EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: MrOmnos on February 05, 2016, 11:42:25 am
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The old yellow lights bulbs are lone gone and even the refrigerators are smart and everything we look around these days is DC powered. So, why are we still using AC? Why are we taking DC power from the large solar panel fields, converting them to AC again and dealing with losses in double digits? When we are turning all our electrical appliances "smart" , why are our grids still in the 50s?? Were the engineers so focused on the transistors that they forgot about the transformers?? What the hell happened??
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take less (fill in what makes you high) .... and you will see the reasons.. :D
just a few of them:
- vast majority of generators producing electricity are 3 phase AC
- vast majority of electric motors in factories are 3 phase AC
- vast majority of the grid uses transformers - again AC
you solar panels and your house - which might be ready to accept DC are just a small fraction of the system (like a grain of sand in a desert).
and don't forget that many home appliances are still AC.
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Distance, and corrosion across dissimilar metals,
How do you step up a DC voltage from 300V to 11KV at 100A?, With DC, it involves multiple electronic parts with a low margin of overload, with AC its a block of metal with wire, that has an efficiency bloody close to 100%
what happens when you mix aluminum clad steel wire with copper wire or joiners, with AC, as the current flow reverses many times, you end up with only environmental corrosion, with DC, it is amplified and things break down faster,
And finally the most important one for the power grids, AC sparks are self extinguishing in most cases, the current falls to zero 50 or 60 times a second, and providing there is enough gap it will stop, with DC it wont, this is not just relevant in faults, think what happens when you open a switch contact, you need a more complex design to ensure the larger required gap is provided when the contacts are separated,
For local in house, sure rectify and use to your hearts content, but play with big power and its not worth it, even a little,
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There are no technical reasons to keep the AC grid. The only reasons are economical and political.
Technically one advantage of the AC grid was its ability to deliver 3 phase rotating field to motors. These days nearly all motors are driven from inverters anyway, despite the availability of 3 phase power. So this advantage of AC is gone.
Second advantage is switching. Switching big DC currents is difficult. The contacts of the switch tent to arc and weld together. AC power passes trough 0V 100 times a second. This tends to extinguish arcs and makes switching easier.
The political aspect is centralization. Politicians like centralization and hate decentralization, despite what they say on TV.
The economic aspect is mainly related to coal and steel. You need three tons of coal to produce a ton of steel. For this to work you need cheap coal.
If the demand for coal fell the price of extraction would go up, which would make steel expensive and the industry would grind to a halt. Many coal burning coal powerplants keep the demand for coal high and price of coal low.
A DC distribution grid would encourage decentralization which would make coal burning plants unnecessary. This simply won't be allowed to happen.
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"why are we still using AC?"
For historical reasons, as the end devices are designed to work with a.c..
Long distance transmission is hvdc now. To make matters worse. Lots of wasted energy in a.c. to dc conversions.
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You say historical reasons, but DC transmission is only just getting economically viable in some situations. The length of the transmission line is a factor. And, despite being around for ages, photovoltaics are only becoming more common in the last few years. A few years... I wouldn't call that 'historical'.
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I don't think you guys got the point I am trying to make. It's not that we can't generate DC power. Why not generate DC power?? AC transmission lines were outdated long time ago. They were never designed to for high power transmission. Only reason we have large AC grids because small grids got interconnected with each other and grew over time. As the population grew the power demand increased and today we are dealing with losses in double digits. According to Siemens for a power input of 3500Mw, at 600miles and 2x500Kv volts the power output of an AC line is around 2,975Mv i.e loss of around 15% per 600miles. Where as High-Voltage dc lines at 600Kv has the output of 3,342Mv i.e loss of about 5%. I understand the High Voltage DC conversion equipment is costly, but the overall cost when the losses are factored in are less than AC lines. And with the advent of better technology it is possible to decrease the conversion costs as well. A DC line would require less conductor, no need to deal with phases. No skin effect. We wouldn't have to worry about big pylons to put the cables on. We could run them underground. And it would be compatible with all other forms of energy generation. I think if we had done the same amount of research in electrical systems as we did in the semi-conductors department than we would have better grids today. We left it as it was because it was working but it won't for long. I mean think about it, how convenient it would be to have 75v DC line running in your house. With the efficiency of DC-DC converter, we could save lots and lots of energy. No need of those crappy AC adapters with crappy step down transformers.The possibilities are endless.
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I get the point and I do agree but I have no say in power distribution research :-//
Google had some articles published about running their servers with 48V DC. They did some analysis on that. Might be interesting reading.
It would be interesting to see how a 75V DC grid would stack up economically with the ever increasing prices of copper.
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This will be the future. Some large companies are working on a home DC standard for home electronics. The utilities will soon become fed up with devices that only take power at the peaks. A single PFC converter for each home will solve that. That demand may be driven by LED lighting. Getting tired of my LED lights flickering with every surge in the grid. The grid won't get more reliable. Considering people would rather starve than have their cell phone and internet go dead, a few solar panels on each house feeding a DC buss will have momentum. I live in a community where 1 in 4 homes has solar panels. In US tanks of hot water are still the norm. Heating water with a few hundred watts of PV is Is the most cost effective use of PV. I heat all my water with just the PV energy that would be normally wasted. Think of the effect of every home having just a few solar panels powering the wall wart suckers of our existance. It is a huge dent in electrical usage. Grid tie may be the sensible option, but the cards are quickly being stacked against it. Payouts will continue to drop when you feed power back. The home DC grid is likely the future.
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Electronics yes. But how would you go about designing an ordinary 2500W kettle? It would have to draw 33.3 Amps.
The on-off switch would have to interrupt this much current without arcing and without welding shut, and it would have to cost pennies. Same for the thermal cut-off.
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I don't see why it has to be one or the other. Utility power is here to stay for big items. A secondary 500W DC grid for the home that is solar ready is in everyones future.
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Smart kettles!! Solid state relays/switches (cost few bucks these days and can handle 40 amps with ease), use digital temp sensors and controllers. Make everything digital. Doesn't cost much!! Encourage people to not use stuff that uses so much power. Why use a 2500W watt kettle to boil water when you can do it with a 1000w kettle and save money by waiting 5 more mins.
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Why use a 2500W watt kettle to boil water when you can do it with a 1000w kettle and save money by waiting 5 more mins.
More trolls! :popcorn:
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with AC its a block of metal with wire, that has an efficiency bloody close to 100%
what you said is correct but 100% efficiency transformer really !!!
AC transmission lines were outdated long time ago. They were never designed to for high power transmission.
really !!
a debate 200 years old ! i think Tesla and Edison argued about this :-DD
1-historical reasons, first distribution grid were DC and abandoned for power loss. and making higher voltage require a lot of equipment as well. AC easier to generate and distribute.
2-while we can't argue much about solar power futur use, we still can argue about it's efficiency in a large scale implantation.
3-Most massive electric load run easily on AC.
4-protection circuit on a DC load are tricky, your circuit breaker will not work fine on a DC line
5-i think but not sure, 400VDC will certainty kill you or cause much more damage than the AC.
6-Your appliance convert AC to DC with a single bridge rectifier, most power is lost in the isolated step down converter , and you will still have to use it even if your grid is DC
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with AC its a block of metal with wire, that has an efficiency bloody close to 100%
what you said is correct but 100% efficiency transformer really !!!
He said close to. 98% is quite reasonable..
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I don't see why it has to be one or the other. Utility power is here to stay for big items. A secondary 500W DC grid for the home that is solar ready is in everyones future.
This is quite a sensible idea actually.
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with AC its a block of metal with wire, that has an efficiency bloody close to 100%
what you said is correct but 100% efficiency transformer really !!!
He said close to. 98% is quite reasonable..
well, yes am sorry for this but it's true it's close to 100%, i always knew transformer had horrible efficiency due to joule loss and core losses. but yap they are pretty much efficient
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AC transmission lines were outdated long time ago.
The point you seem to have trouble comprehending is that the grid is the least of the problem. The consumption is. Most of today's electronics is designed to take ac power, because of historical reasons. While the grid is mostly DC now from a transmission point of view, the delivery is still AC, because the demand is in AC. And that's not going to change any time soon.
