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Thread: DC over AC.

  1. #1 DC over AC. 
    Forum Professor leohopkins's Avatar
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    Most overground trains within the london area and the underground run on 750-850 volt DC current as opposed to 25,000 volt AC current that other networks use. Any ideas why british rail networks in the south seem to favour DC current over AC ?


    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  3. #2  
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    The overhead trains run at 25KV this keeps the wire size down, since at these voltages only a few amps are needed. Were to to try that stunt with the rails it would arc everywhere especially when wet, frosty or snow covered. Conductor seperation for overhead 25KV is about 15 feet or more. Each way has it's own distinct advantages and pitfalls.


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    Sure. im not disputing voltage's. Im speaking of the delivery method, why they favour DC over AC delivery.

    I am going to ask now what voltage actually is. I know its "potential difference" but that term means nothing to me.

    Amps, I know is electrical "bandwidth", right ?
    But what is voltage ?
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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    The overhead way is slightly cheaper but far more prone to breakdowns, the rail method is more expensive yet far more reliable. It is a left over from when pre-beeching days when seperate rail companies existed, some chose one way other chose the other, when the rail network became British Rail I think the ratio was about 50/50 - each system was so large that it was more cost effective to keep both systems rather than modify 1000's of miles of track. in the underground 'overhead' was never an option not only are the tunnels too small but any mishaps would have been incredibly dangerous to sort out and due to the nature of the pantograph would almost certainly have resulted in a fire and/or explosion.

    In terms of Amps and volts 1 amp is 6.28*10^18 Electrons passing a fixed point in one second.

    Voltage is effectively the 'speed' at which they pass that point.
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    Understood.

    Can you explain to me what voltage is ?
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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    Yeah I just added it in for you.

    How easy was that!

    Imagin three rows of soldiers passing by marching, say 20 soldiers per second, if they were in a single line (still twenty per second) they'd be running, but both ways would represent the same amount of 'power'.

    So voltage times current = power.
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  8. #7  
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    Oh I see, it makes sense now. So would I be right in presuming that you could decrease the amps by adding a thinner wire and like-wise increasing amps, at the same time decreasing volts by using a thicker wire ?
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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    No if you have a set current flowing and just reduce the thickness of the wire, it will start to get warm as all the electrons are squeezed closer together and 'collide' with more atoms causing them to emit photons, keep on reducing the wire and it will burn out. The wire size is selected for the maximum current required to flow through it [in normal conditions]. The amount of insulation around the wire is selected to match the maximum operating voltage.

    SO the more current the thicker the wire, the higher the voltage the thicker the insulation.

    Your spark plugs have a very thick insulation yet need only a conductor less thick than a human hair (in practice it's thicker because of engine vibration) - The spark plug voltage is around 8KV - 20KV.

    Whereas your tv Mains lead has relatively thin insulation.
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    Oh I See.

    Couple of other questions....first one, sometimes when I switch the light off in our room on the light switch I see a spark. Is that "back EMF" that we spoke about a while back ?

    Also...I want to fit a dimmer switch to my light socket (which i have done before by turning the power off at the mains before hand) - As we live in a shared house, id rather not turn the power off, although ofcourse unadvisable, if I was to say attempt to fit the dimmer switch with the power still running, that would be okay as long as i didnt touch the copper part of the wire, yes ? And as for the screws inside the switch, id be undoing them using a plastic handled screwdriver so I will be safe too, yes? (eerring on the side of caution, I shall also stand on a thick block of polystyrene).

    My point being, I will have to touch the plastic part of the wire whilst it is still live and put one wire into the dimmer switch at a time. I figured it would be best to put the neutral wire in first then the live ?
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  11. #10  
    Cooking Something Good MacGyver1968's Avatar
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    Quote Originally Posted by leohopkins

    Amps, I know is electrical "bandwidth", right ?
    But what is voltage ?
    The way it was described to me in my first year of electronics school uses the "water pump" analogy. Just like a water pump creates pressure that pushes water down a hose, measured in p.s.i. (I don't know the metric eq.) voltage is the "pressure" that pushes the electricity down the wire. The more "pressure", the more electricity will flow down a given wire. Current (measured in amps) is the amount of electricity that flows through the wire in a given amount of time. In the water ex. it would be measured in liters per second. So a 100 psi pump (voltage) attached to a 10 cm hose (resistance) would yield 10 liters per second of flow (current). Increase the pressure with the same size hose, and the flow increases. Increase the size of the hose, (lowering resistance) and keep the pressure the same, and the flow increases. Does that make any sense?

