Monday, November 14, 2011

Why Does AC Electricity Operate at 60 Hz?

As I briefly touched on the other day in the post about switching transformers, pretty much all AC power "alternates" at a rate of 60 cycles per second, or 60 Hz (in some parts of the world it's 50 Hz, but that's not much of a practical difference).  There are a number of reasons why this is a total pain in the ass and/or deadly:
  • 60 cycles per second is about at the limit of what the human eye can detect, which means things like fluorescent lights that are driven by a 60 Hz supply and have fast on/off times will have a barely perceptible flicker (regular light bulbs, which take more than 1/60th of a second to stop glowing when you cut the power, don't have this problem nearly as much).  Depending on how good your eyes are, this may be unnoticeable, subconsciously noticeable (do fluorescent lights give you headaches?  This is why), or incredibly goddamned annoying.
  • The general rule of screening out electromagnetic interference is that high-frequency noise, since it doesn't travel through conductors very well, is easy to shield, while low-frequency noise is notoriously difficult.  60 Hz is, in a world where our computers clock almost a billion times faster than that, about as low-frequency as you're going to get without being DC.  As a result, keeping sensitive electronics (like the audio and science gear I spend most of my time around) from picking up a 60 Hz "hum" is a highly frustrating black art that's not for the migraine-prone. 
  • As we learned when discussing switched-mode transformers, stepping voltages up and down and converting AC to DC can be done much more efficiently with much smaller components at high frequencies.
  • Possibly most problematically, your sinoatrial node (the thing that tells your heart when to beat) operates at frequencies close to 60 Hz.  People will tell you it's the current, not the voltage, that you want to worry about when you receive an electric shock, and that's generally true, but even low-current 60 Hz shocks have a decent chance of scrambling your SA node and stopping your heart if they travel through you right.
Whether you're an easily-annoyed scientist with exceptionally good eyes or just a fan of not being electrocuted, that seems like a pretty damning case against using 60 Hz as our power transmission frequency.  So why do we do it?

A bit of history first of all: in one of science's most awesome/ridiculous moments, Thomas Edison and Nikola Tesla got into a big thing at the end of the 19th century over whether America's new and growing electrical infrastructure should use Edison's DC power standard or Tesla's AC design.  Edison, who was more of a dick than history books usually give him credit for being, insisted that DC power posed significantly less risk of electrocution than AC power, and illustrated his point by using AC power to electrocute a goddamn elephant and film it.  When that inexplicably failed to win him the argument, he also secretly funded the construction of the first (AC-powered obviously) electric chair to further prove his point, even though he was technically against the death penalty.  Like I said, he was a bit of a dick.

Thomas Edison, the Michael Vick of science
Thomas Edison, the Michael Vick of 19th-century science.

Despite the fact that Edison obviously wanted it more, Tesla's AC system ended up winning out for the simple fact that AC power is much, much more efficient to transmit over long distances than DC.  The hell of it is that Edison was technically correct; it is much harder to kill yourself with DC current, but this was Gilded Age America and "safer" never stood a chance against "cheaper."  Anyway, 60 Hz eventually got settled on as the alternating frequency, both because it was convenient for the large industrial motors, electric trains, and incandescent light bulbs that were the main things using the power grid at the time and because electricity travels over a transmission line more efficiently at lower frequencies than high ones.

As every engineer knows, standards are like a candiru fish; once they're in place for awhile, there usually isn't any getting rid of them.  So in spite of the fact that it wants to murder you and is very inconvenient for most modern electronics, the fact that it's been around for awhile, coupled with its hard-to-beat long-distance transmission efficiency, are why we're stuck with 60 Hz power for the forseeable future.  Apparently driving innocent engineers insane with flickering lights and impossible-to-get-rid-of interference is a small price to pay for saving some cash on delivery costs.

There are a couple of random exceptions to this.  The "third rail" on most subways is actually a high-voltage (~750V) DC supply, which even though it's DC is still way more than enough to kill you.  Apparently Alcatraz Prison, back when it operated, got all its electricity from an onsite DC generator too.

UPDATE 11/16/11: The wonderful Matt Taibbi, in a totally unrelated article, reminded me that Edison only fried that elephant after electrocuting a bunch of random dogs and cats didn't attract enough attention.  That Michael Vick comment seems even more appropriate now.


  1. Great article thanks for posting it. I love the side notes on Edison. I would say he was one of the first to take capitalism to the negative side. Smart but not a role model. Thief too!

  2. Hi, good artlicle, I would like to know why 60 and not 120Hz or 500 por example, it would be just a twists in the coil after all.. why Tesla choose 50-60 ? could it be related to the material used to transmit?

  3. Engineering compromise. If you use too low a frequency, transformers and AC/DC conversion need bigger components. If you go too high, your transmission costs increase because the electricity is only traveling along the outer surface of its transmission line. 60Hz, which is what a lot of old motors and things ran at, is a reasonable happy medium, so we've just kind of stuck with it.

  4. Engineering compromise. If you use too low a frequency, transformers and AC/DC about-face charge bigger components. If you go too high, your manual costs access because the electricity is alone traveling forth the alien apparent of its manual line. 60Hz, which is what a lot of old motors and things ran at, is a reasonable blessed medium, so we've just affectionate of ashore with it. AC Electric Motors

  5. I'm seeking to better understand why the standard 60hz on over head contact system is used for electrification of trains. When I watch a You Tube review of 60 vs 120 hz I can see the difference but I don't know why 60 is the standard for trains. Do you know?

  6. Simplest reason, power generation. To get 60 Hz AC you need 3600 RPM for 2 pole generator, 1800 RPM for a 4 pole generator, 1200 RPM for a 6 pole generator, etc. For double the frequency you need double the speed. The way AC generators work is a drive shaft and to generate a lot of power you need a big, heavy, generator so high speed isn't going to happen meaning you use a lot of poles to get even 60Hz.

    Now, for the important part, converting from one frequency to another WILL involve power low and when dealing with the MVA power ranges even .1% is a huge loss.

    1. Hadn't even thought of that, excellent point. There are also probably a lot of really good reliability-related reasons to want to spin your generators as slowly as possible.