A viable solution is for a transitional period with both AC/DC deliveries, with AC being phased out.
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really !!
a debate 200 years old ! i think Tesla and Edison argued about this :-DD
1-historical reasons, first distribution grid were DC and abandoned for power loss. and making higher voltage require a lot of equipment as well. AC easier to generate and distribute.
First of all, this is not a 200 year old debate. :palm: Telsa and Edision argued about which was better for their time. This is a debate about which is better for our time. As you can see, things have changed lately.
I don't think you read what I wrote. Let me explain it to you as a story. Easy enough in the beginnings, wires weren't that long so losses seemed acceptable. Today though, we have so high power demands and few sources we need to shuffle big power long ways. What they discovered was that with long wires came really big losses because of the AC. Every shift in polarity needed some extra current that was dissipated as capacitive losses due to the proximity to ground and other wires. With really long transmission wires NO power could be delivered! It was all lost in capacitive charging. So they began experimenting with thicker wires and they soon found out about skin effect, the current that is rising or falling prefered to travel mostly in outer parts of wires, so the wire conduction capacity didn't scale with cable thickness. They tried to increase voltage, but that lead to corona discharges, meaning more losses. So their only remedy was really tall and wide pylons to carry the thick cables that had to be finely stranded and partly isolated from each other strands(to combat skin effect) and just live with the losses of about 30-40% per 1000km. I have mentioned the data from Siemens. When you use dc, you need less conductor, no need to worry about phases, no skin effect, no large pylons.
Most massive electric load run easily on AC.
They can be made to run easily on DC as well.
protection circuit on a DC load are tricky, your circuit breaker will not work fine on a DC line
People are making quantum computers and you are worried about circuit barkers. I think we humans are advanced enough to design a good circuit breaker. Most probably they already have because DC lines are already being considered by some countries. Some countries already have HV DC lines.
i think but not sure, 400VDC will certainty kill you or cause much more damage than the AC.
This is just stupid. :palm: 220 volts AC is not going to kill you?? Actually AC an pass more easily through your body than DC. (That doesn't mean DC isn't dangerous)
Your appliance convert AC to DC with a single bridge rectifier, most power is lost in the isolated step down converter , and you will still have to use it even if your grid is DC.
No!!
DC-DC power conversion is more efficient than AC-DC. Low power transformers are very inefficient.
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AC transmission lines were outdated long time ago.
The point you seem to have trouble comprehending is that the grid is the least of the problem. The consumption is. Most of today's electronics is designed to take ac power, because of historical reasons. While the grid is mostly DC now from a transmission point of view, the delivery is still AC, because the demand is in AC. And that's not going to change any time soon.
A viable solution is for a transitional period with both AC/DC deliveries, with AC being phased out.
I know that bit. That's why I am asking the question. We discovered semi-conductors in the 50s. After a decade or so,most of the appliances had started using DC. After that we had 4 decades to change things. But for some reason we didn't. It's like the entire engineering community just ignored the transmission bit. We have made no progress. Now the grid is so complex and broad that it going to be hard to change things.
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There are no technical reasons to keep the AC grid. The only reasons are economical and political.
There is. As rerouter pointed out switching DC at high current is a royal pain in the ass. A small relay can easely switch 220V AC at 8A but the same relay is usually limited to 100mA or so at 30V DC.
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with AC its a block of metal with wire, that has an efficiency bloody close to 100%
what you said is correct but 100% efficiency transformer really !!!
AC transmission lines were outdated long time ago. They were never designed to for high power transmission.
5-i think but not sure, 400VDC will certainty kill you or cause much more damage than the AC.
Back to those cruel animal demos.....
http://knowledgenuts.com/2013/10/19/edison-publicly-tortured-animals-to-discredit-ac-power/ (http://knowledgenuts.com/2013/10/19/edison-publicly-tortured-animals-to-discredit-ac-power/)
Is the OP a descendent of Edison? ;)
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Still not sure if I get your point, DC HV transmission lines are pretty old news. I first heard about them in the 70's. In the early 70's the IEEE contest for most innovative use for a microprocessor was for a toilet. Look how long that took to become everyday knowledge.
Trivia question. According to the IEEE, where was the first high voltage transmission line. If you don't know, you will never guess it.
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Back to those cruel animal demos.....
http://knowledgenuts.com/2013/10/19/edison-publicly-tortured-animals-to-discredit-ac-power/ (http://knowledgenuts.com/2013/10/19/edison-publicly-tortured-animals-to-discredit-ac-power/)
Is the OP a descendent of Edison? ;)
This time it's opposite. AC is accusing DC to be dangerous. :P
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Most massive electric load run easily on AC.
They can be made to run easily on DC as well.
Really? DC motors are much less efficient than three-phase AC ones. Alternatively, you need a complex driving circuitry which converts the DC to multiphase AC (brushless motors ...). The largest loads in a home are typically various heaters and motors - exactly where DC doesn't have any advantages.
protection circuit on a DC load are tricky, your circuit breaker will not work fine on a DC line
People are making quantum computers and you are worried about circuit barkers. I think we humans are advanced enough to design a good circuit breaker. Most probably they already have because DC lines are already being considered by some countries. Some countries already have HV DC lines.
So basically your argument is that because we can fly to the moon we shouldn't be worried about global warming. The argument makes about that much sense.
Some countries do use DC high voltage lines for point to point distribution connections, indeed. But that is not the same thing as delivering DC to premises!
Your appliance convert AC to DC with a single bridge rectifier, most power is lost in the isolated step down converter , and you will still have to use it even if your grid is DC.
No!!
DC-DC power conversion is more efficient than AC-DC. Low power transformers are very inefficient.
The transformer is not there because of AC-DC conversion but because there must be galvanic isolation!
So you will always have that transformer there, you can't connect your low voltage appliance to the 200-400V mains directly only using a non-isolated converter, regardless whether it is DC or AC! This is a complete red herring unless you are also proposing using safe low voltage - and enormously thick cables to carry the currents that go with it.
BTW, those "inefficient" low power transformers (= switching supplies in common appliances like computers or TVs) have efficiences above 80%.
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Back to those cruel animal demos.....
http://knowledgenuts.com/2013/10/19/edison-publicly-tortured-animals-to-discredit-ac-power/ (http://knowledgenuts.com/2013/10/19/edison-publicly-tortured-animals-to-discredit-ac-power/)
Is the OP a descendent of Edison? ;)
This time it's opposite. AC is accusing DC to be dangerous. :P
Indeed, all going round in circles......
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First of all, this is not a 200 year old debate. :palm: Telsa and Edision argued about which was better for their time. This is a debate about which is better for our time. As you can see, things have changed lately.
what changed is what you use in the END of the line, and still you talk about home appliance basically. Again most industrial equipment need that 3 phases AC.
With really long transmission wires NO power could be delivered! It was all lost in capacitive charging. So they began experimenting with thicker wires and they soon found out about skin effect, the current that is rising or falling prefered to travel mostly in outer parts of wires, so the wire conduction capacity didn't scale with cable thickness. They tried to increase voltage, but that lead to corona discharges, meaning more losses. So their only remedy was really tall and wide pylons to carry the thick cables that had to be finely stranded and partly isolated from each other strands(to combat skin effect) and just live with the losses of about 30-40% per 1000km.
am quite sure alot of this is as true as inaccurate
They can be made to run easily on DC as well.
define easily.
People are making quantum computers and you are worried about circuit barkers. I think we humans are advanced enough to design a good circuit breaker. Most probably they already have because DC lines are already being considered by some countries. Some countries already have HV DC lines.
they did ABB did it, and guess what it's full of semi-conductors.
and in the era of quantum computers , overload and protection are still a problem ;)
Actually AC an pass more easily through your body than DC.
it should be the same cause as a load it's the RMS that matter, but i clairly said "am not sure" cause am not saying it's scientific truth, it's just some cases i saw that mention electric shock burns are much more severe when DC is the cause.
