A forum designed mainly for high school physics students in New South Wales, Australia.
kanny
Posts: 1
Joined: Sun May 20, 2007 11:56 am

well we all know the disadvantages of it...eg industrial scale inefficiency due to inability to be easily stepped up using a conventional transformer to minimalise power loss etc etc however whats the actual advantages of it? and similarly then whats the disadvantages of an AC generator?
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

- No good for power supply for electronics
- Not useful for electrochemistry, eg charging batteries
- The fields produce electronic noise
- Has zero potential twice per cycle

I can't think of any others, and the first two of these is overcome by rectification and smoothing and the last is overcome by three phase.

Joe
charithjperera
Posts: 8
Joined: Fri May 02, 2008 4:11 pm
Wouldn't AC power lines induce eddy currents in any metallic objects nearby because of the changing magnetic field?

This will cause power loss and make AC a bit more inefficient.

AC also travels closer to the surface of the conductor because of the skin effect, from P=I^2 R, this would mean more power loss from heat generation on the conductor.

In my opinion, DC would be better if it could just be inverted and transformed easily.
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney
Wouldn't AC power lines induce eddy currents in any metallic objects nearby because of the changing magnetic field?
True. And it also loses energy by radiation. But these are quite small compared to the loses in converting DC voltages.
AC also travels closer to the surface of the conductor because of the skin effect, from P=I^2 R, this would mean more power loss from heat generation on the conductor.
That is true for high frequencies. For that and the radiation reason, very low frequencies (50 or 60 Hz) are usually used.

The I^2R losses and the eddy currents are both minimised with AC by using very high voltages -- 220,000 volts is common. This is difficult with DC. New power circuitry make it possible to convert low DC voltages, but it's very difficult for high voltage. So DC is rarely used for power transmission.

Joe
Mini
Posts: 66
Joined: Sat Mar 06, 2010 10:33 am

I think, but I'm not 100% sure, that transmitting electricity using DC will be better for underground cables.
Mini
Posts: 66
Joined: Sat Mar 06, 2010 10:33 am

I thought of something interesting to note. The overhead transmission lines in Sydney (ie. CityRail) are 1500V DC.

I wonder how much the actual power losses due to this are.
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

I think, but I'm not 100% sure, that transmitting electricity using DC will be better for underground cables.
Around a cable carrying AC, there are varying magnetic and electric fields. These produce currents in nearby conductors and thus can lose energy. In air, such currents are very small.
I thought of something interesting to note. The overhead transmission lines in Sydney (ie. CityRail) are 1500V DC.
I wonder how much the actual power losses due to this are.
I don't know, but they would be less than if they had used a lower voltage for the same cables. If they had used say 300 V, the required current would be 5 times greater, so the ohmic heating 25 times greater.

Joe