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USA converting back to Direct Current

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    #31
    Originally posted by dereckbc
    Y... Now with a solid state voltage regulator to step down means you have the same current on the input as the output. ...
    I don't think that this is an accurate statement. e/m.
    A kwh saved is 2 kwhs earned

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      #32
      Originally posted by tom baker
      A question came up on why we don't use DC for traffic signal cabinets, as most signal heads are DC leds. The answer was there is a significant corrosion issue with DC, and there is a voltage drop issue that you don't have with AC.
      Can somone elaborate on this please (the corrosion issue)? I have never heard of it. The closest thing that I have seen is that they actually put a small DC potential on long distnace metal gas pipes to prevent or slow down corrosion. The voltage drop is probably a function of low operating voltage of these lights and not AC/DC issue. Anytime you tranmit through conductors at a low voltage, you incure higher voltage drops due to higher required current. e/m.
      Last edited by Energy-Miser; 12-18-07, 06:36 PM.
      A kwh saved is 2 kwhs earned

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        #33
        Originally posted by dereckbc
        ... I will say the VD problem is more pronounced with AC because of the added reactance in addition to resistance. Could it be the lower voltage natue of traffic light bulb reuirements? I can then see VD as a problem as with the Telecom industry using 48 and 24 VDC systems. We have to upsize conductors to compensate for VD.
        Other than the added reactance, AC faces slightly higher ohmic resistance as well due to the skin effect. This is more relevant for higher frequency signals, but even at 60 hz it still produces measurable change in the effective resistance of the conductor. The reason for higher votage drop at 48 and 24 volt DC is that at low voltage you need to pump higher currents through to transmit the needed power (compared to when operating at a higher voltge). This higher current is the reason for the higher voltage drop (IR), and not the fact that you are using DC. e/m
        A kwh saved is 2 kwhs earned

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          #34
          Originally posted by Energy-Miser
          I don't think that this is an accurate statement. e/m.
          It is 100% accurate. A series voltage regulator is nothing more than a electronic controled varible series resistor.

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            #35
            Originally posted by Energy-Miser
            Can somone elaborate on this please (the corrosion issue)? I have never heard of it. The closest thing that I have seen is that they actually put a small DC potential on long distnace metal gas pipes to prevent or slow down corrosion. The voltage drop is probably a function of low operating voltage of these lights and not AC/DC issue. Anytime you tranmit through conductors at a low voltage, you incure higher voltage drops due to higher required current. e/m.
            It is called electrolysis, open the hood of your car and look at the battery terminals for an example.

            What you are referring too in pipe lines is called Cathodic Protection system. Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell.
            It is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect steel, water/fuel pipelines and storage tanks; steel pier piles, ships, offshore oil platforms and onshore oil well casings.
            Reverse the polarity and the pipes or whatever is being protected will be gone in short order. Telephone companies used the same thing when the decided to reference the positive polarity to ground. The reason was the lead sheathed cables leaving the central office. If they grounded the negative polarity the lead sheath would be gone within a year. With a positive reference the cable actually grow more lead sheath.
            It is the same principle as electro-plating. You take a carbon or copper cathode and a gold anode, submerge it, run a DC current and you get a copper clad gold cathode.

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              #36
              Originally posted by dereckbc
              It is called electrolysis, open the hood of your car and look at the battery terminals for an example.

              What you are referring too in pipe lines is called Cathodic Protection system. Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell.
              It is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect steel, water/fuel pipelines and storage tanks; steel pier piles, ships, offshore oil platforms and onshore oil well casings.
              Reverse the polarity and the pipes or whatever is being protected will be gone in short order. Telephone companies used the same thing when the decided to reference the positive polarity to ground. The reason was the lead sheathed cables leaving the central office. If they grounded the negative polarity the lead sheath would be gone within a year. With a positive reference the cable actually grow more lead sheath.
              It is the same principle as electro-plating. You take a carbon or copper cathode and a gold anode, submerge it, run a DC current and you get a copper clad gold cathode.
              Thanks yes cathodic protection, I could not think of it for the life of me. Very intersting use of electricity. Thanks, e/m.
              A kwh saved is 2 kwhs earned

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                #37
                Originally posted by dereckbc
                It is 100% accurate. A series voltage regulator is nothing more than a electronic controled varible series resistor.
                I was referring to the statement that input and output currents have to have the same mangnitude. They may be in a specific case, but do not have to, depending on the load. e/m.
                A kwh saved is 2 kwhs earned

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                  #38
                  Originally posted by Energy-Miser
                  I was referring to the statement that input and output currents have to have the same mangnitude. They may be in a specific case, but do not have to, depending on the load. e/m.
                  That is exactly what I am referring too. So let me clarify what we are talking about; A DC-DC converter stepping down in voltage using solid-state electronics. All those devices are serial or series circuits meaning Current In = Current Out, well actually the input current is a little higher than output to power the control electronics plus source the load current. There is just no way around the physics.