The scientific truth is you can feel a 30mA AC but it's 300mA DC that can hurt you.
No!!
DC-DC power conversion is more efficient than AC-DC. Low power transformers are very inefficient.
am not talking about transformers ( used in old linear ps), am talking about DC-DC converter in SMPS they will cause as much power loss as if they are powered from a DC source. Beside a transformer will still be in a DC-DC converter for isolation.
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https://en.wikipedia.org/wiki/High-voltage_direct_current
In the USA, we do use HVDC (400+VDC) for many interstate backbone power grid connections. It is used primarily for it's efficiency.
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Really? DC motors are much less efficient than three-phase AC ones. Alternatively, you need a complex driving circuitry which converts the DC to multiphase AC (brushless motors ...). The largest loads in a home are typically various heaters and motors - exactly where DC doesn't have any advantages.
I was wondering how many of the houses in this world receive 3-phase AC?? Not many!! Most of the motors used in appliances are universal motors. You won't find a 3-phase motor in a house unless the residents are running a small industry from their house. And heaters and kettles are not a problem if you have a 120v or 75v dc lines.
So basically your argument is that because we can fly to the moon we shouldn't be worried about global warming. The argument makes about that much sense.
Do you understand I have posted this because I care about global warming. I am pretty sure if we run a design competition today, tomorrow we will have 100s of design submission of better circuit breakers. Plus somebody might have a good solution out there if I do a better research. Digital circuit breakers might be the answer.
The transformer is not there because of AC-DC conversion but because there must be galvanic isolation!
I was talking about the step down part and I am pretty sure you knew that but still wanted to make a pathetic argument. You do know transformer efficiency varies?? Then you have the loss in switching circuit. Where as you can completely remove the transformer loss when converting DC to DC. And DC-DC converter can reach the efficiency of over 90%. So, we have less loss in transmission compared to AC, and even less loss in consumption. And if do it only for the houses and skip the industry and other sectors still we will be saving a huge amount of energy.
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And DC-DC converter can reach the efficiency of over 90%.
So can AC-DC converters.
So, we have less loss in transmission compared to AC, and even less loss in consumption. And if do it only for the houses and skip the industry and other sectors still we will be saving a huge amount of energy.
Like we save energy by boiling kettles slower?
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Go read the books, everything I said is true expect I made it as a story and sequential. In fact you can read it on Wikipedia. Search HVDC And why and how would I use an isolation transformer when I am not using AC at all. I am not talking about industrial sector here. Only the houses. Like houses get 220v in my country, but it might be different for industries!!
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Go read the books, everything I said is true expect I made it as a story and sequential.
Saving energy by boiling a kettle slower is not true.
And why and how would I use an isolation transformer when I am not using AC at all.
Because you are using AC (look up how an SMPS, that is, a DC-DC converter, works.), and because AC or DC, you need to keep the higher, more efficient voltages for transmission away from things people touch.
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Like we save energy by boiling kettles slower?
No by using low power kettles. In a DC system we have converters, so we don't wanna decrease the efficiency of those converter by drawing unnecessary current. You didn't think of it that way did you??
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Because you are using AC (look up how an SMPS, that is, a DC-DC converter, works.), and because AC or DC, you need to keep the higher, more efficient voltages for transmission away from things people touch.
How will a transformer work in a DC system please explain it to me!!
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Like we save energy by boiling kettles slower?
No by using low power kettles. In a DC system we have converters, so we don't wanna decrease the efficiency of those converter by drawing unnecessary current. You didn't think of it that way did you??
Think of it incorrectly? No, no I didn't. DC-DC converters tend to be more efficient at their design power, not less.
Because you are using AC (look up how an SMPS, that is, a DC-DC converter, works.), and because AC or DC, you need to keep the higher, more efficient voltages for transmission away from things people touch.
How will a transformer work in a DC system please explain it to me!!
Do you or do you not actually know how an SMPS works?
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How will a transformer work in a DC system please explain it to me!!
I suggest you learn something about switching power supplies, namely the flyback topology. If you have used a computer to write the above, then you have one right there. Switching PSU can work with either AC or DC input, with AC there is that rectifier that was mentioned by @hamdi.tn to first change it to DC.
Otherwise, if you don't want to use this type of power supply, I would love to see your solution for the necessary galvanic isolation.
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Go read the books, everything I said is true expect I made it as a story and sequential. In fact you can read it on Wikipedia. Search HVDC And why and how would I use an isolation transformer when I am not using AC at all. I am not talking about industrial sector here. Only the houses. Like houses get 220v in my country, but it might be different for industries!!
:-DD OK, I think this explains everything we need to know :-+
I hope you realize that HVDC stands for high voltage DC (emphasis mine). By definition not anything related to houses. Unless you want 600kV DC coming to your house, that is. It is actually explained in that Wikipedia article! |O
You asked about the 3 phase voltage - pretty much every single house in Europe has a full three phase connection. That there isn't a 3 phase outlet in every room doesn't mean that 3 phase power is not available - typically various circuits in the house are connected to different phases to spread out the load and then you have a full 3 phase outlet for large things, such as central heating/air conditioners, electric heating, if installed, etc.
The nonsense about 75-120V being sufficient - just LOL. Why do many places in the US (which is normally a 110V country) have 220V outlets installed when 120V would be sufficient? I guess they don't know what to do with money ...
Did you ever try to calculate how much thicker the cables would have to be to carry the same power compared to a three phase AC for something like an elevator motor in an appartment block? Never mind that even if you used DC to power it, you would still need a big ass converter to change the voltage to multi-phase AC - you certainly don't want to use a brushed motor at those currents!
The rest is pointless to try to reply to because you obviously don't have a clue what you are talking about.
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In the early days of power distribution and use, it kind of HAD to start with generation, then distribution, then end user. It took a very long time for the system to develop into what we see today.
The difference today is that we can choose to have major changes at any point in the process from generation to distribution to end-uses. On the end-use side, it requires no government or political hurdles to start the process of creating DC system. I was always thinking that having 2 isolated 48VDC busses per house would be a good solution. Low power devices would be 48VDC and high power would be 96VDC. As for fuses and switches - the talk about relays and contact distance is simply non-sense. I have been designing and manufacturing eFuses and solid state DC switching for a while and its easy and cheap. You can fuse and switch very high current DC in solid state very reliably. Think of any SMPS topology - it is switching DC at hundreds of Khz or more 24/7/365. If you only need a light switch that will operate maybe a 100k cycles over decades, who cares? With 48VDC, most applications would not need galvanic isolation, but it is easy if you need to.
eFuses are the same thing, solid state and easy. Much easier than the mechanical breakers we use today. At household and most commercial current levels, switching and managing DC is easy. Look at the power levels seen in a Tesla car, there is not a single relay or breaker (there are fusible links for safety I think). They are dealing with high voltage and high current and switching it to a super high power vector drive to make the car faster than a Ferrari.
The reason I would want local household DC is for the ease and efficiency of distributed generation. Wind, solar, battery backup, etc are much easier to tie together when everything is DC. Local batteries charged by solar and wind can take peak loads off the grid and provide some level of backup in the event of grid failure. As for commercial use, I recently sold my CNC machine shop. We of course had a ton of high power motors up to 30HP - All of which ran off of HVDC around 320V. The air compressor, the CNC spindles, everything. There was not a single motor in the whole facility that ran off of AC directly. This includes the variable speed blowers on the HVAC system - HVDC also. We did not plan that, it's just the way it is.
As for utility scale generation and distribution, I have few opinions because I have no experience on what it takes to deal with long distance DC. I will say that if there is a push for big growth in local micro-generation, there will be a diminishing need for high-power centralized generation that needs to travel 100's or 1000's of Kms. I have enough sun exposure to generate about 60% of my electricity at my house. If all houses did that, the grid would have a lot of excess capacity. There is a large storage warehouse close to me that has a 1MW solar installation, but they only use about 25% of that. The rest is being pushed back out.
