AC Efficiency: Fact or Fiction?

In summary, AC is more efficient than DC for power transmission because it can be easily transformed to higher voltages, which reduces energy losses. However, transformers are expensive and there is potential for DC voltage changers to become more cost-effective in the future. Additionally, three phase systems are more efficient than monophasic ones for constant power transfer.
  • #1
Guidestone
93
5
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!
 
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  • #2
Guidestone said:
I've heard is easier to transport than dc current. Is that true? Why is that?

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 can't be done with DC
 
  • #3
davenn said:
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 can't be done with DC

Isn't DC capable of generating magnetic flux in the core?
 
  • #4
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
Guidestone said:
Isn't DC capable of generating magnetic flux in the core?
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.
 
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  • #6
This sounds like a homework question.
 
  • #7
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
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
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
Averagesupernova said:
This sounds like a homework question.

No homework Sir. Just killing my ignorance
 
  • #11
Guidestone said:
No homework Sir. Just killing my ignorance
Guidestone said:
Isn't DC capable of generating magnetic flux in the core?
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
rootone said:
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.
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.
 
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  • #13
Interesting thought. I always regarded HVDC as Tinkertoy technology, plausible but impractical. But so were the automobile and computer .

It might work if we can interest enough young people in taking up careers maintaining that complex stuff.http://electrical-engineering-portal.com/8-main-disadvantages-of-hvdc-transmission
 
  • #14
jim hardy said:
Interesting thought. I always regarded HVDC as Tinkertoy technology, plausible but impractical. But so were the automobile and computer .

It might work if we can interest enough young people in taking up careers maintaining that complex stuff.http://electrical-engineering-portal.com/8-main-disadvantages-of-hvdc-transmission
#7; A DC step voltage? Never thought about the possibility of getting your legs blown off. :oldsurprised:
 
  • #15
jim hardy said:
Interesting thought. I always regarded HVDC as Tinkertoy technology, plausible but impractical.
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
sophiecentaur said:
owever, 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.

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
anorlunda said:
So let me add my own speculation. Suppose that thermo-electric devices were a million or billion times better than today's.
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 can't 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

Gulf16.jpg
Gulf20.jpg


old jim
 
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  • #18
anorlunda said:
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. ..
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.)
 
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  • #19
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 don't intend to learn those big converters.
I like lube oil and steam, wish i'd lived in days of recips and flyballs...
 
  • #20
jim hardy said:
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 don't intend to learn those big converters.
I like lube oil and steam, wish i'd lived in days of recips and flyballs...
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)
 
  • #21
sophiecentaur said:
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.)

I wasn't trying to ridicule your argument Sophie. I think it is a fun speculation, and a reminder that the proven ways of doing things need not be eternal.

But I thought you meant only the last single-phase parts of the distribution, not the three-phase parts.

But if there is any benefit it must be at the consumer level, not the distribution system. Simply swapping transformers for converters while using the old wires is hardly worth the trouble. The transformers are not a big cost in power distribution.

But in the homes where so many devices convert AC to DC, is where we could make savings. LED lights and electronic gadgets work best around 5V. High power devices like stoves would use higher voltages. Two sets of house wiring, 5V and 220V might be enough.

The power company would have to pick up the costs of buying all customers new appliances and gadgets as part of an AC to DC conversion project. We already have experience with that, for example when Mexico converted from 50Hz to 60Hz.

I also smile at the thought of Edison's ghost smiling from his grave. His debate with Tesla would ultimately swing back to Edison's side.

Apologies to the OP. We thououghly hijacked this thread.
 
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  • #22
sophiecentaur said:
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.

Hmmm I'm stuck in the sixties... still thinking symmetry and not used to smart substations..
IF
one just replaced substation transformers with HVDC converters and suitable DC breakers
AND
L1 and L2 were opposite polarity and equal load
THEN
their return currents would cancel
so an existing 3 phase transmission line's neutral would only have to carry L3's return current
might one base load L1&L2 to actual demand and use L3 for the short term fluctuations?

