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February 26, 2009

AC vs. DC – The Westinghouse / Edison War Continues...

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AC High Voltage Power Lines Did you know if Edison had his way, all generation and transmission of electrical power including the outlets in your house would provide direct current (DC) instead of alternating current (AC) that we have today?  Around the turn of the 20th century, Nikola Tesla invented alternating current generation, transmission and AC induction motors.  He then licensed his patents to George Westinghouse and the war with Edison began.  Edison went as far as electrocuting animals with AC power to show how lethal it was compared to direct current. The fact is ANY electrical current can be fatal. It does take more current to place your heart into fibrillation with DC than AC (around 60 milliamps for line power AC, and 300 to 500 milliamps for DC). Above 200 milliamps muscles contract so violently, the heart cannot pump at all...  Thus the reason you should always throw off the circuit breaker when working on an electrical project... I do (well, most of the time).

We all know that Tesla and Westinghouse won the battle. AC power has the advantage of easily being "transformed" to higher and lower voltages allowing transmission over vast distances.  Additionally, AC power propagates down a wire with lower loss than direct current.  DC power suffers seriously from Ohm’s Law (R = V / I where "R" is resistance in ohms of the wire, "V" is the voltage drop across the length of the wire in volts, and "I" is current flowing through the wire in amperes).  To calculate the power lost for DC power due to the resistance of a wire, you simply use ohms law plus the power equation (P = I * V) and find P = I^2 * R where P is power in watts.  If you consider a transmission line carrying DC power with a current of 10,000 amperes and a transmission resistance of only 0.1 ohm, you will be losing 10 million watts of power! Also, there would be a voltage loss (a drop in voltage) of over 1,000 volts from one end to the other. Depending on the length of the wire it will either get warm, catch on fire or explode! Since it was known that transmission losses would be much higher than zero ohms (unless the wires were made from super conducting materials), DC transmission was considered impractical and abandoned. But interestingly, the battle still rages on in pockets of our industry.

There are complexities with AC power namely maintaining the correct frequency (50 or 60 Hertz depending on your country) and phase synchronization.  When generators are brought on-line, they must exactly match the phase and frequency of the "grid" otherwise "seriously bad things happen".  Consider what would occur if a 100 megawatt generator was switched into the grid with as little as 1 degree of phase difference between the generator and the grid. The phase angle of 1 degree at the zero crossing (the point where the sine wave power goes to zero before reversing) would be equal to a power loss of over 1.74 megawatts! Well, in reality the power wouldn’t be lost... it would show up somewhere you wouldn’t want it to -  like a high voltage transmission transformer (i.e. imagine a large boom followed by much panic). That’s why our transmission grids have safeguards - like high power circuit breakers the size of automobiles. There are other problems with large distributed networks that span a nation - the phase of the power will be different along the grid and there is always the issue of Power Factor.

With all the problems associated with AC power, our modern world runs on it.  What’s interesting is that in most homes, the electronics (including your PC) immediately turn the AC power into high voltage DC and then using a switching power supply convert the power into lower DC voltages required by the system.  Most electronic subsystems run on DC voltages that range from less than 1 volt to around 48 volts.  There are losses with the conversion from one DC voltage to another, but most designs can provide about 80% efficiency with many above 90%.  To learn more about switching power supplies, go check out National’s Analog University tutorial on switching power.  Also check out their WEBENCH tools which allows you to design a complete switching power supply on-line.

Another reason for converting to DC is the ever increasing need for alternative energy sources such as wind and solar. For instance, photovoltaic panels used for solar installations supply DC power which must then be converted to AC. As LED lighting begins to overtake the traditional incandescent bulbs and CFLs, they will require direct current.  This again is supplied by switching power supplies that convert the power into a constant level direct current for the LEDs. 

But this begs the question, "what about our existing infrastructure?"  I doubt anyone would say, "sure, come on over and tear up my entire house and rewire it for DC power."  Just the issue with appliances is enough to stall any initiative.  However, a dual power system might actually have some merit.  For those systems that can benefit from DC power (such as charging your electric vehicle’s batteries), making a DC gateway into the home might provide some benefits.  You would have one very efficient DC power supply that would reduce the AC line current to around 48 volts DC.  Then, any appliance or electronics that would require DC could start at the 48 volt point and easily convert it to what ever the system requires.

There is a silent movement to move back to DC power for some of the above reasons at least at the final destination. I seriously doubt that Edison will finally win the war which is pretty much over at this point. But as applications for direct current emerge in the home a master DC home gateway may one day show up in your garage.  Something to think about... till next time...


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Lyle Gentry

As a nation, why don't we develop a system that utilizes the best of both the AC and DC worlds?
I envision a system where we use AC to span large distances and carry electricity to the home and then, once inside the home, convert over to something like a 48-volt DC system. Wouldn't that give us the best of both worlds? We'd get the long distance capability of AC outside the home and the saftey and efficiency of DC inside the home.
My ideal system would be a set of photovoltaic cells on the roof connected to an in-home battery bank to use as a buffer. Everything inside the home would be connected to that battery bank--which would also be connected to the AC grid in case it was being discharged faster than your photovoltaics could keep up.
I'm sure there's something wrong with this scenario but I can't think of what it might be - with the exception of converting all your lighting, appliances, electronics, HVAC and everything else to run on DC. But that's a one-time sunk cost and then you're good to go.
Or am I crazy?


I loved reading your article.

You described well the reason that DC loses power over long lines, but what makes AC so efficient in comparison - and ohms law does not apply.
Could it be that the reason for this is that once you oscillate the voltage (you now have a signal) you are now dealing with inductance and not resistance.
Perhaps High Voltage Switching Power Supplies would be useful. I do believe all power in the home and most industries today should be DC. There is no need for AC in the home with the technology available today.


Very good and Useful topic.
How can be the coeffiecient of Adhesion explained?


Good topic - after attending APEC recently and hearing several presentations on the methods / advances / challenges of converting solar energy to AC I also thought: So much of the loads in the home would actually like a DC source... why not have a DC bus in your house instead and save the DC-AC then AC-DC conversion losses? As solar gains ground I think we will eventually have that - give it ~10 years. Thanks again.

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