Why our homes are supplied with an AC not DC???

Thread Starter

adn07

Joined Sep 25, 2007
16
:rolleyes:this is a qustion that i allways look for an ansewear to it !!!!
why just we use DC , is the load have something realated to it??
any response would be appreciated .....
 

mik3

Joined Feb 4, 2008
4,846
:rolleyes:this is a qustion that i allways look for an ansewear to it !!!!
why just we use DC , is the load have something realated to it??
any response would be appreciated .....
In our homes we use AC because you can change the value of the voltage very easy with transformers. Its very important to step up the voltage (and step down the current, so power remains the same) generated from the power station and transmit it at this stepped up voltage to minimize losses. Then we have to step down the voltage down to safe levels to be able to use it with safety (this doesnt mean that it cant still kill you) at home. As i said this is achieved easily with transformers. With DC is more complicated and expensive to make this conversion in voltage levels thus we use AC.
 

S_lannan

Joined Jun 20, 2007
247
yes, the above is also true.

in china they have dc power lines apparently in some areas.

there is an australian magazine called silicon chip that has an article on High voltage dc power transmission this month.
 

Wendy

Joined Mar 24, 2008
21,848
This was a major debate at the beginning of wiring grids between Dr. Tesla and Thomas Edison. Edison had invented a lot of DC components and had a financial stake in using DC, but ultimately transformers was what make the difference.

Remember, efficient electronics wasn't around then, not even as a theoretical concept, but transformers were off the shelf tech.

Where it really makes a difference is long distances. I'm not sure of the figures, but they boost the voltage on high tension lines to really high voltages (440KV?) and send it down the wires. This is because the current drops by an equivalent ratio, and current is what heats up wires, and is waste. By boosting the voltage accordingly the transmission efficiencies go really up. Even in your home in the USA the input voltage is 220 VAC, and is usually dropped to 120 VAX at the pole.

Because the standards vary widely from place to place in the world you will see many members post their country of origin.
 

Thread Starter

adn07

Joined Sep 25, 2007
16
thanx guys for fast response ....all what you have said is fine,but
in my opinion i think that the reason behind using AC not DC ...because
simply we have different loads at home ,such as tv , Air conidition
referegrator , light ,and so forth ,this load take different voltage, so
we cant use DC ,because dc has constant value that will not turn on all these devices .
i hope that its true.........and thanx agine.....
 

blocco a spirale

Joined Jun 18, 2008
1,535
thanx guys for fast response ....all what you have said is fine,but
in my opinion i think that the reason behind using AC not DC ...because
simply we have different loads at home ,such as tv , Air conidition
referegrator , light ,and so forth ,this load take different voltage, so
we cant use DC ,because dc has constant value that will not turn on all these devices .
i hope that its true.........and thanx agine.....
Not quite. The advantages of AC generation and distribution outweigh those of DC. AC is easier to generate, distribute, step up, step down....

Before power stations, industries that required electrical power would generate their own and sell the surplus which in some cases would be DC. this was fine for motors and relays and lamps and heaters that could run happily on the same DC voltage but electronics require different voltages and transformers and AC are the easiest solution. Having said that, most switch-mode power-supplies, as found in most modern electronic equipment, would be fine with DC.
 

Wendy

Joined Mar 24, 2008
21,848
There is also the fact that a lot of electric motors run better off AC. A lot of these were invented by Dr. Tesla too.

The the step up and step down in voltages is the big reason, as has been mentioned, there simply wasn't any other way to do it back then (they didn't even tubes when electricity was starting to be used by homes).
 

SgtWookie

Joined Jul 17, 2007
22,201
There is also the fact that a lot of electric motors run better off AC.
Most AC motors won't run on DC.
A lot of these were invented by Dr. Tesla too.
Yep, at least the first few. I'm sure Thomas Edison was very upset by the fact that Tesla had created motors that ran just fine without brushes or commutators.

Edison was stunned when Tesla put on a huge light show at the 1893 World's Fair in Chicago, even though Edison had denied Tesla the use of his patented light bulb base just six months before.
Link: four of the 12 generators build for the 1893 World's Fair exposition:
http://www.neuronet.pitt.edu/~bogdan/tesla/chicago.htm
Link: Many more photos from that exposition, and other current events of the day.
http://www.teslasociety.com/columbia_expo2.htm [eta] Forgot to put in this link!
Keep in mind, most people of the time knew little to nothing about electric lamps; streetlights were powered by gas, and they used candles or natural gas lamps after the sun went down.

I found the photos of the naval review quaint. :)

The the step up and step down in voltages is the big reason, as has been mentioned, there simply wasn't any other way to do it back then (they didn't even have tubes when electricity was starting to be used by homes).
Absolutely true.

DC-DC converters have only recently become viable. AC transformers are still more efficient, but they require far more in the way of material, are heavy, bulky and expensive.

I don't see DC-DC converters replacing the AC power grid anytime in the near future; there is simply far too much infrastructure already in place. It would take much more than an act of Congress to effect a changeover to DC power.

Edison was a brilliant man - but boy oh boy, was he stubborn.

He would sit at the end of his dock all day fishing, with strict instructions that no-one was to bother him. To ensure that he really was not bothered, he wouldn't put a hook on his line, either. ;)
 
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Wendy

Joined Mar 24, 2008
21,848
Just curious, do you know what bulb base Tesla used? The Edison socket really is a universal standard as far as I know, even if there are other ways.

I probably should have said, there are better electric motors using AC.
 

SgtWookie

Joined Jul 17, 2007
22,201
Bill, I've spent the last hour trying to find the page on which I saw the photo a few weeks ago, to no avail.

But if I remember correctly, it's similar to a design used nowadays for halogen lamps; a couple of pins that lock into the base when twisted. If I run across the photo again, I'll let you know.
 

SgtWookie

Joined Jul 17, 2007
22,201
Thanks, Beenthere. The GU-10 was indeed the base that I was thinking of. However, I really wanted to find the photo of Tesla's actual bulb. I'll probably run across it again in the next few days.
 

Wendy

Joined Mar 24, 2008
21,848
Happens to me all the time. Tesla was better than Edison in his specialties, his understanding of electricity was light years ahead of most people of the time. Betcha Edison spent a lot of hours trying to figure out how Tesla's system violated his patents.
 

jerone

Joined Aug 30, 2008
15
When transmitting dc, i think this will create a higher power loss than ac transmission since its length matters,really matters.
Loads in our home is designed based on the available supply.
 
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subtech

Joined Nov 21, 2006
123
Some common transmission voltages in the U.S. are:

115KV
138KV
161KV
230KV
345KV
500KV
765KV

The voltage that a particular line operates at is normally determined by the amount of energy that needs to transmitted and the distance involved.
Large blocks of power are transmitted to substations where the voltage is usually stepped down to 2.4 to 34 KV again depending on the distances or amount of area
that the energy must be distributed to. As near the customer as practical, this distribution voltage is stepped down once again to a level that is required by the
customer. Common secondary voltages (used at homes or businesses)in the U.S. are:
120
208
240
277
416
480
600

Best wishes to all.
 

mik3

Joined Feb 4, 2008
4,846
When transmitting dc, i think this will create a higher power loss than ac transmission since its length matters,really matters.
Loads in our home is designed based on the available supply.
What do you mean its length matters?
 

Wendy

Joined Mar 24, 2008
21,848
The longer the wire, the higher the resistance. Decreasing the current by increasing the voltage compensates.

To Jerone, AC and DC are the same as far as how they react to resistance and power dissipation. The reason AC is used is ease of conversion between voltages.
 
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