Question for somebody closer to power systems than me. Real time sub-cycle telemetry opens new worlds for them.

Of course I'm assuming HVDC current is continuous, not pwm or something. I was never around any and just don't know how it works .
I do know one has to build into his converters protection against any modulation of load at frequency anywhere near the mechanical resonance of turbine shaft and blades , our was seven hz. Subsynchronous Resonance (SSR) is the term. That can cause a turbine to sling blades or snap a shaft and pretty much ruin your day.

sophiecentaur said:
(Sorry about the lack of Oil and steam in my future world)
It's still driving our pushbutton world. Does 1927 movie "Metropolis" ever come on the tube over there ? I recommend it. Could have been the inspiration for Bolero.
[PLAIN said:
http://www.npr.org/templates/story/story.php?storyId=3335230]As[/PLAIN] [Broken] romantic as it may seem, Ravel said the pulsing, rhythmic composition was inspired by one of the factories he had visited with his father, who was an engineer.
old jim
 
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  • #23
I have to admit that the changeover would be decidedly lumpy. Luckily, most domestic equipment is compatible. Edison had the wrong solution at the time and Tesla earned a lot of cred by forcing AC on us. But that was then and in the future things could well be different - along with more sensible use of electrical power (a more economical regime)
We already think nothing of DC-DC conversion at low power. All I'm suggesting is that the same thing that makes power supplies cheaper these days (i.e. no transformers) could well apply at higher and higher power levels. Transformers are familiar and cuddly but they will always cost what they cost today. Solid state continues to get cheaper and cheaper and more reliable. Given the trends in power generation, can we be sure that the alternator will be the main source in a hundred year's time?
The steam locomotive era finished with a bang in the UK - less than a decade from start to finish.
I would like to have a crystal ball.
 
  • #24
sophiecentaur said:
I have to admit that the changeover would be decidedly lumpy. Luckily, most domestic equipment is compatible. Edison had the wrong solution at the time and Tesla earned a lot of cred by forcing AC on us. But that was then and in the future things could well be different - along with more sensible use of electrical power (a more economical regime)
We already think nothing of DC-DC conversion at low power. All I'm suggesting is that the same thing that makes power supplies cheaper these days (i.e. no transformers) could well apply at higher and higher power levels. Transformers are familiar and cuddly but they will always cost what they cost today. Solid state continues to get cheaper and cheaper and more reliable. Given the trends in power generation, can we be sure that the alternator will be the main source in a hundred year's time?
The steam locomotive era finished with a bang in the UK - less than a decade from start to finish.
I would like to have a crystal ball.
Yea. The tube may return. :devil:
450px-Mercury_Arc_Valve%2C_Radisson_Converter_Station%2C_Gillam_MB.jpg

image compliments https://en.wikipedia.org/wiki/Rectifier
 
  • #25
That's got a cheaper life cycle than a transformer? Counting the education of people to keep it running ? Transformers can last a hundred years.
 
  • #26
jim hardy said:
That's got a cheaper life cycle than a transformer? Counting the education of people to keep it running ? Transformers can last a hundred years.
lol Jim?

edit: you did see the :devil:. Right?
 
  • #27
jim hardy said:
Does 1927 movie "Metropolis" ever come on the tube over there ?
The sexyist robot ever in that film.
 
  • #28
dlgoff said:
lol Jim?

Well there is a natural 'Law of Diminishing Returns' .