The simplicity and ubiquity of installed AC generation and distribution mean that it will be around for a very long time, but there is no reason the push for change can't start at the end-user side of the equation. Most of the changes would not need any real government or political permissions. The scale will tip at some point while people seek better (more efficient and reliable) solutions to energy needs. The interstate grid system will be around for a very long time, but if it is supplemented with new ideas and technology, it will never need to expand and the demand for it will slowly diminish.
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Go read the books, everything I said is true expect I made it as a story and sequential. In fact you can read it on Wikipedia. Search HVDC And why and how would I use an isolation transformer when I am not using AC at all. I am not talking about industrial sector here. Only the houses. Like houses get 220v in my country, but it might be different for industries!!
well thanks for the advice, i will read a lot about it :-+ i said it's as true as inaccurate because it's true what you said, it's just not the problem and while DC can be a solution for those problem it's not the solution for the all the other problems that lead to this choice of using the AC grid.
the funny part is the wiki article talking about HVDC explain the particular use of such system and when it can more advantageous than the AC. so i really really suggest that you read some books or at least the same article cause it clearly say how Siemens in unclear about the projects cost (for commercial reasons), if simens write about it , they expect some money to be made.
You aware that even if this happen it will only be useful for 18% of electricity users, this is the pourcentage of residential users.
50% of electricity consumers are industrial
(source US energy information administration)
transformer work on DC, by switching. SMPS use it all the time.
The transformer is not there because of AC-DC conversion but because there must be galvanic isolation!
I was talking about the step down part and I am pretty sure you knew that but still wanted to make a pathetic argument.
no it's not a pathetic argument, he just thought that you knew what you talking about and you knew about switching transformer that run on DC :) that what you expect from someone sure that all our appliance are in DC ! you not talking about 5V USB grid voltage don't you ?!
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How will a transformer work in a DC system please explain it to me!!
I suggest you learn something about switching power supplies, namely the flyback topology. If you have used a computer to write the above, then you have one right there. Switching PSU can work with either AC or DC input, with AC there is that rectifier that was mentioned by @hamdi.tn to first change it to DC.
Otherwise, if you don't want to use this type of power supply, I would love to see your solution for the necessary galvanic isolation.
Don't they converted back to AC by the chopper part in a SMPS?? Transformer can't work in DC that is what I am trying to say.
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all this been said , it's not wrong that in a particular case of home , a DC power supply can be much useful, TRUE. but can't be as easy for everyone else, and not as easy to distribute so basically that's why power grids still in AC.
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Go read the books, everything I said is true expect I made it as a story and sequential. In fact you can read it on Wikipedia. Search HVDC And why and how would I use an isolation transformer when I am not using AC at all. I am not talking about industrial sector here. Only the houses. Like houses get 220v in my country, but it might be different for industries!!
well thanks for the advice, i will read a lot about it :-+ i said it's as true as inaccurate because it's true what you said, it's just not the problem and while DC can be a solution for those problem it's not the solution for the all the other problems that lead to this choice of using the AC grid.
the funny part is the wiki article talking about HVDC explain the particular use of such system and when it can more advantageous than the AC. so i really really suggest that you read some books or at least the same article cause it clearly say how Siemens in unclear about the projects cost (for commercial reasons), if simens write about it , they expect some money to be made.
You aware that even if this happen it will only be useful for 18% of electricity users, this is the pourcentage of residential users.
50% of electricity consumers are industrial
(source US energy information administration)
transformer work on DC, by switching. SMPS use it all the time.
The transformer is not there because of AC-DC conversion but because there must be galvanic isolation!
I was talking about the step down part and I am pretty sure you knew that but still wanted to make a pathetic argument.
no it's not a pathetic argument, he just thought that you knew what you talking about and you knew about switching transformer that run on DC :) that what you expect from someone sure that all our appliance are in DC ! you not talking about 5V USB grid voltage don't you ?!
If you read my previous post, I have mentioned houses.
I am aware about switching transformers. I was not aware thy were called "switching transformer". I was thinking of indcutors all this time. Never met anyone calling them transformers. May be I should start talking to people more often.
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I am aware about switching transformers. I was not aware thy were called "switching transformer". I was thinking of indcutors all this time. Never met anyone calling them transformers. May be I should start talking to people more often.
In inductor is half a transformer and used for inductive storage. Sending pulsed DC into a transformer is a normal everyday thing to isolate one DC circuit from another.
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If you read my previous post, I have mentioned houses.
I am aware about switching transformers. I was not aware thy were called "switching transformer". I was thinking of indcutors all this time. Never met anyone calling them transformers. May be I should start talking to people more often.
Or read on the subject..
As far as houses go.. why would I want to use an awkwardly low DC voltage to feed my larger loads?
My oven doesn't want to run on 75VDC. Nor does my computer (although my laptop would be quite happy on a sane value like 48VDC), my microwave, my welder (yes, people use welders at home!).. Don't get me started on the aircon.
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I was always thinking that having 2 isolated 48VDC busses per house would be a good solution. Low power devices would be 48VDC and high power would be 96VDC. As for fuses and switches - the talk about relays and contact distance is simply non-sense. I have been designing and manufacturing eFuses and solid state DC switching for a while and its easy and cheap. You can fuse and switch very high current DC in solid state very reliably.
Didn't know this about the reliability of DC switching. If the reliability and the price is right it is definitely feasible.
You are also right about the motors. All motors are installed with inverters these days so the form of the input energy doesn't matter any more.
The only thing lacking is standard outlets and wiring code.
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I guess the incoming voltage for the house could be anything - ~300VDC.
Very high power devices like stoves, heaters, welders, etc can operate at high voltages. Maybe take the incoming HVDC and create 4x isolated 48VDC. The stove would run from all four legs at 192v. While the coffee pot uses 2 legs at 96v and the TV uses a single leg at 48v.
Could be best to just have a single HV line for the stove, heater, welder. If you start from 300VDC or so, you can make any voltage that makes sense. Solid state switching and fusing.
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I like the idea of isolated rails in 48V increments. But if you connect all 4 to a single resistive element you break the isolation for the other loads. It has to have 4 separate heating elements so the isolation is maintained.
Also this would make the power outlet design very complicated. I suppose each outlet would supply all 4 rails and the wiring arrangement on the load side will decide which voltage is selected. This means at least 4 contacts per outlet + Ground for non-isolated or 8 contacts for isolated outlets. Too complicated (read expensive).
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Do the DC fanboys here realize that a typical 16A household circuit breaker (of which you will have dozens in a typical distribution box for a family home) for 230V AC here is rated to reliably interrupt short-circuit currents of up to 10000A, has the size of a box of cigarettes and cost 2.95EUR retail. I wonder how you do that with DC? Keep in mind, matching interrupt capability, size, and price!
Oh, and so far noone has mentioned the shaded-pole ac motors used in fans, pumps (washer, dishwaher...). For sure converting all of them to brushless motors with electronics will offer new opportunities to shorten their lifetime by design >:D
Oh, and regarding power conversion: Also with a DC network you would need to convert voltage levels and provide isolation for low-voltage DC. So the power supplies in all electronic devices basically would look the same as nowadays. The input rectifier might be omitted and the input capacitor might be smaller. A few cents saved for a few hundred to thousand items you might buy in a lifetime. Have an electrician come and install a second distribution network in you house: Couple thousand $/EUR minimum. Savings -> negative. Oh, and modern power supplies ar quite efficient also with 120/230VAC input, not much power to be saved if one supplies them with 48VDC instead...
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I like the idea of isolated rails in 48V increments. But if you connect all 4 to a single resistive element you break the isolation for the other loads. It has to have 4 separate heating elements so the isolation is maintained.
Also this would make the power outlet design very complicated. I suppose each outlet would supply all 4 rails and the wiring arrangement on the load side will decide which voltage is selected. This means at least 4 contacts per outlet + Ground for non-isolated or 8 contacts for isolated outlets. Too complicated (read expensive).
That is true about the isolation, and maybe an argument for a dedicated high power rail. Although 4 element resistive heater is not that big of a deal. For the connectors, there is no need to have all of them available on all outlets. Only the high power loads need the high power. Normal around the house outlets would be fine with only 4 wires - unfortunately 1 more wire than normal household wiring. :-- Running all new 4 wire system is not practical for millions of homes.
