DC vs AC?

[cessna152]

Is DC voltage more efficient than AC Voltage? Is there any reason left to stay with AC voltage? What are the pros/cons of AC/DC?

 

[Matthias99]

"efficiency" is a hard term to nail down, and you're using it very vaguely here.

Generally, on a small scale, DC power is easier to work with. Electronics, in particular, use pretty much 100% DC power (thus the need for AC-DC transformers or power supplies with most electronic devices).

However, it's hard to get very high-voltage DC power, and transmission tends to be quite lossy. AC power is easier to transmit over long distances, and it's far easier to build a stepup transformer for AC power, which then lets you transmit it over long distances with less power loss. There's a *lot* more to it, but that's a start. If you're at Rutgers, take an EE/physics class or two.

 

[Varun]

A rotating generator makes an AC sin wave. I would have to think that rotating generators are still the most common. You could rectify that to DC, but at a cost of power since the diodes would have to take some power.

Transformers are a very very easy way to step voltage up and down, and they only work with an AC wave.

As far as I know, AC electric motors are more efficient than DC ones (someone correct me if I am wrong here).

Inductors and Capacitors do not function the same with DC (Caps go to open and Inductors go to a short after 5 time constants)

Let's not forget about 3 Phase transmission.

Basically I think we need AC, at least for the time being.

 

[compudog]

All the comments here are correct. There are few instances in industry or industrial applications where DC is the power of choice. For example, on large plastic extrusion lines, DC motors are quite common though much less efficient than AC.

I see more and more applications of DC being replaced with AC as the AC drives become more capable. AC drives have the ability to now run full-torque at ZERO RPM without damage for short periods of time, this is something that was always well tolerated by DC motors.

I can purchase a two horspower DC-field wound motor and drive for about $800.00-$1,000.00 while I can get two, two horse AC motors and two AC drives for about the same amount.

 

[Cogman]

If I remember my history right, back in the 1800's DC was the power of choice as most electricity came from batteries, but the problem was, how do you get the electricity from say Idaho to Calafornia (even though not states of the time.) Edison and Tesla both fought over what power source should be used. Tesla believed that DC Power had to be used for any sort of common person to be able to use an electrical device. While Edison believe that AC would work best. Edison was right, AC could travel over long distances with very little power loss, while DC could only travel relitivly short distances before loosing all power. Out of this did come an interesting device, The Tesla coil. A device designed to send DC electricity wirelessly from one spot to another, and, if I remember right, had very little degration.

Anyways AC is the power outlet of choice now, and nobody thinks twice about it.

 

[jhu]

if we have superconducting wires, would the difference matter anymore?

 

[Cogman]

no It should not matter if the wire was a true superconductor. However, even if we did, the standards are in AC and it would seem pointless and a waist of money to most to switch.

 

[compudog]

It was actually George Westinghouse and not Tesla that pioneered the use of AC. Tesla patented the idea of AC motors, but Westinghouse bought up all the patents. Edison was the champion of DC. Edison and Westinghouse had great debates over which was the better method. IIRC, Edison demonstrated an electric chair using the more "lethal" AC voltage in an effort to convince people that DC was the way to go.

It all goes back to Ohms law. In order to transmit energy over great distances, you need a much higher voltage to overcome the resistance. In DC, the higher voltage would be unusable for something such as lights. AC voltages can be generated very high, transmitted over wires and then transformed to a usable voltage.

 

[Cogman]

opps. Thanks for the correction. I was thinking It was tesla because of the Tesla coil which I thought was used to transmit DC electricity wirelessly.

 

[compudog]

Wasn't meant as a correction to your post cogman just more information. It was an interesting period of history, 1860 through 1900 or so. Westinghouse, Edison, Tesla, Sprague, Oersted. Lots of neat stuff invented.

 

[Varun]

Edison was a great mind, but he was really stuck on the whole DC thing. It's too bad because it cost him his company.

 

[PowerEngineer]

As others have already noted, AC has several advantages over DC. These include the ability to build transformers (that allow you to step up volateg and so deliver more power over the same (current-limited) conductor) and simpler motors (where the AC voltage variation can induce torque on a rotor without requiring brushes).

In some special cases where you want to carry a lot of power (say >1000Mw) a long ways (>500 miles), then DC can be a better choice. This is because with DC you can essentially hold your voltage and current at the peak values your conductors and insulation will withstand 100% of the time, while with AC you reach peak values for only small fractions of each cycle (and even then the two waves aren't in phase). This means that a conductor can carry about twice the power using DC instead of AC (and with a lower loss percentage). In this sense, then, DC can be more efficient than AC. The downside for DC transmission is the high cost of the inverter and rectifier stations needed at the ends of the DC line (to integrate it into the AC tranmission grid).

 

[Mark R]

Originally posted by: PowerEngineerIn some special cases where you want to carry a lot of power (say >1000Mw) a long ways (>500 miles), then DC can be a better choice. This is because with DC you can essentially hold your voltage and current at the peak values your conductors and insulation will withstand 100% of the time, while with AC you reach peak values for only small fractions of each cycle (and even then the two waves aren't in phase). This means that a conductor can carry about twice the power using DC instead of AC (and with a lower loss percentage). In this sense, then, DC can be more efficient than AC. The downside for DC transmission is the high cost of the inverter and rectifier stations needed at the ends of the DC line (to integrate it into the AC tranmission grid).

An important point, which you beat me to, because I was distracted by trying to imagine what a 2000MW inverter looks like. I'm thinking that it might look a bit like the one in a UPS but with bigger thyristors.

 

[PowerEngineer]

Originally posted by: Mark R
I was distracted by trying to imagine what a 2000MW inverter looks like. I'm thinking that it might look a bit like the one in a UPS but with bigger thyristors.

Look here or here.

:beer:

 

[pm]

There was a book review that I read about in IEEE Spectrum several months ago that talked about Edison and Tesla and the fight over DC and AC power as a distribution system in the US. I can't recall the name of it. It would probably be interesting reading. Anybody know the name of the book?