                  This is exactly why switch mode rectifiers and converters were conceived to get around the inefficiency of linear DC power supplies and rectifiers or DC>AC>DC. Switch mode also has some other benefits such as size and weight, but efficiency is the main driving force. Therein lays the magic of AC because it is very easy, economical, and efficient to step-up or step-down voltages using a very simple transformer.

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                    #39
                    Originally posted by dereckbc
                    What you might also be overlooking even if you do have something like Solar PV, you still have to be connected to the grid. Solar PV systems cannot power high-wattage items like dryers, HVAC, ovens, your blow dryer, etc. Well I guess you could if you are foolish enough to spend 6 or 7 figures on the system and had a acre or two to devote to it.
                    Well, yes and no, IMO. Provided a decent battery storage and a strong inverter, and efficient appliances, the everyday comforts can be attained without a grid connection.

                    I wired a house up in the mountains, miles from grid power, where they had no other choice with their desired site. I don't know if the system reached six figures, but I know it wasn't cheap.

                    Despite more and more appliances available for 12VDC for RV use, the designer of the system (the homeowner) opted to stay with a 120/240VAC system throughout the house, with a scant three 12VDC receptacles for certain specific appliances.

                    I felt the system would have been more sound if they had left the lighting DC, circumventing the inverter - but in retrospect, I am glad I didn't have to think about the voltage drop that would have been present with only 12 volts to begin with. There were enough headaches in that house without adding to it (it was also a SIP wall home).

                    When it came to running the larger motor loads, they "made hay when the sun was shining", literally: The water well pump only ran when he went to the basement and manually turned it on, to pump into the cistern, on sunny days. He also relies on a propane generator that the inverter can activate or he can manually as well.

                    I remember clucking my tongue at the dryer, but I don't remember what it was. It was not particularly efficient, IIRC, it was more of a nod to the extravagance of the other appliances of the home.

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                      #40
                      Originally posted by georgestolz
                      I wired a house up in the mountains, miles from grid power, where they had no other choice with their desired site. I don't know if the system reached six figures, but I know it wasn't cheap.
                      George that is kind of my whole point about Solar PV, it is a niche application where either grid power is not available or prohibitively expensive to bring in.

                      I have designed and installed about 6 of the systems in remote cell radio sites in TX and NM. These systems only powered a 250 watt load, (radio operating continuously 24x7x365). There had to be enough panels to be able to supply the load and charge the batteries in 3-hours to be able to carry through the remaining 21 hours. The system has to be designed for worst case which happens to be short winter days. All the systems ran around $90K, and there was no inverter involved because the radio operates on 24 VDC. The batteries alone are $30K and that does not include the installation. Also a LP generator had to be used for those cloudy days.

                      So for this small application it took the same size or number of panels being touted for a full size home of 3 KW in panel output during peak sun.

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                        #41
                        so maybe this thread should be renamed "US Not Convertig back to Direct Current"

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                          #42
                          so maybe this thread should be renamed "US Not Convertig back to Direct Current"
                          That's what I said early on.

                          the idea of an Energy Star home on solar is admirable, but expensive and impracticle unless it is remote and not serviced by the utility grid.
                          Living in the sunshine, staying contented most of the time.

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                            #43
                            Originally posted by tom baker
                            A question came up on why we don't use DC for traffic signal cabinets, as most signal heads are DC leds. The answer was there is a significant corrosion issue with DC, and there is a voltage drop issue that you don't have with AC.
                            But if you had LEDs would you not end up only using half the AC waveform? In theory you'd only have current flowing one direction, though one head might be hot > neutral and another might be neutral > hot.

                            Of course now someone is going to tell me that LED signal heads employ bridge rectifiers to avoid this issue.
                            "The Doppler effect means red lights will appear to be green if you drive fast enough..."
                            1) Wait for disaster 2) Determine it's not your fault 3) Blackmail the culprit 4) Profit

                            Caution: Classless when exhausted from a tough day at work, may attempt to compensate with poor humor and Christmas cheer...

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                              #44
                              Originally posted by Rampage_Rick
                              Of course now someone is going to tell me that LED signal heads employ bridge rectifiers to avoid this issue.
                              Either that, or front-to-back pairs.
                              Master Electrician
                              Electrical Contractor
                              Richmond, VA

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                                #45
                                This link talks about a city in NY(I believe to be Buffalo) that has been supplied with direct current for 125 years, how did they deal with corrosion? Also alot of you are only considering PV as the sole sourse, wind energy will charge a system when the it cloudy or at night when the sun may not. Also consider natural capacitors, thermal, hydro, what if every time you flushed the toliet,or took a shower you were generating electricity. I think that one day in the very near future it will all come together and be reality a system that is is independant even if it uses the grid as a form of redundancy, AC or DC or both

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