What are the advantages of alternating current over direct current for long range transmission?

Question

Also, what is meant by frequency of alternating current?

Answer 1

Kia ora
The advantage of AC over DC is quite simple. It has to do with the size of the current you need, and the fact that you cannot transform DC but you can transform AC.
Say you had a house that you wanted to supply with DC (over long cables) with 240V, 20A.
That's 4800W of power since P=VI.
A reasonable sort of cable might have a resistance of 5Ω over a distance of several km.
Now using DC at 20A current, the Voltage loss would be found using V=IR=20*5=100V.
So to compensate for this, you would have to generate at 240+100=340V. The generated power is therefore P=VI=340*20=6800W. So that's a 4800/6800 = 70% efficiency.
Now let's say you use AC. You use a 100:1 transformer to step down the Voltage to the house from the transmission lines and a 1:100 transformer to step-up the Voltage from the generator to the transmission lines.
Remember the transformer formula: Ns/Np=Vs/Vp=Ip/Is.
Start from the house: getting 240V at the house means you need 24,000V (24kV) from the transmission lines. Increasing the Voltage means a reduction in current: you would only need 1/100 of the current (0.20A) in the lines. That means the Voltage loss across the lines is V=IR=0.20*5=1V.
That means you need 24001V in the secondary coil of the step-up transformer from the power supply.
That means you need 240.01V at 20A from the generator.
In terms of power, that's P=VI=240.01*20=4800.2W.
That's an efficiency of 4800/4800.2=99.996% efficiency.
It may help to draw a circuit diagram, but this is the correct answer to your question!

You also asked about frequency - this is just the number of AC cycles per second.
AC is not necessarily sinusoidal, BTW!

Answer 2

Alternating current can be converted into direct current using a transformer and it experiences minimum loss due to transmission.
A/C changes it polarity many times in a second. This is called frequency of the alternating current.

Answer 3

AC current maintains power over much greater distances than DC, due to the nonideal nature of transmission lines and long conductors. Alternating current means that the voltage (and current) are alternating between two extrema sinusoidally. The frequency of alternating current refers to the frequency of the voltage wave. Household AC current operates at 60Hz, meaning it alternates between maximum and minimum voltage 60 times every second.

Answer 4

Current is the flow of electrons.(charges). If this rate is at one coulomb / second, it is said to be "one ampere". This flow of current takes place because of the difference in potential.- voltage. If the current flows only in one direction and the rate of flow is steady such a current is called "Direct Current". In
another type of current, the rate of flow of charges varies from zero to maximum and again from maximum to zero. This process continues. and the direction of the current periodically changes- alters. This type of current is called "Alternating Current". In the transmission of the current , some heat energy is dissipated due to the resistance of the conductor. In the long distance transmission the resistance of the conductor will be very high. If the current is also very high a lot of heat energy will be wasted. In the long distance transmission we can not avoid resistance. The only possibility is to reduce current. This can be done only with AC, using transformers. This process is called "Stepping Up" or "Stepping Down". Keeping the power constant, if the voltage is increased , the current will be reduced. ( P = Vx I ). As the current is reduced the power loss can be minimised.
The alternating current produced at the generating station is stepped up(voltage) to reduce the current. This high tension (HT) voltage is carried on very tall HT towers (to avoid danger) At the consumers end it again stepped down to the required voltage (220 Volts / 110 volts). This transferring is done by the phenomenon called "Mutual Induction" which can be done only with AC (not with DC)
The number of change of direction in one second.is called the frequency. Its unit is Hz (hertz).

Answer 5

I asked this exact question in July:
"DC vs AC for long-distance power transmission"
http://answers.yahoo.com/question/index?qid=20070722071916AAqggSd
It is not a trivial question and depends on line voltage and line length.

See the best answer by Steve W.
You could inbox Steve W before your asking of this question closes to see if he wants to contribute the power transfer number data he mentioned previously.

As to frequency, yes f is the number of times per second that the AC voltage (and thus current) change from +ve to -ve and back to +ve (units: Hertz).
f=60Hz in North America and most of Latin America, also Saudi Arabia, and 50Hz in Europe, Australia and most of Africa and Asia. Japan is the only country in the world to split between 50 and 60Hz (a legacy of WWII).
[see Wikipedia map and table link at bottom]


You will also see circular frequency ω = 2πf (units: radians) used in formulas.
They're just two ways of looking at the same quantity.

Answer 6

frequency of an alternating current is that number of times the direction of current changes in one second.

Answer 7

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Answer 8

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