Alternating Current vs Direct Current

[h25b]

The whole reason for the introduction of AC power was to transport electricity over long distances.... ASqaured, read this...

http://www.teslasociety.com/ac.htm

or...

http://en.wikipedia.org/wiki/War_of_Currents

The push for AC power was a direct result of the inefficiencies of DC power transmission. Specifically, this all came along when they were developing the hydroelectric power plants at Niagra Falls...DC power requires a much higher current than AC to push it through the transmission lines. This higher current (otherwise referred to as amperage) yields a much higher resistance due to the higher heat that results in the lines. AC tranmission is of a much higher voltage but much lower current which yields lower power loss due to line resistance developed as heat build up.

In other words...

AC power is transmitted at higher voltage but lower amperage and equals less resistance.

DC power is transmitted at higher amperage yet lower voltage and equals more resistance. This is why if we had a DC power transmission system we'd need power stations everywhere. The transformers that we see mounted up on our electric poles are a simple way to simply change to the high voltage being transmitted in the main lines down to the lower voltage commonly used for household applications.

 

[A Squared]

The whole reason for the introduction of AC power was to transport electricity over long distances.... ASqaured, read this...

Right, both of the articles say exactly what I've been saying. It's more efficient to transmit high voltage electricity than low voltage electricity, and DC is extremely difficult to step up to higher voltages. I didn't read every word, but I don't beleive either of the articles says that the fact that the current alternates makes transmission more efficient (and if it does, it is wrong)

 

[h25b]

Right, both of the articles say exactly what I've been saying. It's more efficient to transmit high voltage electricity than low voltage electricity, and DC is extremely difficult to step up to higher voltages. I didn't read every word, but I don't beleive either of the articles says that the fact that the current alternates makes transmission more efficient (and if it does, it is wrong)

I think we may be missing each other's points and are both correct. In order to get DC power to travel at a length you must transmit it at a very high current (amperage) to achieve a usable amount of power at the other end. As a result, you suffer quite a bit more loss of power due to the increased resistance at the higher amperage (current). The higher the current, the more resistance, and hence more power lost over the course of its transmission. AC however can deliver a high voltage and requires lower current to do so and thus generates less resistance. You just have to end up adjusting the voltage with a transformer to get it to a useful voltage (which is very simple). The less resistance equates to less power loss from the heat that is a natural product of the resistance that goes along with amperage...

So if we're measuring how much power is lost along a given length of transmission line, AC power is clearly MORE efficient. Especially since resistance is the measure of efficiency most would be interested in.

Keep in mind when I speak of efficiency I speak of which form (AC or DC) can move a given amount of power a distance with less resistance.

 

[A Squared]

A small quiz:

Which is more efficient in transmitting power?

5 volts DC or 5 volts AC?

12 volts DC or 12 Volts AC?

115 Volts DC or 115 Volts AC?

If your answer for any of these was "the AC", you're wrong. Period.

If your answer for all of these comparisons was "the efficiency is the same", you are correct. Now, this leads us to the inevitable conclusion that the efficiency is a result of the voltage, not whether or not it alternates.

 

[DaveJ]

A Squared has got it right

h25b you're missing the point. It's not whether the current is AC or DC that matters. Low voltage/high current will have more losses than high voltage/low current regardless of whether it's alternating or direct current. AC is much easier to step up or down in voltage than DC and so is the more commonly used.

AC has some other advantages as well with respect to constant speed electric motors.

 

[h25b]

h25b you're missing the point. It's not whether the current is AC or DC that matters. Low voltage/high current will have more losses than high voltage/low current regardless of whether it's alternating or direct current. AC is much easier to step up or down in voltage than DC and so is the more commonly used.

AC has some other advantages as well with respect to constant speed electric motors.

Maybe I am confused. But I am speaking in terms of power transmission. If DC is as efficient to move along a given medium then you'd be saying that placing power stations every couple miles would be a good way to distribute electricity ??

Power is what you're trying to move from the power plant to the customer. So if you'd like to get X amount of power to the end of a power line AC does this more efficiently because you will not lose as much to resistance along its route. This is the whole point of using AC in the power grid.

 

[h25b]

Nope. There are a lot of reasons why AC power is used for transmission, Efficiency isn't one of them. A 510KV DC transmission line would be exactly as efficient as a 510KV AC transmission line.

I don't think this is true... To end up with 510KV at the end of the line you'd need to start with 510KV at a much higher amperage at the beginning of a given power line... To end up with that same 510KV at the end of an AC power line you don't have to do it at nearly as high of amperage. This is why they use AC instead of DC. If they used DC they'd have to have power plants along the way to keep the amperage high enough so you end up with the proper voltage at the end.

 

[mzaharis]

I don't think this is true... To end up with 510KV at the end of the line you'd need to start with 510KV at a much higher amperage at the beginning of a given power line... To end up with that same 510KV at the end of an AC power line you don't have to do it at nearly as high of amperage. This is why they use AC instead of DC. If they used DC they'd have to have power plants along the way to keep the amperage high enough so you end up with the proper voltage at the end.

The point is not that 510KV DC is less efficient than 510KV AC for the same amount of power (current X voltage). ASquared is correct - they are approximately as efficient (although I think there is a second order effect based on the fact that AC is sinusoidal, not square wave - it's not a big thing for this discussion, and if anything, may favor DC). It's not that you need power stations every 2 miles for 510KV DC - it's that you cannot easily convert power station output to 510KV DC, then step it down to 120V DC. Therefore, you're relegated to transmitting it at 120v, which IS very inefficient.

Go back to my earlier post. It covers the equation for resistive loss for transmission (Power lost = current squared * resistance), and the equation for the amount of electrical current (current * voltage). This is why transmission lines are very high-voltage, low current (for the amount of power they carry). The problem with DC is not that this relationship is any less true than it is for AC, but you cannot transform DC to a higher voltage, then transform it back down at the point of use. Therefore, you're left with transmitting it at low voltages (say, 120 volts). This is very inefficient, whether you do it with AC or DC.

So, to sum up:

1. High voltage, low current transmission is much more efficient than low voltage, high current, whether for AC or DC - about the same for the same voltage and current (therefore power)
2. You can easily change the voltage and current of AC, but not DC (transformer)
3. For most uses, due to 2., you're left with transmitting DC at (inefficient) low voltage, high current, while AC can be transmitted at (effecient) high voltage, low current
4. If you could go out and invent an efficient, cost-effective DC "Transformer", you could transmit DC as easily as AC.

 

[A Squared]

Maybe I am confused. But I am speaking in terms of power transmission. If DC is as efficient to move along a given medium then you'd be saying that placing power stations every couple miles would be a good way to distribute electricity ??

DC *is* as efficient as AC, when comparing similar voltages. It's just that it's D@mned difficult to produce 510KVolt DC without a thunderstorm. At any rate, mzharris said pretty much what I was going to say, perhaps better.

 

[h25b]

DC *is* as efficient as AC, when comparing similar voltages. It's just that it's D@mned difficult to produce 510KVolt DC without a thunderstorm. At any rate, mzharris said pretty much what I was going to say, perhaps better.

I think I'm on the same page now, the other guys description I think makes sense. Not trying to be difficult, but just want to understand clearly.