AC vs. DC

Discussion in 'General Electronics Chat' started by alva, Dec 14, 2005.

  1. alva

    Thread Starter Member

    Dec 14, 2005
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    Since being bit by an AC appliance many years ago (before 3 prong grounded) I never trust Ac power. I recently found Thomas Edison didn't like it either!
    Anyway since so many things are DC now and DC motors are more efficient than AC and appliances can be made for DC, do you think sometime in the future that
    power companies will use converters from AC to DC off the AC transmission lines and then use that for household power instead of AC?

    A 48v DC standard or thereabouts could be used, sure current would be a little more but I'd rather get a burned hand than have a few milliamps of AC that cause the heart to go into cardiac arrest! Anybody agree with me that household 120V AC must go!
     
  2. pebe

    AAC Fanatic!

    Oct 11, 2004
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    In my youth I lived in a town that was fed electricity at 220VDC. The town's supply was biphase - alternate houses being fed with a supply whose live lead was either +ve or -ve WRT ground. We had the -ve feed. Consequently we had a 'universal' radio with a 220V live chassis! Highly dangerous. DC shocks are worse than AC shocks because muscle spasms make you hold on instead of being thrown off.

    If someone wanted to use an AC-only radio, then it had to be fed by a Philips vibrator invertor. They were expensive and had a 3 month life expectancy so only the rich could use them. All modern equipment requires voltages other than 220V, so imagine the added cost and bulk of an invertor in every piece of equipment.

    The other consideration is the enormous wear and tear on switches because of the arcing caused by switching DC loads - anything inductive will produce a sizeable arc. The mains trip that we now use in lieu of a fuse would grow in size and expense.

    It will never happen. It's a no no.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    Yeah, old Thomas A. had lots of problems with AC. It happens that they had to do with profit rather than any actual advantages of DC. He went as far as electrocuting an elephant on stage to "prove" the dangers of AC. Nicola Tesla's AC motor just worked better. No brushes to wear out, for one thing.

    The fact that your house is fed by AC should tell you that AC is lots more flexible. Your utility doesn't have any continuing interest in putting one in Mr. Edison's eye. For instance, you can reduce the voltage with a transformer. With DC you have to use dropping resistors. The load voltage is dependant on the power draw, and does not regulate without circuitry(or lots of wasted power).
     
  4. alva

    Thread Starter Member

    Dec 14, 2005
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  5. pebe

    AAC Fanatic!

    Oct 11, 2004
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    I don't know about the medical tests and it may be that DC and AC currents have different effects on the heart under laboratory conditions. But we live in the real world where it is difficult to make comparable tests. Having received many shocks from AC supplies, including being loosely on the receiving end of a contact with 6.6Kv overheads, I can vouch for being thrown off!

    Dr. Frankenstein was long dead when the electricity supply was installed in my town at the turn of the last century. It was installed to provide power for tramcars. The fact that it was made available to the public was a spinoff which they welcomed. Anything was considered better than gas. It remained until the mid 1950s when the nation's electricity supply came under government control and was changed to AC.

    You mentioned 40VDC. Have you considered the cable sizes required? My cooker is fed from a 240V supply fused at 30A and I believe the incoming mains is fused at 100A. So the supply to the house would then need to be 600A. What sort of domestic circuit breaker could handle 600A DC?

    The points made about transformers by 'beenthere' are valid. There is no simpler way to change the incoming supply to the voltage you require.

    You say 'Of course electricity transmitted at long distances at AC would continue' but that is an concept that is already changing. In France, I believe, high voltage supplies are being carried across the country as DC because it is more efficient. But that's another story.
     
  6. Firestorm

    Senior Member

    Jan 24, 2005
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    This isn't a debate over which is better, just that each is used for different purposes. Have you ever touched an electric fence alva? If you press the fence to the ground you will hear the pops in AC. The reason it is AC and not DC is due to muscle tension. DC will only draw your grip tighter around the wire. Yes, both AC and DC are dangerous as are both 12 guages and .308's. They have different purposes. It is and will probly remain easier to transmit power in means of alternating current and switch it over inside the appliance. The DC theory is like communism, only good in theory.

    -fire
     
  7. buddyengineers

    Member

    Mar 19, 2005
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    Well, i dnt get this one right.., how come High Voltage DC transmission is more efficient than AC Transmission. Wont it let more power loss than our normal High Voltage AC Transmission. Kindly please elaborate on it.
    Rgds,
    Dilpreet



     
  8. pebe

    AAC Fanatic!

