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March 19, 2009

AC / DC Wars Continue... Part II

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AC or DC? You decide... My previous post "AC vs. DC - The Westinghouse / Edison War Continues..." has created some very active feedback and thus compelled me to create a "part two" post on the subject.  Surprisingly, there are individuals on both sides of the fence.  Some are very pro-DC, others pro-AC.  It’s fascinating to see the reasons for each point of view.

Some readers are promoting DC for use in HV transmission systems where the higher voltage (as in AC transmission systems) lowers the resistive loses (see my previous blog for the math).  For example, ABB (a manufacturer of HVDC equipment) makes very good statements on the advantages of HVDC transmission such as connecting grids of varying frequency (60 Hz to 50 Hz) and lower cost over long distances (only 1 wire is required not 3 or 4 - makes sense).

Others are promoting DC in the home providing a universal DC bus for equipment such as PCs and other electronics.  Since there would be only one DC supply, it would be much easier to back it up with batteries - possibly fed from solar panels.  Why have Uninterruptible Power Supplies (UPS) on every computer, DVR or gaming console when you can have one, low voltage DC supply.  Again, this makes sense.

On the other side are some very good reasons for alternating current.  AC power transmission systems are extremely reliable, well understood, fairly universal and ubiquitous.  We have had AC with us for over 100 years and to universally convert our transmission systems to DC would be unrealistic (for now... in 100 years, who knows).  Home and commercial systems are all engineered to work with AC from circuit breakers to florescent lights (an incandescent bulb would work either way).  Today’s incandescent bulb dimmers require AC power to work... a completely different technology would be required to dim them using DC power.  LEDs will eventually replace CFLs and incandescent bulbs and could greatly benefit from a DC lighting bus.

I take the position there’s a place for both.  In many applications, local DC buses could provide a uniform, uninterruptible supply of power that easily integrates with local power generation (solar, wind, etc.).  An AC/DC "gateway" can provide the interface between the energy producers (e.g. the power company) and the local generation capability (e.g. solar panels) and manage the power flow.  The direction of flow can be from the grid to the home or visa versa when the sun is shining with low local consumption.  This device can also provide a gateway to the power consuming devices in the home to help manage and lower the consumption.

The future of energy management is bright and will require a rethinking of how we use power whether it is AC or DC in nature.  I believe that AC and DC power can live in harmony where each has a place that simplifies the application.  Let me know what you think!  Till next time...


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im very much interested in this paper.i would like to know that if we use low voltage dc at house(<50v),how can we reduce it to further diffrent low voltages (eg. in case of mobile charging,tv,lighting system .etc........).kindly reply soon............

Active Noise Cancellation

Great article, I didn't know AC and DC were fighting. Thanks for the great info.

Tony Bell

Nice article. I'm not sure about your HVDC transmission logic though. For a start you would still need 2 wires (not one) to transmit DC, and unless my maths brain escapes me you get 1.7 x more power from the 3 wire AC as you do from the 2 wire DC for the same current rating, making AC more advantageous by 14%. And of course, as mentioned in your previous post AC is far easier to convert voltage. However, thats as far as the distribution goes. Inside the home/office, DC would be more advantageous for a host of reasons. You mentioned UPS Systems, and batteries, but noise filtration and clean power are easy to achieve on DC, not to mention the benefits obtained by not wasting so much power on conversion losses.

EZ notes: The "single" conductor is due to the use of an earth return. See

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