As most consumer electronic devices now use switch-mode power supplies, and many AC electric motors can be replaced by DC motors, is it time to consider replacing AC domestic power by DC?
The main advantage would not be electrical, but one of safety. There can't be many who would argue that 230v DC is far less liable to cause injury and death than 230v 50/60Hz AC.
Distribution to the residence would still be AC, but then it would be rectified to DC. Power rectifiers aren't that expensive now, so the additional cost wouldn't be that high. There would probably no need for smoothing at that stage (but if that was necessary, it would not be cheap - smoothing caps which can deal with 50 or 100A ripple are expensive).
What other advantages would there be? Well for a start, you wouldn't need diodes at the front of all SMPS. There would be less electrical noise if power could be controlled without switching (or pulse modulation).
OK, I doubt that changing the domestic supply from AC to DC is even vaguely likely for residences which already exist, but what about future builds?
On a sunny day (Fri, 01 Apr 2011 10:05:45 +0100) it happened TE wrote in :
DC circuit breakers can make arcs that are difficult to stop. Some clocks use the mains frequency. Still many motors use AC. Triac controls would not work. Transformers are actually cool. I have heard the suggestion you made many times, even Edison believed it. But it sucks big time in practice. Not that it cannot be done, but it is not very practical.
Yup. A lot of us are still alive due to the decision to use lowish-voltage AC for house wiring. The Japanese might have it slightly better at 100V.
Cheers
Phil Hobbs
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1) A diode is a cheap and easy way to convert A.C. to D.C. when that is needed, there is no equivalent cheap device which converts the other way. If D.C. were supplied, the cost of conversion to A.C. when it was needed would be much higher than the combined cost saving of all the small rectifiers in consumer electronics.
2) Switching D.C., especially at high currents, needs bigger and more expensive switches than A.C.; if the load is inductive, they must be equipped with blow-out coils. Fuses for D.C. are also much bigger than their A.C. equivalents.
3) Small isolation transformers could not be used on D.C. except in SMPS, so power supplies for small items could become expensive.
4) Existing motors and other equipment would become redundant and would be scrapped long before the end of their useful life, causing a huge waste of resources.
The idea only makes sense (if at all) by taking a very narrow viewpoint, in the wider picture it would be a disaster.
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I've heard the claim that the increased current causes more deaths from fires than the electricutions saved. Now that the europeans are putting GFIs everywhere, the safety balance may be swinging towards
240. Best would be 240, but balanced to ground.
Geez, just got a call from The Brat. It's a beautiful day in SF, and we had a good month, so she decided we'll have a company BBQ. She's emailing me a shopping list, and directing me to go to Tower Market on my way in to get steaks and salmon. Kids these days!
It's fine until you want to run high power devices and you also need thicker wiring cable. Having said that, here in the uk, I run 115-0-115v to the lab for safety reasons and also to make it easy to run stateside kit that doesn't have psu taps at 240v...
As a survivor of two near-electrocutions in my misspent youth, a bit of extra copper is not a worry.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net
I used to use a variac for powering up suspect kit. I used the tiniest 1 or 2A type from RS, for "safety". Every few months I would short it out one way or another, which welded the wiper or blew the winding, resulting in another purchase. Now I have a huge 16A one, when that is shorted it just trips out at the house MCB box :)
Statistically, I read once, you are more likely to be killed with 120 volts than 240 volts... The reasoning behind that was that 240 volts more often kicked the person away while 120 allowed them to 'grab' more firmly and not be able to release.
But I'm not going to test that...
As to lower voltage, I think all houses should be wired at 1.5 volts maximum.
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Tim Wescott
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Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
Or those "wireless power" microwave gizmos that MIT was pushing so hard a few years ago.
How come MIT can come out with the power distribution equivalent of cold fusion and not get laughed off the continent?
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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
A lot of the time, these days, you create DC voltage then invert it to create AC again. The filter capacitors are bulky and can be expensive and limit the life of the equipment.
Very few supplies these days are linear. There is an issue with power transmission though- pole pigs cannot be easily replaced at an economical cost, nor can the larger distribution transformers.
