Posted on 11/20/2011 9:56:06 PM PST by neverdem
Thomas Edison and his direct current, or DC, technology lost the so-called War of the Currents to alternating current, or AC, in the 1890s after it became clear that AC was far more efficient at transmitting electricity over long distances.
Today, AC is still the standard for the electricity that comes out of our wall sockets. But DC is staging a roaring comeback in pockets of the electrical grid.
Alstom, ABB, Siemens and other conglomerates are erecting high-voltage DC grids to carry gigawatts of electricity from wind farms in remote places like western China and the North Sea to faraway cities. Companies like SAP and Facebook that operate huge data centers are using more DC to reduce waste heat. Panasonic is even talking about building eco-friendly homes that use direct current.
In a DC grid, electrons flow from a battery or power station to a home or appliance, and then continue to flow in a complete circuit back to the original source. In AC, electrons flow back and forth between generators and appliances in a precisely synchronized manner — imagine a set of interlocking canals where water continually surges back and forth but the water level at any given point stays constant.
Direct current was the electrical transmission technology when Edison started rolling out electric wires in the 19th century. Alternating current, which operated at higher voltages, was later championed by the Edison rivals Nikola Tesla and George Westinghouse.
The AC forces won when Tesla and Westinghouse figured out how to fine-tune AC transmission so that it required far fewer power plants and copper cable.
DC didn’t die, however.
AT&T adopted direct current for the phone system because of its inherent stability, which is part of the reason that landline phones often survive storms better than the electric grid...
(Excerpt) Read more at nytimes.com ...
I think org’s point was that the conductors on trunk circuits would have to be as big around as Rosie’s backside.
With vacuum tubes, we had (and still have) to remind ourselves that the EE’s current inside the tube goes from the cold thingie to the hot thingie, not the other way around.
I've wondered about that a bit. Do the line installers have current-adjusting resistors available at the C.O. when provisioning a particular dialup subscriber loop?
Seems to me I ran into at least 1800 ohms loop resistance on about a 4 mile leased pair. We had to calibrate the current for a remote-reading meter, and the current sender had enough adjustment range to juuuust barely, nearly, almost, bring the meter into cal.
Where is it written that if you're a EE you can't hit it with a big hammer?
Of course, in IBM field service, we had to call them "impact forming tools."
I meant that my pitiful knowledge of AC electricity is about tapped out at this simple circuit. Thankfully, I'm almost competent at certain other things.
P.S. Always glad to meet a Freeper who has a "1" as the first digit of their join year and a "7" as the last.
We just call them HAMMERS! The bigger the better. There is a lot of satisfaction and even good that can come from giving something a good BASH! I’ve thrown my wireless mouse on the floor with extreme prejudice at least three times (Logitech M505) parts fly all over the place and the dang thing revives and works better every time. Can’t say the same for my last PC laptop though. It did not survive the trip out the front window, across the porch and out into the yard. The MAC is faring somewhat better.
Electrons? You mean the thingies that go from positive to negative? Or is it the other way around? I can never remember for sure. Why is it that sometimes you hook up the neutral wire on 220 and other times you don’t? Is that important?
—”What good is it if you can’t hit it with a big hammer?”
When I was in the Air Force, I was an Airborne Command Post Communications Equipment Repairman.
We had an automatic telephone switching system on the EC-135C Looking Glass and E-4B Nightwatch aircraft. The MUX (multiplexer) rack used with the telephone system had four sections along the bottom with Frame Relay Switching units which consisted of not much more than a bunch of relays crammed into a metal box.
Sometimes the relays got stuck and would not drop out the telephone calls. When this would happen, guess what the inflight checklist’s recommended procedure for unsticking the relays was? Yup, take a big, rubber mallet and give the malfunctioning relay unit a good whack on the front cover!
Most of the time, that was more than enough to unstick the relays.
Of course, there were lots of hard stares and several freakouts from the flight crew and newbie officers whenever they caught sight of me whacking away at the MUX rack with a rubber mallet—especially since the equipment which was used to remotely launch nuclear missiles out of the ground silos was nearby! LOL!
Yup, good times.
Thanks for bringing back the memory.
Cheers
DC makes a comeback only when subsidized.
Step-down transformers that supply 240VAC from the transmission line have center tapped windings on the secondary. The center connection is the neutral, the outer connections supply 240VAC across them, but to the neutral, its 120VAC from each.
The neutral from the transformer is grounded to the earth at the base of the pole and also wired to the incoming electrical distribution box, where in turn, it is grounded once again through a copper coated rod into the earth. Due to the “grounding” of the neutral it is commonly known as “ground.”
In a single phase home there will be 3 large wires coming into it. If you open the distribution box you will see that the 2 of the large incoming wires are attached to the section of the distribution box that feeds the circuit breakers while the other large incoming wire is attached to the bare metal terminal block in the distribution box. That one is the neutral and the other 2 are the 240VAC inputs to the house.
You will notice in the distribution box that it has black, white and bare copper wires attached to breakers and terminal blocks. The bare copper wires are all attached to terminal blocks while the black wires are attached to circuit breakers. (NOTE: if the bare wires are aluminum, you have a potential a fire hazard in your wiring system.) The white wires are also attached to the same block as the bare copper wires. Seems strange about the copper and white wires being attached to the same terminal block doesn’t it? Well, the neutral, the copper wires and the white wires are physically the same...they are the “ground.”
Now, the “split” is explained. Remember, the 2 large incoming wires that are attached to the circuit breakers? If you were to measure across them you would read approximately 240VAC. If you were to read from each one to ground (neutral) you would read 120VAC in a balanced system.
Some very simple applications are listed:
Your clothes dryer is a perfect example of why a 3 terminal plug/receptacle is used for the 240VAC. The heating coils are attached to the 240VAC while the blower motor and drum turning motor (the same motor) use 120VAC from the split 240VAC to ground. If there were no ground in the wiring that could not be done.
The same thing applies to an electric stove. The heating coils and oven element is 240VAC while the electronics and clock are 120VAC.
In applications where only 240VAC are required the third wire (ground) is still required for safety purposes only.
Hope that explains a little about the power distribution system to a common house.
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