Alternating Current vs Direct Current

[GogglesPisano]

I prefer matter/anti-matter. Much cleaner. 500 lightyears per crytsal.

 

[ePilot22]

Go make crazy faces and wave your arms wildly in front of a TAZER toting traffic cop and let us know how you do.

I would imagine it would be something like this... KRUMPING.




eP.

 

[A Squared]

A Squared: You are scaring your compadres - Not to hack on you, but Tarp is exactly correct.

No, he is absolutely *not* correct.

AC power has 2 principal advantages:

1) Less efficiency loss in transmission (very significant over long distance) and

2) Smaller conductors required to transmit the same power (equals smaller wire or cable size, and smaller and lighter motors as compared to the DC equivalent)

No, sorry, you are every bit as mistaken as Tarp. Efficiency of transmissin and the use of smaller wires is absolutely not a characteristic of AC.

Here's what *IS* correct:

It is more efficient to transmit power (and you are able to use smaller wires) using higher *VOLTAGE* not "because it is AC". This effect has absolutely *nothing* to do with being AC and has everything to do with the voltage. The exact same is true with higher voltage DC. THat's the reason that many small airplane manufactureers have switched from 12 volt DC systems to 24 volt systems; Because it is more efficient to transmit the same power at higher voltages. If you need 120 watts at the wingtip (just for example) , you need 10 amp in 12 volt system, but only 5 amp in a 24 volt system. You can use smaller (therefore lighter) wire to transmit the same power with the 24 volt system.

The effect is completely independent of whether it's AC or DC. In fact, if you had an airplane which had 115 Volt DC power and 26 Volt AC power (unlikely arrangement, but it's a hypothetical situation) it would be much more efficient in terms of power loss and wire weight to transmit power to the high power items using the DC rather than AC.

Once more, in case you missed it the first five times: It has nothing to do with being AC.

This is why AC is used on larger aircraft, as well as for bulk power transmission (ie across the country). Precisely for the purpose of limiting heating losses (and transmission line sag), voltage for most transmission systems is 345000 volts AC (or even 510kV way out west).

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.

So why is AC used? As mentioned, it's primarily because it it's easier to change it's voltage. Some other reasons are because it's nicer to work with in instrumentation. It's easier to make an AC gyro which spins at a very stable speed than it is a DC gyro. DC motors tend to vary speed with voltage fluctuations more than AC motors. It's easier to make more accurate sensors and indicators with AC than DC. A capacitance fuel guage, for example requires AC. There are Capacitance guages on some DC planes, but the instrument itself will have to have a small ac power supply, often built into the unit.

So why aren't *all* planes AC powered? Because AC power is more difficult to make. Actually it's not, AC power is very easy to make, much easier than DC power. All rotating generating devices inherently produce AC. That's the only thing they *can* produce. In DC alternators and generators, that AC power is rectified, either using diodes or a commutator (which is essentially a mechanical rectifier) What *is* difficult to generate is AC power with a stable frequency. It requires a Constand Speed Drive for the generator, which is heavy, expensive and complex .... Or it can be made from DC which requires an inverter, which adds weight and expense, and loses energy.

SO for small airplanes, where the electrical power requirements are small, the benefits of AC don't outweigh the extra weight, cost and expense.

On larger airplanes, with higher power demands and complex electrical systems, the benefits of AC make it worth the added complexity to produce it.

 

[GogglesPisano]

And remember, when it comes to gauge needles: AC lies, DC dies.

 

[DaveJ]

Why 400 Hz for aircraft power?

Some of you might be wondering why the aircraft that use AC power use 400 Hz instead of the 60/50 Hz that is generated by land based power systems. The reason is to save weight. In order to change the voltage of an AC power system a transformer is used. A transformer has two coils of wire wrapped around an iron core. The voltage change is proportional to the ratio of the number of turns of wire in the two coils. A 2 to 1 turns ratio means a 2 to 1 voltage ratio.

The efficiency of a transformer depends on the amount of magnetic field that saturates each of the coils of wire. The magnetic field depends on how much iron is used in the transformer. More iron means a more efficient transformer. It turns out however, that if you use a higher frequency, you can get the same efficiency with less iron (and weight) than you could using a lower frequency current and a bigger iron core. So aircraft systems use 400 Hz power instead of 60 Hz power so that they can use smaller lighter transformers.

 

[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.