• David Armes

What’s the difference between AC and DC power?

Updated: Dec 5, 2020

This Fun Fact Friday we’re going to take a look at the differences between alternating and direct current. We’ll start with Edison and Tesla’s Current War in the 1880s where each battled for their type of electricity. Then we’ll compare the two and take a look at the pros and cons of each, including why both are needed for today’s technology. Last, we’ll talk about the differences in electricity standards around the world.


The War of the Currents started in the 1880s between two very well-known names: Thomas Edison and Nikola Tesla. Thomas Edison developed direct current, or DC, electricity that for a time was the only available source of electric power. The problem with DC is the difficulty in converting it to higher or lower voltages, limiting its ability to travel long distances efficiently. DC power stations had to be located with a mile of the buildings they powered, which wasn’t at all feasible for rural areas. Tesla came up with AC, or alternating current, which rotates frequency (in the U.S., 60 times per second) and can be converted to other voltages with a transformer. Edison had patents for DC and didn’t want to lose money, thus, he moved to discredit AC - he lobbied state legislature, spread disinformation, and even attempted to sway public opinion by electrocuting stray animals with AC current to prove its dangers.


Nikola Tesla

Nikola Tesla. Images Credit: WorldStandards

Thomas Edison

Thomas Edison. Images Credit: WorldStandards


This all changed at the Chicago World’s Fair in 1893. According to Energy.gov, “General Electric bid to electrify the fair using Edison’s direct current for $554,000, but lost to George Westinghouse, who said he could power the fair for only $399,000 using Tesla’s alternating current. […] That same year, the Niagara Falls Power Company decided to award Westinghouse -- who had licensed Tesla’s polyphase AC induction motor patent -- the contract to generate power from Niagara Falls.” In November, 1896, General Electric was fully on board with AC and their newly built hydroelectric dam at Niagara Falls lit up Buffalo, New York. It seemed AC power had finally triumphed over DC.


If AC power won out over DC in 1893, why do we use both today? For a while, AC was the dominant form of electricity, but today we use DC every day, especially in portable devices. Let’s take a look at a direct comparison of AC to DC. At its simplest, electrical current is the flow of electrons. The main difference between the two types of current is direction: DC is direct, meaning it only flows in one direction. You can think of this as the battery with the positive and negative ends, it must be installed one way so current can flow through it (I know what you’re thinking, spring to the flat end of the battery. Unless you’re an electrical engineer, now you’re rolling your eyes). AC has an oscillating repetition that looks like a wave pattern on a graph, compared to DC that looks like a flat line. The wave pattern is actually the Hertz, or number of oscillations per second. AC power is more convenient for reaching the high voltages for transmission long distances, so our lamps and most household electronics that plug into the wall use AC current. However, with a phone, AC current flows to your house, which then charges your phone that has a battery running on DC. This is the same with laptops, which is why they have a brick between the wall and the computer - the power needs to be converted to DC for the internal battery.


Sine Wave graph, amplitude vs time

AC Sine Wave Voltage. Image Credit: Sparkfun


DC Voltage graph, voltage vs time

DC Voltage. Image Credit: Sparkfun


But it wasn’t always this way, AC dominated for a long time. The big turning point for DC power was in the 1970s when semiconductor electronics were invented. It was much more economical to transform AC to DC and some equipment could generate very high DC voltage. Additionally, DC current is required for many electronics; according to Sciencing.com: “Direct current is used in any device that has a circuit board because the chips within these devices require a steady, unidirectional flow of electrons to operate and store data.” This means that your desktop computer, gaming system, and TV all use DC current, and contain a full bridge rectifier to transform the AC power from the wall outlet to DC power. Some of these devices also contain hard drives, powered by motors which, you guessed it, require DC. With the surge of electronics that has continued since: personal computers, cell phones, smart TVs, electric cars, and a whole lot more, DC is back with a vengeance and being used everywhere.


A short side note about sending power over long distances. Since most of the world is wired for AC, that’s what we use, but surprisingly, some high voltage DC lines, or HVDC, can actually deliver electricity with less loss than AC over a long distance. These lines can even allow multiple AC systems such as Europe’s 50Hz and the United States’ 60Hz to the same line. However, increased cost and decreased reliability means AC lines are still much more common.


Tesla at a 240v outlet in a garage.

Tesla on a 240v charger. Image Credit: JSR Electrical.


If everyone uses AC, why do different parts of the world use different standards, like Europe and the United States? For example, in the Americas and Japan a 100-127 volt system is standard, but Europe and most of the world uses twice that voltage, 220-240 volts. Nikola Tesla did careful calculations to come up with the 60 Hz number as the best frequency. To make matters even stranger, the lower voltage systems run at 60 Hertz, or 60 oscillations per second, while the rest of the world runs at 50 Hz. According to WorldStandards, “When the German company AEG built one of the first European generating facilities, its engineers decided to fix the frequency at 50 Hz, because the number 60 did not fit the metric standard unit sequence (1, 2, 5). At that time, AEG had a virtual monopoly and their standard spread to the rest of the continent. In Britain, differing frequencies proliferated, and only after World War II the 50-cycle standard was established.”


The voltage standard was originally 120v in Europe as well. Working with a certain copper wire diameter, Europe thought it necessary to increase voltage for fewer losses and less voltage drop. The United States wanted to do the same, but when this was being considered in the 50s and 60s, many American households had appliances like washing machines and refrigerators and the cost of replacing them was prohibitive. Most European households did not yet have those appliances, and it was an easier switch. Staying at this lower voltage actually caused problems in the U.S. with high and low voltage spreads. The solution was to run 240 volts to each building, and then split it into two 120V lines. Although some appliances like dryers and ovens use 240V in the Americas, some of these appliances still do not work with European power due to the phase difference of 60 Hz vs 50 Hz.


World map of different voltages used in different countries

Map of voltages with 120v in red and 240v in blue. Image Credit: WorldStandards.


Without the great work of Edison and Tesla, our day to day lives as we know it today would not be possible. It turns out that AC is great for some purposes like long distance travel and DC is great for other purposes like computers, LEDs, and electric vehicles. Either way, it’s all power!



Sources:

https://www.energy.gov/articles/war-currents-ac-vs-dc-power

https://learn.sparkfun.com/tutorials/alternating-current-ac-vs-direct-current-dc/all#:~:text=In%20direct%20current%20(DC)%2C,because%20the%20current%20changes%20direction.

https://engineering.mit.edu/engage/ask-an-engineer/whats-the-difference-between-ac-and-dc/

https://sciencing.com/uses-direct-current-7394786.html

https://www.worldstandards.eu/electricity/history/why-no-standard-voltage/

https://jsrelectrical.com/electric-car-chargers/

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