Complexity . It confuses our tongues.

old jim
 
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  • #29
last two threads by the OP got so hijacked that the Chinese government hacking into US files can be considered a minor inconvenience :D

as to what you guys said , first of all, simply changing all the worlds grid to some adapted DC which can run through the existing wiring is not realistic atleast not in the near term and not in a capytalist world we live in.All the equippment running in the world today would simply have to be either modified or thrown away , excep for incadescent bulbs and heater elements.I doubt that is an investment someone is willing to make for a few percent decrease in grid loss , and that's mainly the long HVAC lines across countries from power stations to cities , not the small local grids which start at the secondary side of a transformer in a local substation and hen runs to your house.

second of all I don't know why you sophie said that the DC thing could eliminate transformers, some of them yes but not all, firs of all there are many ways were rotating mechanical power has to be converted to electricity and the majority of all the devices used to do that (generators) work on AC, and quite frankly I would like to point out that the only true DC generator that is mechanical and outputs a pure DC voltage without ripple is the homopolar/faraday (disc) generator, all the others work on induction and induction means something has to alternate either way.
Also I'm no expert on HVDC but as much as I have been reading the substations still use a transformer and quite frankly every power supply even run on DC needs a transformer if it wants to isolate the supply voltage from the output voltage with galvanic isolation. and the majority of SMPS that I have seen open on my desk also have a transformer except for a few topologies usually for low power specific devices.maybe I've missed something but how else can you isolate galvanically between two circuits ? a thyristor or a transformerless converter can't do that.
 
  • #30
Switch mode power supplies do use inductors or transformers, true but, because the frequency can be so high, the inductor doesn't need to be massive. mH rather than H.
The advantage of massive generators is due to the efficiency of big turbines - implying large alternators. Alternative systems which don't use heat engines could be nothing like as big. Small scale networks do have advantages when the control systems are up to it.
Listen, guys, I am not an Electrical Engineer but you experts in the present system could do well to consider alternatives that could result from new technology and future Energy problems. I am only flying a kite - which is one of the purposes of PF.
As for compatibility, look at the lifetime of most electronic and electrical goods. Our fridge is 18 years old and that's the oldest thing we own. Given a decade of warning, manufacturers could easily make their power supplies suitable to change from AC to DC. We have seen the transition from 405 line TV to 625 line Analogue Colour TV to Digital TV in 40 years. How many generations of computer have we been through since our first ever micro?
 
  • #31
I'm complicit in the fantasizing about what might be.

If i ever build another house it'll have alongside the 120VAC system a 12VDC one with a light in every room and a couple DC outlets . Or whatever voltage automobiles use by then...

Appliances are already halfway there. New efficient appliance motors turn house AC into rectified DC which is then turned by an embedded computer into 3 phase AC with field oriented vector control , which i consider unspeakably complex. You'll not see any more thirty+ year appliances like my old GE Potscrubber dishwasher, the DC3 of kitchen equipment.

SMPS's are not too complex until you add power factor correction circuitry to them to make them compatible with the AC distribution system.

I think local transmission will remain AC for same reason it replaced DC in mid 1900's; it's simple and reliable and flexible and we're already mighty far up the learning curve..
 
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  • #32
Consolidated Edison in New York City continued offering DC residential service until the 1980s. Are there any Con Ed customers there who remember living with that?
 
  • #33
sophie is correct on the short lifespans of most modern electronics and I just want to say its a business model these days than anything else , theoretically our stuff should last longer and longer as we progress towards and as I do understand the need for PC's and other digital devices to develop with new ones coming in and old ones going out , I kinda oppose the same consumer thinking on all other brands simply because whenever we throw out things that still work or recycle stuff that could have went on for some extra 10 years , we have to produce new stuff and reuse the old stuff and that takes energy and resources are being spent and it damages our ecosystem simply for big corporation profit.
sorry for off topic I just had to say that.

as for the computer generations and tv's , I'd say fully reorganizing the grid with everything attached to it is way more fundamental of a task than simply giving consumers a " better" box to play their games or watch stupid youtube videos every year.
 
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  • #34
jim hardy said:
I think local transmission will remain AC for same reason it replaced DC in mid 1900's; it's simple and reliable and flexible and we're already mighty far up the learning curve..

I agree it's unlikely residential power will be converted to DC distribution internally. Almost none of the commonly used and installed 120VAC electrical parts have UL approval for DC current. Every switch, breaker, socket and receptacle in the house would need to be replaced with parts designed to snuff the sustained DC arc when contacts open.


