streets and replace it with something safe and orderly.. and DC.

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Jul 18, 2013
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It has been around for a long time and is in current use in the province I live in for Hydro-electric distribution.
Since the advent of semi conductor switching it is now more economical to convert from AC to DC for long distance distribution, there are enormous savings in lower radiation losses and the need for only two conductors instead of three.
One other smaller advantage is the reduction in lightening strike damage being carried through to consumer equipment.
Max.

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7. RBR1317 Active Member

Nov 13, 2010
268
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For a respite from technical readings, recommended alternative is "Empires of Light" by Jill Jonnes, 2004, subtitled "Edison, Tesla, Westinghouse, and the Race to Electrify the World"

"At the close of the nineteenth century, three visionaries strove to establish mastery over the electrification of America. Within the endeavor lay the promise of vast fortunes, but what was really at stake was the chance for one of these men to make his mark on history."

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8. pigpen Thread Starter New Member

Jan 26, 2016
23
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Thank you so much! You guys pretty much covered every angle.
I think I got confused by the graph of 3phase AC and its similarity to the middle stages of rectifying to DC, which is why i put my question in those terms.
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Pls clarify how a DC distribution system means lower radiation loss and could reduce lightning strike damage to eqpt. (sorry if this is a naive question).

Side questions:
-inside homes, it would still have to be AC because ...washing machines...will always make more sense using AC?
-(a hypothetical question) if we ever wanted to make DC wall sockets standard, and get rid of adaptors forever, what do you think would be a reasonable amount of voltage coming out of those sockets: around 20V DC? And design our gadgets accordingly? Or would such a socket have a voltage selector?

9. pigpen Thread Starter New Member

Jan 26, 2016
23
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thanks for the book recommendation!