    Oct 11, 2004
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  9. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    Loss due to the cable resistance in transmission of equal DC and AC voltages are similar. AC transmission loss due to other things is a little bit higher than DC transmission, but it is not decidedly so. It is true that most people regard that for equal voltages DC is more dangerous to us than AC as pointed out by the postings above. But it is also true that with AC the shock current will be a little bit higher. It is however a moot point, because the transmission is largely governed by cost and power loss instead of safety.

    The main reason the transmission has to be through high voltage is the cable loss, it is just more economical to carry high voltage and low current than vice versa. Hence the very high voltage of long distance power transmission (on pylons). If we to lower the voltage the cable would have to be larger in diameter, increasing the cost and also the power loss would be greater (Ohm's Law).

    The reason AC is used is because it is easier to step AC voltage down or up. High power, high voltage transformers are relatively cheap to produce. If we to use DC instead, we have to convert the DC voltage through switching converters. These are way more expensive than AC transformers, do not even capable of handling the kind of power and voltages that are common on transformers and the efficiency is not that great.

    There are a few examples where DC transmissions are necessary. One of these is the channel link between England and France. The main reason cited is because the phase synchronisation between the two countries. It is just not possible to synchronise both power grids. DC transmission can be converted and synchronised easier.
     
  10. n9xv

    Senior Member

    Jan 18, 2005
    329
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    AC power transmission is much more efficient than DC primarily due to I^2*R power loss. For a given line loss resistance, the higher voltage at lower current is more efficient over a low voltage at high current. Transformers not only step down the voltage at the end user point but they also step up the current to meet the current requirements of the end user. In the US the pole mounted (or ground mounted) transformer steps down some 6900-Volts to the required 240-Volts for the residence. Thats a ratio of 28.75 : 1. Your 200-Amp service is derived from a source of 6900-Volts @ 7 or 8-Amps. Imagine the power loss that would occur if you tried to push 200-Amps down a line with a resistance of several hundered Ohms.

    DC would be just fine over relatively short distances. BTW, I understand the US automotive industry is looking to move to a common 48-Volt DC system in the future.
     
  11. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    One of the most dangerous things I've done as an electrician is to work around high-ampacity 48v supplies!! There are very strict protocols in place to keep any metal from accidently coming near the busses. Only uthorized & trained personel are allowed near the stuff. There are documented accounts of screwdrivers exploding due to the current going through them. Men have been permanantly blinded by such explosions. Other men have been killed.

    Give me a twenty-amp breaker, thank you. If a forlk gets dropped in the toaster, the breaker trips. If a hair-dryer gets knocked in the toilet, the GFI trips. As long as I don't stick my tongue in the outlet, I'll be just fine!


    As far as what electricity does to human bodies...

    http://www.emedicine.com/derm/topic859.htm
     
  12. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
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    I'll add my 2p to this discussion.

    As mentioned, for DC transmission, power loss on a transmission line is given by:

    P = I^2*R(line)

    Therefore to reduce losses there are two options: 10 reduce the line resistance or 2) reduce the current.

    1) To reduce the resistance would require a an increase in cross-sectional area of the transmission line - whihc incurs a cost penalty.

    2) For reducing the current:

    I = P(load)/V(load)

    To reduce the current while delivering the same amount of power requires an increase in the voltage:

    V(load) = V(source) - R(Line)*I

    So reducing the current would also reduce the voltage drop between the source and the load.

    So above is a mathematical analysis of why DC transmission has problems relating to power loss on the transmission line. There are other issue which make DC transmssions quite inefficient compared to AC transmission:
    - There is no easy way to step-up or down DC voltage, something which is relatively simple in AC transmissions.
    - DC generators can only produce at most a few hundred volts (this may not be the case now, if anyone knows any better please correct me on this one)
    - DC generators tend to require more maintenance han AC machines
    - DC motere tend to be more expensive and less robust

    The relative advantages of AC transmissions have been mentioned above, however the abilty to step-up and down voltages in the key to AC transmissions in the key factor. Higher voltages lead to a reduction in the I^2*R losses and also allow for the reduction in conductor size. There are alos stabilty benefits. There are disavdavntages, however, the transformers increase in cost, they require more insulation to prevent flashovers and the Corona losses (power losses due to V^2) increase.

    This may add to some of the comments above.

    Dave
     
  13. pebe

    AAC Fanatic!

    Oct 11, 2004
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    I assume you are referring to HV transmission. The line losses would be the same for AC as DC. But among the advantages put forward for DC, are these.

    1. The maximum voltage that can be used is limited by physical spacing of conductors in overhead wiring. For any given safe peak AC voltage, the same DC voltage would allow a 41% increase in power.
    2. For transmission within a cable, a given dielectric would allow a higher DC then AC voltage working (note that any non-electrolytic capacitor has a higher DC than AC rating).
     
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