Our "rule of thumb" for an AC rated switching contact is 10% of voltage and current for a DC application.
 
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  • #35
that's a great video over there, sadly i didint know of this video earlier because we had an argument over here in another thread about whether rectified mains or any rectified AC into DC produces more load through the same load because of a higher voltage and it does , i made such a thing for a soldering iron I had wound a bit too high of a resistance.

as for the video , the first thing I thought was Oh Snap! when they used the 310v DC or thereabout (rectified AC) and the contact was open the switch was like 1cm or more apart and the arc still formed , how is that possible? isn't that arc a bit big for 310v?
Or does DC behave differently in this manner as it's capable of striking longer arcs at the same voltage level?
 
<h2>1. Is it true that higher SEER ratings mean better AC efficiency?</h2><p>Yes, the Seasonal Energy Efficiency Ratio (SEER) is a measure of how efficiently an air conditioning unit operates. A higher SEER rating indicates that the unit can cool a space more efficiently, resulting in lower energy consumption and cost.</p><h2>2. Does regularly maintaining my AC unit improve its efficiency?</h2><p>Yes, regular maintenance such as cleaning or replacing air filters, checking refrigerant levels, and cleaning coils can help improve the efficiency of your AC unit. This allows it to operate at its optimal level and use less energy to cool your space.</p><h2>3. Can the size of my AC unit affect its efficiency?</h2><p>Yes, the size of your AC unit can impact its efficiency. If the unit is too small for the space, it will have to work harder and use more energy to cool the area. On the other hand, if the unit is too large, it may cycle on and off frequently, leading to energy waste.</p><h2>4. Are there any myths about AC efficiency?</h2><p>Yes, there are some common misconceptions about AC efficiency. One myth is that leaving the AC on all day is more efficient than turning it off and on. In reality, turning off the AC when you're not home can save energy and money. Another myth is that closing vents in unused rooms will save energy, but it can actually disrupt the balance of your AC system and decrease efficiency.</p><h2>5. How can I improve the efficiency of my AC unit?</h2><p>Aside from regular maintenance, there are a few other ways to improve the efficiency of your AC unit. These include using a programmable thermostat, keeping your windows and doors closed, and using ceiling fans to circulate cool air. Additionally, ensuring proper insulation and sealing any air leaks in your home can also help improve AC efficiency.</p>

1. Is it true that higher SEER ratings mean better AC efficiency?

Yes, the Seasonal Energy Efficiency Ratio (SEER) is a measure of how efficiently an air conditioning unit operates. A higher SEER rating indicates that the unit can cool a space more efficiently, resulting in lower energy consumption and cost.

2. Does regularly maintaining my AC unit improve its efficiency?

Yes, regular maintenance such as cleaning or replacing air filters, checking refrigerant levels, and cleaning coils can help improve the efficiency of your AC unit. This allows it to operate at its optimal level and use less energy to cool your space.

3. Can the size of my AC unit affect its efficiency?

Yes, the size of your AC unit can impact its efficiency. If the unit is too small for the space, it will have to work harder and use more energy to cool the area. On the other hand, if the unit is too large, it may cycle on and off frequently, leading to energy waste.

4. Are there any myths about AC efficiency?

Yes, there are some common misconceptions about AC efficiency. One myth is that leaving the AC on all day is more efficient than turning it off and on. In reality, turning off the AC when you're not home can save energy and money. Another myth is that closing vents in unused rooms will save energy, but it can actually disrupt the balance of your AC system and decrease efficiency.

5. How can I improve the efficiency of my AC unit?

Aside from regular maintenance, there are a few other ways to improve the efficiency of your AC unit. These include using a programmable thermostat, keeping your windows and doors closed, and using ceiling fans to circulate cool air. Additionally, ensuring proper insulation and sealing any air leaks in your home can also help improve AC efficiency.

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