I am not sure if its practical, but I have a system I designed with a multi-voltage output power supply and smart loads on the other side. The power supply outputs only a pilot voltage so that the load can communicate what it is and how much voltage/current it needs. The power supply gets this confirmation and send the correct power. It's for a commercial application and works well, requiring no effort on the part of the user. It only uses 3 wires - 2 for power and 1 for communication. Could work on a large scale and dynamically allocate the outlet to multiple voltage options with DC switches in a central controller similar to a fuse box. When not being used, the outlets are only 12v at a few mA for the communication so danger is non existent as the entire run is cold until a proper load is verified.
That would be enough to cover any time of lighting, TV, vacuum cleaner, etc. High power appliances already have dedicated plugs on dryers and stoves so it would not be too crazy. There are only a few of those in most houses and they rarely get touched/moved/replaced.
I am not really advocating a move to DC, just enjoying the conversation so far. Blindly sticking with old ideas will get us nowhere while change for the sake of change is not helpful either. The question is: With all the new tech and knowledge from the past 100 years, is AC outdated so bad that there is a real tangible benefit to such a massive change? Clearly there are some benefits and some drawbacks.
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Do the DC fanboys here realize that a typical 16A household circuit breaker (of which you will have dozens in a typical distribution box for a family home) for 230V AC here is rated to reliably interrupt short-circuit currents of up to 10000A, has the size of a box of cigarettes and cost 2.95EUR retail.
That a PIA* (1 phase or 2 phase)cost 2.95€ will be from the china, because in Spain it costs over 12€ of the Merlin-Gerin corporation
http://www.portalelectricidad.es/fabricante/merlin-gerin-automaticos (http://www.portalelectricidad.es/fabricante/merlin-gerin-automaticos)
* PIA = Small Automatic Interrupt
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I am not really advocating a move to DC, just enjoying the conversation so far. Blindly sticking with old ideas will get us nowhere while change for the sake of change is not helpful either. The question is: With all the new tech and knowledge from the past 100 years, is AC outdated so bad that there is a real tangible benefit to such a massive change? Clearly there are some benefits and some drawbacks.
There is a demand towards incorporating renewables and making local storage easier. We also have many small gadgets that need energy. Whoever meets that demand will make money.
Other than that we already have time proven DC distribution onboard every vehicle, the outlets suck though.
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Do the DC fanboys here realize that a typical 16A household circuit breaker (of which you will have dozens in a typical distribution box for a family home) for 230V AC here is rated to reliably interrupt short-circuit currents of up to 10000A, has the size of a box of cigarettes and cost 2.95EUR retail.
That a PIA* (1 phase or 2 phase)cost 2.95€ will be from the china, because in Spain it costs over 12€ of the Merlin-Gerin corporation
http://www.portalelectricidad.es/fabricante/merlin-gerin-automaticos (http://www.portalelectricidad.es/fabricante/merlin-gerin-automaticos)
* PIA = Small Automatic Interrupt
That's because you're getting screwed, not because they're expensive.
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That is true about the isolation, and maybe an argument for a dedicated high power rail. Although 4 element resistive heater is not that big of a deal. For the connectors, there is no need to have all of them available on all outlets. Only the high power loads need the high power. Normal around the house outlets would be fine with only 4 wires
What a regression. Here in Germany we have one type of outlets at home. We use them for low power devices up to kettles, washing machines, dryers, (mobile) space heaters, etc. This is a flexibility I didn't want to give up.
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That is true about the isolation, and maybe an argument for a dedicated high power rail. Although 4 element resistive heater is not that big of a deal. For the connectors, there is no need to have all of them available on all outlets. Only the high power loads need the high power. Normal around the house outlets would be fine with only 4 wires
What a regression. Here in Germany we have one type of outlets at home. We use them for low power devices up to kettles, washing machines, dryers, (mobile) space heaters, etc. This is a flexibility I didn't want to give up.
A 30A dryer uses the same plug as a 10A kettle?
In the US, we have 15A, 20A 120v plugs in houses but mainly 15A. Then there are 30A, 40A 220v types for dryers and stoves. At least those are the common ones seen in houses. 3-4 different types.
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Do the DC fanboys here realize that a typical 16A household circuit breaker (of which you will have dozens in a typical distribution box for a family home) for 230V AC here is rated to reliably interrupt short-circuit currents of up to 10000A, has the size of a box of cigarettes and cost 2.95EUR retail.
That a PIA* (1 phase or 2 phase)cost 2.95€ will be from the china, because in Spain it costs over 12€ of the Merlin-Gerin corporation
Ripoff. I'd think Eaton a reasonable brand, and here you go:
http://www.pollin.de/shop/dt/Mjg5OTM3OTk-/Haustechnik/Installationsmaterial/Schaltgeraete/Leitungsschutzschalter_EATON_PXL_B16_1.html (http://www.pollin.de/shop/dt/Mjg5OTM3OTk-/Haustechnik/Installationsmaterial/Schaltgeraete/Leitungsschutzschalter_EATON_PXL_B16_1.html)
Retail 2.50 in a shop selling to private customers.
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A 30A dryer uses the same plug as a 10A kettle?
Yes dryers, kettles use the same plug. We have 3 types of plugs which fit into the same type of outlet: 10A/16A with PE, 10A/16A without PE and the smaller 2.5A europlug.
Our dryers at home don't need 30A because of 230V mains voltage.
edit: I didn't noticed that your 30A dryer sockets are 220V. So our dryers didn't have as much power available.
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They electric dryers here are on 220-240v at 30A. Maybe we are less patient and feel the need to blast our clothes with near thermonuclear heat. :-DD
In our daily life though, the vast majority of people are only seeing one type of plug - the NEMA 5-15 15A/120v for just about everything. The high power plugs for stoves or dryers are hidden and rarely noticed.
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They electric dryers here are on 220-240v at 30A. Maybe we are less patient and feel the need to blast our clothes with near thermonuclear heat. :-DD
This explains why dryer lint catching fire and burning down the house is so common ;D
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Back to the DC only powered home. IMHO a standard outlet should provide 2kW or more.
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Back to the DC only powered home. IMHO a standard outlet should provide 2kW or more.
Here in the US, we get 1875W from a normal outlet and it does not feel like a major limitation.
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They electric dryers here are on 220-240v at 30A
That bestly, you leave more profitable have a thripahsic conection
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Back to the DC only powered home. IMHO a standard outlet should provide 2kW or more.
Here in the US, we get 1875W from a normal outlet and it does not feel like a major limitation.
That's about 2kW. We have nominal 2.3kW (10A) continuously.
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Back to the DC only powered home. IMHO a standard outlet should provide 2kW or more.
How about this?
(http://i.imgur.com/TQRvcyK.png)
At 16A per phase it could provide 3000W total. Accepts any combination of loads. Just an idea.
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The downside would be load balancing, if all the loads were on a single leg that leg would be over loaded while the others idle.
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That would be a problem on an AC system but not as much here. The real problem would be the number of conductors.
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If you have 4 isolated supplies and only one of them is loaded with the all the things in the house, you would need supplies that are each capable of supplying the whole house = a huge waste. Some sort of smart management would be needed OR you would need 48v supplies that are 4x the needed size.
The load, ideally, would be reasonably distributed among the (4) 48v rails.
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Like we save energy by boiling kettles slower?
No by using low power kettles.
Energy is a function of power and time. Using a lower power kettle will use at least as much (probably more) energy as a higher power kettle.
I think something else DC fanatics are overlooking is infrastructure - for Australia at least (European homes are probably very similar) your typical circuit for 2.3kW socket outlets will be on a 20A circuit breaker ($20-30) with approx. 100 metres of 2.5mm2 twin core and earth cable ($100-150). By going from 230VAC to even 100VDC, you will need a 50A DC circuit breaker (unknown, at least triple the cost of AC C/B) and at least 10mm2 cable ($500). The cost difference is going to be similar for all circuits as well as your consumer mains, sub mains, etc. so for a new house, what might have been an $8000 invoice from the electrician is now going to be $40000-50000 simply due to extra cost of materials. Galvanic isolation and fault current protection are additional issues that have already been mention and will cost more on top of that. From there, the local distribution network is going to be nowhere near big enough, and neither is the transmission system feeding it, not to mention lots utilities actually generate power in AC so either massive rectification plants or new utilities would have to be built. Costs for that would run a long way into the billions for a place like Australia or Europe. Never going to happen.