    It's not an exact description, but it gives a basic idea....I'm sure others may elaborate...
    Fixin' shit that ain't broke.
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    NO! F&ck the water pump analogy it's crap, cut open your hose the water flows unrestricted, cut your mains cable - no flow...

    No not back emf, as the contacts break an arc forms which lengthens and eventually stops, - can't explain fully now -too late.

    Get a proper electrician.

    I would take the bulb out then even if I shorted the wires out no sparks, dont stand in a bucket of cold water on copper nails - dont' touch the copper part - Iv'e done it loads of times BUT I know what to expect - and how to react if I get a shock. Keep your left hand behind your back, then you won't get a shock across the heart region. - As I said get asn electrician :wink:
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  13. #12  
    Cooking Something Good MacGyver1968's Avatar
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    I didn't say it was a perfect example, MB...the hose is not a good example of resistance...but it does describe flow.
    Fixin' shit that ain't broke.
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  14. #13  
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    Quote Originally Posted by MacGyver1968
    Quote Originally Posted by leohopkins

    Amps, I know is electrical "bandwidth", right ?
    But what is voltage ?
    The way it was described to me in my first year of electronics school uses the "water pump" analogy. Just like a water pump creates pressure that pushes water down a hose, measured in p.s.i. (I don't know the metric eq.) voltage is the "pressure" that pushes the electricity down the wire. The more "pressure", the more electricity will flow down a given wire. Current (measured in amps) is the amount of electricity that flows through the wire in a given amount of time. In the water ex. it would be measured in liters per second. So a 100 psi pump (voltage) attached to a 10 cm hose (resistance) would yield 10 liters per second of flow (current). Increase the pressure with the same size hose, and the flow increases. Increase the size of the hose, (lowering resistance) and keep the pressure the same, and the flow increases. Does that make any sense?

    It's not an exact description, but it gives a basic idea....I'm sure others may elaborate...
    Not quite, because what you are saying is that amps is the amount of electricity per second that passes through the wire. If that were the case and voltage was pressure then by increasing the voltage you would automatically be increasing the amps ?!?

    Anyway......can anyone give me advise on the dimmer switch thing.
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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    read back I added it in again...

    Sorry mac your analogy is so far out of date nobody uses it, except bloody school teachers. There are more holes in it than in your average garden hose.

    As I said 1 Amp = 6.28*10^18 electrons passing a point in one second.
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  16. #15  
    Cooking Something Good MacGyver1968's Avatar
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    Quote Originally Posted by leohopkins
    If that were the case and voltage was pressure then by increasing the voltage you would automatically be increasing the amps ?!?
    Yes...as long as the resistance stays the same.

    Ohms law:
    Voltage=current * resistance

    Power (watts) = Voltage * current

    As for working on a hot circuit, it's not advisable. Don't y'all use 220v for household power? If you absolutely have to, I would use needle-nose pliers with insulated handles. You don't need to stand on anything as your floor is made of insulating material like wood and is not grounded. Wearing a pair of gloves would be a good idea. You should have a circuit breaker box somewhere in the house that would let you kill power to just that circuit.

    Back to the original question, why use DC over AC? That's maybe a question for MB.. I know main power lines use AC because over long distances the resistance of the wire causes a voltage drop, and with AC you can use a transformer to boost it back up.
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    Personally I'd get the wife to do it, tell her the juice is off, that it needs the delicate hands of a woman, then when she's jiggling about all over the place and can't let go, well I'd be in there like a rat up a drain-pipe! - Let her do all the movin around for a change....
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  18. #17  
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    LMAO !
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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    The London Underground is one of the few networks in the world that uses a four-rail system. The additional rail carries the electrical return that on third rail and overhead networks is provided by the running rails. On the London Underground a top-contact third rail is placed beside the track, energised at +420 V DC, and a top-contact fourth rail is located centrally between the running rails at -210 V DC, which combine to provide a traction voltage of 630 V DC

    http://en.wikipedia.org/wiki/Railway...ication_system


    Modern trains like TGVs in France use 3 phase AC motors. The 3 phase is electronically derived from the single phase 25 kV.
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