Universal 3-phase would be pretty much the best of both worlds. A simple bridge and you have a fairly low ripple DC source, and you can use transformers to change the voltage for transmission. The problem is the 'last mile' to the consumers' home.
AFAIUI, Europe has more available 380VAC 3-phase power. It's not really much more dangerous than 220VAC (phase to neutral voltage).
200VAC 60Hz 3-phase would be a nice compromise for North America if we were re-engineering everything from scratch.
There were a few places in old-town Scottsdale that had three-phase power in residential areas. The very first house I bought, in 1964, has 3-phase, with a Goettl A/C unit that purred :-) ...Jim Thompson
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Remember: Once you go over the hill, you pick up speed
I thought that 50 or 60 Hz was a very good frequency to provoke heart fibrillation (at 100mA or so), and that you need about 5A to "reset" the heart to normal again. Problem with DC will be electrolysis in your body, also very bad, but in case of short duration it has less change on fibrillation.
On a sunny day (Fri, 01 Apr 2011 12:27:37 -0400) it happened PeterD wrote in :
I am not sure if that is true. Because if you are 'kicked away' or 'grab onto' depends on how you touch the live conductor. Grab it with you hand for example and the muscles will contract over it, If it touches you hand's muscles on the back side you may pull away. I have been hanging from the 220 AND the 240, with the 240 I had ground chassis in one hand and the live wire in the other, Being hardened by shocks over time, I did a quick self check, and noticed I could not move my arms but could still move my legs. So I went through my knees and jumped backwards, that broke the connection. In case of the 220 I had to ask somebody to pull the plug, but I was much younger then. Today I would try the jump thing again. You do not have a lot of time to make a decision, in the 220 case I had serious burns on my hand, In the 240 case I was much too fast to get burns, but a lot of current through the heart region probably does not let you last very long. Some people panic easily it seems, I sort of go into a cool calm and collected state in cases like that, I have that in traffic too, should have been dead some weeks ago, but did the most fantastic steering and came out without a scratch. I can still play it back in my mind, must have looked amazing to the bystanders :-) With electricity I always keep some basic rules, work with one hand if you think something is live. That did not work if some moron left the power on, like I had on a heli deck of a warship, I was much younger and believed the guy, so ALWAYS check, I was very very lucky then, I felt some tinkling on my nose when it was close to the metal of the equipment (my hands were in it adjusting some huge rheostat), that made me suspicious, we checked, power was still on. Probably rubber shoes and good paint isolated me enough. The guy I worked with almost passed out .... hehe he went all white, while it was ME who worked on the stuff. Destiny. Now let me tell you about that ball lightning in front of my open window. We were eye to eye so to speak, for a long time, it went down, exploded and melted an antenna there. Fascinating, what is ball lightning? Electron black hole ? What was is the guys name who calculate the possibility of an electron black hole ?Murat Ozer ? I remember him asking about how to test something for that on Usenet.
This is not a new idea, one proposed +/-750 V DC system
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The 230/400 Vac distribution is not very dangerous. Usually those that are killed by 230/400 V are Darwin award candidates :-).
The 20 kV MV distribution is much more likely to kill lorry and crane drives, when hitting the lines.
The simple 6 pulse rectifier (from 3 phase power) creates quite nasty current peaks close to the voltage peaks. 12 (and 18) pulse rectifiers do not cause so much problems to the feeding network, but this requires 6 (or 9) phases, but this would require extra windings in a transformer (delta/wye), thus it would make more sense to use the MV/LV distribution transformer and add some extra secondary windings to generate the extra phases.
I had a mispent youth playing with ex ww2 transmitter receivers with tube electronics and hundreds of ht volts. After you have been thrown across the room a couple of times, you learn a healthy respect for electricity that lasts a lifetime. The only really near miss (Age 20) blacked me out for a couple of minutes and pulled the heavy piece of kit off the bench on top of me, but fortunately pulled the power cord out as well.
The engineers of today don't get this 'education' and are perhaps more careless as a result. The golden rule: keep one hand behind your back when adjusting or measuring high voltage is applicable even today...
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