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Whaaaa?
20A at 220v = 4.4Kw
100VDC at 50A = 5Kw
That sounds fanatic to me. In the USA, we use 120V at 15A for the vast majority of household needs. Only hairdryers and toasters max a circuit like that. Circuit protection is CHEAP whether its DC or AC.
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If you have 4 isolated supplies and only one of them is loaded with the all the things in the house, you would need supplies that are each capable of supplying the whole house = a huge waste. Some sort of smart management would be needed OR you would need 48v supplies that are 4x the needed size.
The load, ideally, would be reasonably distributed among the (4) 48v rails.
This reduces the number of conductors to 5 but still doesn't solve the load balancing issues.
(http://i.imgur.com/hGsivgv.png)
For load balancing the most sensible way would be to have one single phase. But then the voltage would have to be high.
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Or,
It is as good (or as dumb, can be seen either way really) - start with the DC equivalent of 110/220V really.
Since most of the household appliances are designed for 340V+ DC (in case of 220V AC).
Dispense with the input bridge rectifier (or keep it) and switch to DC grid.
As far as I can forsee (not too far really), the only major concern will be that of switchgear and circuit breakers. Since the rest of the electronics are designed to run off the rectified DC anyway.
[/size][size=78%]It is as good a start as any. [/size]
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In my proprietary system, I use bi-directional communication to determine the voltage/current that will be delivered. It takes all human decisions out, they just plug in and go. My design is for a specific commercial use, but the concept could work in a house. It would allow low-voltage/low power up to high voltage/high power to be automatically negotiated and managed while using existing 3 wires and a single plug.
It does buy a substantial safety benefit because all the lines are very low power until they are activated by a successful negotiation. When the system is active, it sends a heartbeat pulse and if that heartbeat is disturbed, the branch shuts off. I am not saying this is a reason to go DC, its just an existing device that I build that has some merit.
I like the safety component that makes it very difficult to get electrocuted - even if you stick a piece of metal into the connector.
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Whaaaa?
20A at 220v = 4.4Kw
100VDC at 50A = 5Kw
One circuit has more than one outlet on it (usually 10 or more), and miniature circuit breakers are generally only available in common sizes. 4.4kW at 100V is 44A, next biggest size of commonly available protective device is 50A. Smallest cable with a great enough CCC before taking into account derating factors is actually 16mm2 (I made a mistake before) and that is without considering fault loop resistance and voltage drop.
That sounds fanatic to me. In the USA, we use 120V at 15A for the vast majority of household needs. Only hairdryers and toasters max a circuit like that. Circuit protection is CHEAP whether its DC or AC.
As I've said above, that one circuit might have your hair dryer, hair straightener, toaster, TV, xbox, microwave oven, and PC all on the one circuit, all running at the same time. Putting a single outlet on a circuit is just silliness. In Australia we have a similar system to Europe, our general purpose outlet is 230V 10A, and that is what is used for >90% of domestic appliances. You might need a 15A or 20A outlet for welders, small lathes, etc. or large kitchen appliances for example, and generally those outlets have dedicated circuits. I suppose if an entirely new system was introduced you could get away with smaller general purpose outlets, but I (as well as probably a lot of people) really like the simplicity of only needing one type of outlet for most commonly used items
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Since most of the household appliances are designed for 340V+ DC (in case of 220V AC).
Dispense with the input bridge rectifier (or keep it) and switch to DC grid.
My central heating, dish washer, washing machine, refrigerator, fans, audio amp, turn tables, alarm clocks and most of my test gear is definitive AC only. That's way over 20000€ of devices. And probably the SMPS powered gear doesn't work on DC as well because the PFC doesn't like it.
That's the real problem for a transition to DC only. Backwards compatibility. It doesn't make sense to invest tens of thousands for the installation and new devices to save a few hundreds.
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My central heating, dish washer, washing machine, refrigerator, fans, audio amp, turn tables, alarm clocks and most of my test gear is definitive AC only. That's way over 20000€ of devices. And probably the SMPS powered gear doesn't work on DC as well because the PFC doesn't like it.
That's the real problem for a transition to DC only. Backwards compatibility. It doesn't make sense to invest tens of thousands for the installation and new devices to save a few hundreds.
There is little question that a transition would be costly. The only way it will be acceptable on a large scale is that is gradually changes over time. Even that would only happen if the general public saw a benefit - which most wont care.
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There is little question that a transition would be costly. The only way it will be acceptable on a large scale is that is gradually changes over time. Even that would only happen if the general public saw a benefit - which most wont care.
Is there real, big enough benefit? I doubt it. For some niches sure. But not general.
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Again, most of them (modernish in the last 5-7 years) are all inverter driven which means that you have AC-DC-AC conversion happening internally,
Dishwashers, AC, washing machine, refrigerators. The exceptions I see to it are fans.
PFC, I agree is a problem, but then again we could dispense with the PFC altogether if the transmission is DC. No more leading and lagging vectors, no real and apparent power.
As I said before, it is as good an idea as any (also as dumb).
Also, we are not really trading anything here in the name of better,faster,cheaper at all. We are just moving AC generation downstream from the power station to the Point of Load. Each type of transmission comes with it own pros and cons.
Quote from: sync on Today at 09:36:24 AM (https://www.eevblog.com/forum/index.php?topic=62516.msg860953#msg860953)
>Quote from: krish2487 on Today at 09:06:48 AM (https://www.eevblog.com/forum/index.php?topic=62516.msg860936#msg860936)Since most of the household appliances are designed for 340V+ DC (in case of 220V AC).
Dispense with the input bridge rectifier (or keep it) and switch to DC grid.
My central heating, dish washer, washing machine, refrigerator, fans, audio amp, turn tables, alarm clocks and most of my test gear is definitive AC only. That's way over 20000€ of devices. And probably the SMPS powered gear doesn't work on DC as well because the PFC doesn't like it.
That's the real problem for a transition to DC only. Backwards compatibility. It doesn't make sense to invest tens of thousands for the installation and new devices to save a few hundreds.
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In my proprietary system, I use bi-directional communication to determine the voltage/current that will be delivered. It takes all human decisions out, they just plug in and go. My design is for a specific commercial use, but the concept could work in a house.
This is also how EVs charge, J1772 and similar.
Once you've gone to this point, you might as well take it a bit further, like letting someone plug in a generator or PV panel and that, too will negotiate a voltage and current (and, obviously, a direction). This is part of what the idea of "nano grids" are about: http://nordman.lbl.gov/docs/icdcm2015nordmanLPD.pdf. (http://nordman.lbl.gov/docs/icdcm2015nordmanLPD.pdf.) Basically, letting loads and sources work out between each other what's going to happen over a circuit.
And once you have that negotiation, it becomes interesting to put network control on all the outlets, and maybe even allow devices to work cooperatively to maximize the use of a circuit. For example, if you plug in a blow dryer and a toaster on the same circuit, they will negotiate who gets to run and who doesn't. (That was a particularly bad example, as I don't think many people make toast in the bathroom, but some might dry their hair in the kitchen.)
But my point is that intelligence is nice, but it would be semi-tragic in my mind to come up with a ubiquitous intelligent power transfer negotiation standard that did not allow for more features.
Of course, I don't see how any of this militates for either DC or AC, as it can be done either way, though arbitrary voltage conversions for AC are a bit more expense (capital and efficiency costs).
In my mind, the decision for AC or DC is not so much about "what is best" but about "winners and losers." On a DC system devices with power supplies will save a few pennies by eliminating a diode bridge and some filtering (winners). Resistive heating devices won't care (neutral). Large induction motors will require VFDs to synthesize their local AC drive (losers). Whether such a conversion looks appealing to you has a lot to do with who you are and what your main loads are. And of course, if you make stuff that would need to be replaced, you probably would be *thrilled*. If you own stuff that needed to be replaced, much less thrilling.
