Diversions

Alternating Current Kills Elephants

April 8, 2014

Here at EngineerJobs, we’re pro-elephant. Who can resist their tusk-y faces, steadfast wills, and propensity to chew the bark off of trees? How very like us they are! But what do elephants have to do with alternating current?

Alternating Current Scales

First, a few definitions:

Alternating Current Kills Elephants

Credit: Atlant

Direct current refers from power flowing from the negative pole to the positive pole of a given circuit at a constant level in one direction. Alternating current reverses current flow in a circuit at a fixed frequency. (In the US, it is 60 Hz. Elsewhere, you can see 220, 221. Whatever it takes.)

In the early days of electrification, Edison’s DC system was the standard. It was well suited to power incandescent lighting, the alternating current motor had yet to be invented, and Edison had a functional DC power meter. Sounds almost modern, but there were some problems.

First off, DC requires heavy cabling, and even so, there are significant losses. Joule’s Law states that power losses (as heat) in a conductor are a product of the square of the current and the resistance of the conductor. Typical DC transmission lines delivered power at distribution voltage (approximately 100v at the time DC transmission was used) and high current. Therefore each doubling of current incurs four times the loss, making direct current a poor choice for a technology that was expected to scale.

In practice, this meant that cables were heavy. In addition, DC voltage could not be stepped up or down, necessitating separate transmission and distribution lines for each voltage required. Since the incandescent lighting of the day operated on a different voltage than motors, this was of real practical concern. Voltage drop was such that consumers had to be located no more than about one mile from a power plant.

Alternating Current Kills Elephants

Public Domain Image

Alternating current is subject to electromagnetic induction, however, and it is electromagnetic induction that gives us a way to work with the limitations imposed by Joule’s law – the transformer. The transformer at its most basic is a wire coil at the input (the primary coil), a wire coil at the output (the secondary coil) and something to facilitate coupling of the electromagnetic fields (the core.) The ratio of windings between primary and secondary determines the ratio of the step up or down in voltage. As we step up voltage, current decreases in accordance with Ohm’s law (that is, voltage divided by resistance equals current.)

With the transformer, the early grid could be far more versatile. High voltage transmission allowed transmission of power efficiently over long distances, and step down transformers ensured that the voltages demanded by power consumers could easily be obtained.

… But Alternating Current Kills Elephants

As talk of an alternating current power grid grew, Edison launched a campaign to promote AC as a “killer’s current.” He attempted to popularize the term “Westinghoused” as a synonym for being electrocuted, and his assistants electrocuted small animals for the press. Thomas Edison even paid an inventor, Harold P. Brown, to develop the first electric chair.

Even after Westinghouse won the bid to electify the 1893 Columbian Exposition in Chicago, Edison was still promoting the dangers of alternating current. This culminated in the 1903 electrocution of Topsy the elephant (Warning: animal cruelty), who was fed almost half a kilogram of potassium cyanide to make sure she remained “Westinghoused.”

Fortunately for us, George Westinghouse and Nikola Tesla were undeterred.

The New DC Revolution

Alternating Current Kills Elephants

Credit: Lawrence Berkley National Laboratory (Click to Enlarge)

With the nasty War of Currents behind us, direct current seems on the cusp of making a return. Though transmission and distribution of power rely on AC, many of our favorite gadgets use DC. Computers, LCD screens, solar panels, LED lighting and more operate on Direct Current.

Data centers are leading the way in the new DC revolution. Consider the path power takes in a modern datacenter, for instance. AC power from the mains gets stepped down to distribution voltages, converted to DC to suit the batteries in uninterruptable power supplies, is converted back to AC, then sent to the switching power supplies in computers to once again be converted back to DC to actually run the equipment. Each step incurs a small loss, but at datacenter scale, these losses add up. A more direct distribution of power can save 10-20% on power costs, not to mention waste heat and added complexity.

Ship builders too are looking to DC power systems for the same reason. Efficiency. Recently, GE was awarded a development contract to look into simplifying the electrical systems in Royal Navy ships. The U.K. Ministry of Defense is showing interest in DC systems to make its ships greener and easier to maintain, as demonstrated in the Electrical Ship Technology Demonstrator.

 

Are you working on new DC technology? Do you have a great story about switching off the wrong breaker and getting “Westinghoused?” Let us know! Post your story in the comments, below, or tweet us @Engineerjobs to join the conversation.

Featured Image Credit: Jessica Kennedy