Where lies AC efficiency?

1. Dec 21, 2015

Guidestone

I've heard that alternate current is more efficient than direct current. I've heard is easier to transport than dc current. Is that true? Why is that?
Also, why are three phase systems better than monophasic ones?
Thank you very much!

2. Dec 21, 2015

davenn

it's easier to "transport" ( send over a transmission line) because of the ease of use of transformers to step up and step down AC voltages
that cant be done with DC

3. Dec 21, 2015

Guidestone

Isn't DC capable of generating magnetic flux in the core?

4. Dec 21, 2015

davenn

a transformer ONLY works with a changing magnetic field
that that ONLY happens with AC

do some googling on transformers, induction and similar things

Dave

5. Dec 22, 2015

Jony130

If you connect a the DC voltage across the primary winding the primary current and the flux will ramp-up in constant rate of change. So, the secondary voltage will also be induced. This means that ideal transformer work with DC voltage. But with real world transformers this will not work. Why ?? The answer is the core limitation. Because the DC voltage at primary causes the flux to ramp-up in constant rate, and this rise in flux will quickly saturate the core. And when the core is saturated the coils stop working as a inductor (loses all its inductance), now the coil work as a "primer winding resistor". So we have a short in the circuit. And Bmax for a mains transformer cores are in range of 1T ...1.6T.

Last edited: Dec 22, 2015
6. Dec 22, 2015

Averagesupernova

This sounds like a homework question.

7. Dec 22, 2015

Jeff Rosenbury

Three phases are the minimum number of phases for constant power transfer.

In a single phase circuit, the power pulses. This is fine for many applications like small motors, but for large motors the pulsing can lead to oscillation, vibration, and other destructive problems.

8. Dec 22, 2015

meBigGuy

AC is more efficient than DC for power transmission only because it can be easily transformed to higher voltages. It is actually less efficient for the actual transmission.

Once you have high voltage, DC would be more efficient.
1. No skin effect
2. Constant power with single phase
3. reactive effects

From https://en.wikipedia.org/wiki/High-voltage_direct_current
"Depending on voltage level and construction details, HVDC transmission losses are quoted as about 3.5% per 1,000 km, which are 30 – 40% less than with AC lines, at the same voltage levels.[22] This is because direct current transfers only active power and thus causes lower losses than alternating current, which transfers both active and reactive power."

9. Dec 22, 2015

rootone

The reason why we need to step up the transmission line voltage to a very high value (over 100kV is typical), is because this means the line can deliver X watts of power, with less current flowing in the cable.
That results in more efficient transmission since less current = less energy wasted in heating up the line.
Clearly though, we can't have 100kV power supplies directly into homes, so the grid voltage is stepped down to a sub-lethal level at a local transformer close to where the power will be consumed.
As has been pointed out, you can't easily do this sort of thing with DC, whereas with AC you can.

10. Dec 22, 2015

Guidestone

No homework Sir. Just killing my ignorance

11. Dec 22, 2015

meBigGuy

Sometimes your questions seem to be off-the-cuff responses that have not been thought about or researched. It sometimes seems like you are not asking a well thought out technical question. It would be like me answering with "of course it is" instead of a thorough technical answer. You are capable of asking better questions than that.

12. Dec 23, 2015

sophiecentaur

However, transformers are (and always will be) very expensive devices and it may not be too long before DC voltage changers ( step - up and down ), working at very high powers will be cheaper than Iron / copper transformers. I reckon that AC power distribution will then die a surprisingly fast death.

13. Dec 23, 2015

jim hardy

14. Dec 23, 2015

dlgoff

#7; A DC step voltage? Never thought about the possibility of getting your legs blown off.

15. Dec 24, 2015

sophiecentaur

The link from UK to France has used DC for quite a few decades and that use(d/s?) mercury arcs. Cumbersome and expensive but worth it for spreading the load over the time differences.
That link does quite a job on HVDC but, apart from the RFI comment, it's not based on probable future technology but on what's currently available. Having seen solid state move from mW to kW and having more and more complex circuits (Flawless TV displays, for a start), I have to say it's feasible to think in terms of many MW systems. Also, the network could well involve more, lower power cables.

16. Dec 24, 2015

Staff: Mentor

Congratulations Sophie on your fantastically imaginative speculation for the holidays. Even my friends who work in power electronics R&D don't have dreams as broad as that. On the other hand, if we assume that Moore's Law applies to power electronics and lasts forever, then there is no ceiling. ...

So let me add my own speculation. Suppose that thermo-electric devices were a million or billion times better than today's. Then we could move to wireless transmission of power for the last mile. A phased-array laser or maser on the pole top (or mounted on my solar/wind array in the tree top) aims at a receiver on my house and heats the receiver. The receiver(s) generate multiple DC voltages for short distance distribution within the house. Heck, that would even work for me living on my boat 200 m from shore.

Then comes the advanced version, the masers aim at each device's thermo-electric receiver directly (even mobile devices.) thus eliminating in-house wires. We would have wireless power transmission, plus complete electrical isolation eliminating ground and lightning problems.

17. Dec 24, 2015

jim hardy

EDIT Hijack Akert (sorry - i'm scrambled)
Ahhhh my old daydream.
Then we'd place thermopiles in Gulf of Mexico and Arctic Ocean , tapping off power all along the way.
Mother Nature's moving heat that direction anyhow with Gulf of Mexico edit oops, make that Gulfstream and atmosphere - watch your satellite weather this time of year you see huge sheets of clouds carrying enormous heat of vaporization from Gulf up across Tennesseee Valley then offshore at Northeast. US/Southeast Canada.
We'd help her along with her heat transport reducing Δ-temperature, maybe relieving severe weather like yesterday's .

Curious - i just cant seem to imagine decentralizing the grid. I guess i was imprinted by the big machinery.

from http://marine.rutgers.edu/cool/sat_data/?nothumbs=0&product=sst_comp&region=gulfmexico
Look how Gulf cooled between 16-20 Dec

old jim

Last edited: Dec 24, 2015
18. Dec 24, 2015

sophiecentaur

I think you may be extending my argument so far that it loses credibility. (Well known arguing technique). Fact is that solid state could work over small networks now. No new technology needed to provide a home's worth of DC power. No change of conductors would be needed for lower level distribution. I'm not talking in terms of next year and the exponent in the 'Moore's Law' of Power may not be as high. But we all know that 'Wound Components" are phased out wherever they can be. . . . . .
(Sorry Jim but it won't affect either of us so there'sno need to worry too much.)

19. Dec 24, 2015

jim hardy

KVA3phase = Vline*Iline*√3

If earth or neutral is return, same 3 wires could carry √3 more power at same RMS voltage,
though they're insulated for √2 more.
√3 * √2 is 2.45.......

i dont intend to learn those big converters.
I like lube oil and steam, wish i'd lived in days of recips and flyballs....

20. Dec 24, 2015

sophiecentaur

You have brought up a significant problem with three conductors plus a fourth, much thinner one. What volts would you put on each one? I guess the potentials wouldn't need to be arranged symmetrically; the thin conductor could carry less current and, as long as K1 is followed, there would be a range of choices. I am assuming that the converters would be smart enough to divvy up the power in an optimum way.
(Sorry about the lack of Oil and steam in my future world)