That said, I'm not happy with the idea of low voltage DC like 48V. Any savings you make in devices and maybe conversion efficiency will be paid back in i2c losses in the wall and/or more copper.
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Again, most of them (modernish in the last 5-7 years) are all inverter driven which means that you have AC-DC-AC conversion happening internally,
Dishwashers, AC, washing machine, refrigerators. The exceptions I see to it are fans.
My dishwasher and washing machine are not that old but using 230V AC motors for the pumps. Also the relay for the heating element will destroyed by DC mains. And if they don't use a relay for it then they will using a triac.
Also, we are not really trading anything here in the name of better,faster,cheaper at all. We are just moving AC generation downstream from the power station to the Point of Load. Each type of transmission comes with it own pros and cons.
The generators on the big power plans are AC and I think they will stay AC. The same for wind and water energy. I think only PV is DC at the source. For long transmission lines DC makes sense. But that's the problem of the companies which runs them. They can do the calculation.
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Again, most of them (modernish in the last 5-7 years) are all inverter driven which means that you have AC-DC-AC conversion happening internally,
Dishwashers, AC, washing machine, refrigerators. The exceptions I see to it are fans.
My dishwasher and washing machine are not that old but using 230V AC motors for the pumps. Also the relay for the heating element will destroyed by DC mains. And if they don't use a relay for it then they will using a triac.
Yes, this is total rubbish. My late model dishwasher has 12VAC motors for all pumps. The last top loading washing machine also had an 120VAC induction motor. Our new front-loading washing machine has a VFD-driven motor, but the accompanying gas dryer does not.
I would not be surprised to see some high-end models using VFD driven motors, to be quieter or whatever, but it's def not mainstream in, in North America, at least.
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Large induction motors will require VFDs to synthesize their local AC drive (losers).
VFD do have some excellent benefits which is why they are so common now. My AC units have them, my previous business had VFD air compressors, CNC machines, etc. There is no inrush current as they slowly ramp to speed. They can also respond only the demand at that moment. For example, had a 20HP air compressor with an AC 3ph motor, it ran continuously because it was so hard on the motor and drive train to start/stop. When the air was not needed, it unloads the pump but keeps turning all day long.
That was replaced with a VFD based compressor that turned only as fast as was needed to keep up with demand. We saved something like $600/mo on electricity just from that change. My AC units are the same, distributed VFD blowers that only move what is needed which maintains a constant temp and is very easy on the mechanics and associated electric supply. Again, my washing machine is VFD as well. Dishwasher too.
It's not necessarily an argument for an all DC infrastructure, but more and more VFD's are coming to the most normal of appliances.
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My dishwasher and washing machine are not that old but using 230V AC motors for the pumps. Also the relay for the heating element will destroyed by DC mains. And if they don't use a relay for it then they will using a triac.
Yes, this is total rubbish.
Why is that rubbish?
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Like we save energy by boiling kettles slower?
No by using low power kettles.
Energy is a function of power and time. Using a lower power kettle will use at least as much (probably more) energy as a higher power kettle.
It will be more.
Using lower powered kettles isn't good thinking. Unless the kettle contains the water within a environment that is much better insulated than present designs, the longer heating time will result in greater losses through heat escaping into it's surrounds. A far, FAR better idea is to stop people boiling a litre of water to make a single cuppa.
I also have deep reservations about the conductor size required for any low voltage supply. Cost is the main one - but I also wonder how much fun it would be to wrangle in a domestic wall outlet.
As for the power negotiation scheme idea, to truly solve the power balancing issue, each outlet would ideally need to be individually wired back to the controller.
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My dishwasher and washing machine are not that old but using 230V AC motors for the pumps. Also the relay for the heating element will destroyed by DC mains. And if they don't use a relay for it then they will using a triac.
Yes, this is total rubbish.
Why is that rubbish?
Sorry, I must have quoted the wrong thing. What I was rejecting was previous poster's idea that modern appliances all are moving to inverter driven motors. Some are, most aren't. At least here.
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Large induction motors will require VFDs to synthesize their local AC drive (losers).
VFD do have some excellent benefits which is why they are so common now.
This is interesting. I suspect there is an inflection point where VFD becomes more attractive. For example, a motor that runs 24/7 and needs to adjust smoothly to varying loads: very attractive. A motor that runs for 20 minutes once a week (eg, household vacuum) crappy but cheap universal motor (another bad example as those will run on DC -- I'm a fountain of bad examples today) will remain attractive for awhile. But over time, as the controls become cheap, maybe the cutoff point will move to more and more "brushless dc" as they used to say.
The only VFD in my house at this time is our front-loading washer, which certainly does make good use of it, asking quite a bit of the motor: low and high speeds and everything in between, lots of torque at times, etc.
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The new heat pump dryers (in the US, of course) all seem to run on 120V.
I think that for electronics, pulsed DC at 170-340V would be a good compromise between AC and DC. It solves the arcing issue with DC and conversion from DC is much simpler. For large motor loads, PoL inverters (VFDs) are a good solution.
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Industrial 3 phase motors are highly efficient,need hardly any servicing,& are quite likely to remain the device of choice for heavy duty applications where fine speed control is not necessary.
Such motors may well be in use for 30 plus years,so many now in service have no need of replacement with totally new systems for decades..
Even if a motor fails,usually it is only the motor which needs to be replaced (or repaired)---all the other circuitry can be re-used.
Replacement with a VFD controlled motor,driven by a dedicated DC line makes no economic sense,either in direct costs.or in Environmental impact.
In use,losses in a single DC feed are also going to be greater than a 3 phase system,where resistive loss is distributed over 3 lines.
Our front loader washing machine probably has a VFD motor.
It cost more to run than the old top loader------not because of the motor control system,but because the idiots didn't include a hot water inlet to use cheap solar heated water.
The stupid thing uses a resistive element to heat the cold water.
It saves water,though!
Brainless design like this cast large doubts in my mind about the abilities of the current generation of appliance designers to address the possibilities of totally new household power supplies.
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Large induction motors will require VFDs to synthesize their local AC drive (losers).
VFD do have some excellent benefits which is why they are so common now.
This is interesting. I suspect there is an inflection point where VFD becomes more attractive. For example, a motor that runs 24/7 and needs to adjust smoothly to varying loads: very attractive. A motor that runs for 20 minutes once a week (eg, household vacuum) crappy but cheap universal motor (another bad example as those will run on DC -- I'm a fountain of bad examples today) will remain attractive for awhile. But over time, as the controls become cheap, maybe the cutoff point will move to more and more "brushless dc" as they used to say.
The only VFD in my house at this time is our front-loading washer, which certainly does make good use of it, asking quite a bit of the motor: low and high speeds and everything in between, lots of torque at times, etc.
The place where VFD motors have in fact,made inroads in home appliances is in just those that normally used "universal" motors.
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It cost more to run than the old top loader------not because of the motor control system,but because the idiots didn't include a hot water inlet to use cheap solar heated water.
The stupid thing uses a resistive element to heat the cold water.
It saves water,though!
That's done for the ability to reheat water and more reliably control temperature.
It is a bit of a downside on electricity costs though. Win some, lose some. I'd like to see BOTH - take hot water in and mix it, then reheat.
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It cost more to run than the old top loader------not because of the motor control system,but because the idiots didn't include a hot water inlet to use cheap solar heated water.
The stupid thing uses a resistive element to heat the cold water.
It saves water,though!
That's done for the ability to reheat water and more reliably control temperature.
It is a bit of a downside on electricity costs though. Win some, lose some. I'd like to see BOTH - take hot water in and mix it, then reheat.
No, it's done just be cheap,& because many countries don't have hot water on tap in the laundry.
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It cost more to run than the old top loader------not because of the motor control system,but because the idiots didn't include a hot water inlet to use cheap solar heated water.
The stupid thing uses a resistive element to heat the cold water.
It saves water,though!
That's done for the ability to reheat water and more reliably control temperature.
It is a bit of a downside on electricity costs though. Win some, lose some. I'd like to see BOTH - take hot water in and mix it, then reheat.
Install a thermostatic mixing valve and set it to whatever temperature is optimal for the detergent you're using.
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Motors below 5~10 HP can run perfectly fine from DC connected VFDs. In that sense there is little reason to retain AC in domestic settings. Actually a DC VFD could be even cheaper compared to an AC counterpart.
In industrial settings everyone is installing VFDs anyway because they like the control and the efficiency.
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A 30A dryer uses the same plug as a 10A kettle?
In the US, we have 15A, 20A 120v plugs in houses but mainly 15A. Then there are 30A, 40A 220v types for dryers and stoves. At least those are the common ones seen in houses. 3-4 different types.
Our dryers are not 30A. Keep in mind that we have 230V instead of only 115V, so to get the same wattage, we need less current. Stoves/ovens usually are connected to a 3-phase supply. Although the heating elements run on 230V, using 3-phase the load is shared amongs the phases (if more than one element is turned on).
Speaking of 3-phase, this another issue with the idea of a multi-line DC system. When you have a 3-phase supply, and connect eqal loads between all three phases and neutral, there is no current flowing on the neutral anymore. Only if there are unequal loads, a bit of current will flow there. Due to that the neutral conductor can have the same size as the phase conductors. On a DC system, you would need a much larger return conductor if you want to distribute several voltages. Or each rail gets it's own return.
Greetings,
Chris
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It cost more to run than the old top loader------not because of the motor control system,but because the idiots didn't include a hot water inlet to use cheap solar heated water.
The stupid thing uses a resistive element to heat the cold water.
It saves water,though!
That's done for the ability to reheat water and more reliably control temperature.
It is a bit of a downside on electricity costs though. Win some, lose some. I'd like to see BOTH - take hot water in and mix it, then reheat.
No, it's done just be cheap,& because many countries don't have hot water on tap in the laundry.
As I recall having no hot input also improves the water "star" rating, but thats all hidden behind paywalls around Australian standards so I cant check easily.
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I DO like to see distribution voltage become DC SELV for lighting, electronics, plus conventional AC 100~240V for motors and heating elements. AC lines are deeply buried in walls and semi-permanently connected to prevent shocking, while DC lines comes from sockets, poses virtually no hazards if properly fused.
Here the AC lines are buried in the wall for appearance. It's ok to put the cables on the wall. But people don't like to see them. It would be the same for DC lines. They will be buried too. It adds additional costs with very little gain. I see a niche for DC lighting in large non-domestic buildings. But not at home.
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DC would be better for lighting. Since LED's are the best when run on DC. No annoying flickering or tricky drivers pumping harmonics.
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DC would be better for lighting. Since LED's are the best when run on DC. No annoying flickering or tricky drivers pumping harmonics.
Sure. But people won't invest lot of money for a domestic lighting DC distribution. They just buying LED lamps which don't have that problem.
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Here the AC lines are buried in the wall for appearance. It's ok to put the cables on the wall. But people don't like to see them. It would be the same for DC lines. They will be buried too. It adds additional costs with very little gain. I see a niche for DC lighting in large non-domestic buildings. But not at home.
Thin film cable (similar to flex PCB type stuff) can be made the same color as the wall so it's nearly invisible. Great for adding lights where there isn't existing wiring. They use low voltage as the insulation is very thin.
One place where a "nanogrid" would be very easy to implement is a home network center. Most home network equipment already run on 12V so it's just a matter of wiring them to a 12V battery, then add a float charger and possibly some alternative energy sources.
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The old yellow lights bulbs are lone gone
No they're not. I do all my work - soldering, design or even reading a book in a workspace illuminated by a good old incandescent. If you prefer LEDs or CFLs - fine but don't force your flawed solutions onto others.
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eFuses are the same thing, solid state and easy. Much easier than the mechanical breakers we use today. At household and most commercial current levels, switching and managing DC is easy.
The wiring regulations mandate physical separation. eFuses will never be suitable - how would an electrician lock off such a device for safe isolation?
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The wiring regulations mandate physical separation. eFuses will never be suitable - how would an electrician lock off such a device for safe isolation?
Based on most of the sparks I've known - with a 4lb club hammer. :)
I'm kind of amazed that this whole thread has run so far without anyone saying "I2R losses" along with "Have you seen the proce of copper recently?".
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The old yellow lights bulbs are lone gone
No they're not. I do all my work - soldering, design or even reading a book in a workspace illuminated by a good old incandescent. If you prefer LEDs or CFLs - fine but don't force your flawed solutions onto others.
"old yellow lights bulbs" brings a picture to my mind of the sodium street lighting, not incandescent.
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eFuses are the same thing, solid state and easy. Much easier than the mechanical breakers we use today. At household and most commercial current levels, switching and managing DC is easy.
The wiring regulations mandate physical separation. eFuses will never be suitable - how would an electrician lock off such a device for safe isolation?
A fuse and a switch are two different things. There is no reason that a removable maintenance plug or switch be separate from the over current protection for the occasional task that requires a line to be guaranteed cold. Is it practical? Is it a step up from what we do today? Does not seem like it, but not for lack of technology, it's for lack of financial benefit to the consumer on a large scale. My comment earlier was simply to point out that current protection with DC is not a problem as some were saying about arcing contacts, etc.
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Technically one advantage of the AC grid was its ability to deliver 3 phase rotating field to motors.
It also delivers a 3-phase rotating field to generators. As I understand it, this is one of the major reasons that it is even possible to have the grid we have, where multiple power plants can hook together and work as a system.
I'd handwave something about "lead/lag", "frequency regulation" and "rotating masses" but I'd probably end up looking stupid. What I do know is that the grid does synchronize generation as well as transport electrons, and things get complicated when the frequency signal is not there.
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#1) DC Circuit protection is indeed extremely difficult. Especially on breakers. They have to be built to have an extreme opening force, must have heavy magnetic arc blowouts, and have to have heavy physical shielding. They also need to be inspected after any operation where they trip under load. High interrupting capacity fuses are widely used, but they are expensive to replace when they clear. Most high power DC fuses are actually multiple smaller fuses in parallel. Once you get over about 1200VDC, the physical size of a fuse becomes huge.
#2) DC Circuits are indeed very difficult to interrupt. Normal switches and contactors don't work, well, for very long. Contact wear and burn is absolutely a problem, again, only solvable by incorporating heavy magnetics, large blowout chambers, and extreme tensions on the springs for the contacts.
#3) For the equivalent RMS voltage, AC is safer from an electrocution point of view, as AC gives you 100 or 120 chances a second to remove your connection to it. DC does not. DC makes muscles sieze up. If you complete a DC circuit with your body, you are pretty much stuck there until the muscle fiber burns.
#4) Variable frequency drives are indeed used in many places, but not all. That would be incredibly cost in-effective. Here at my facilty, we have approximately 240 motors on variable frequency drives, only 20 of those are over 100 HP. Two are 1200HP, and the drive cost is astronomical. The other 1200 or so motors are direct start across the line. Fans, pumps, conveyors, mixers, feeders... There is absolutely no benefit to putting any of them on VFD's (except of course, to the VFD salesman).
#5) Yes, you cannot ignore the IR losses, and the commensurate heating of a DC facility supply. For that matter, we have motors that are across the line, and are 1500HP, 4160 VAC. Smooth, quiet starting, minimal demand on our on-site substation. They do draw almost 220A at 4160 3Ph on start, full load running current is about 160A. They run happily being fed with 00 copper cables (that is 9.26mm conductor diameter) . Let's see what we would need for DC, even being generous, and using a 1000VDC motor, with all of it's inherent commutation problems:... back of the envelope calculation... That would absolutely require a drive, or a complex staged starter, so I'll just consider full load running current... 1,118 Amps. So, I would need at least 4, 0000 (11.7mm dia) copper conductors per "Phase" to power the beast.
So, what is the advantage?