So why is 60 Hz the power supply frequency in US (50 Hz in the UK).
I believe it is an optimization problem, because lower than 25-20 Hz will result in persistance of vision (flickering light bulbs ?) and higher frequencies cause “skin-effect” losses in power transmission.
So who (if someone did at all) did this optimization and came up with the value of 60 (or 50 for that matter) Hz ? Is it just another of those arbitrary things ?
I believe it was 50 Hz at one time. (Why? I don’t know. My guess is that it’s just above the threshold of the “flickering” effect.) A power plant somewhere (New York?) was operating near maximum capacity, and an engineer figured out he could get more efficiency from the transformers by simply boosting the frequency to 60 Hz.
http://www.thebear.org/essays1.html
God only knows where the writer got their information, but it’s at least plausible.
Pretty arbitrary.
50 - 60 Hz is pretty convenient for building large generators to deliver (Alternators actually).
Also pretty convenient for large scale transformers.
400 Hz is also very popular in the miliary where they own alot of their own generators. As the frequency increases, transformers can be built that are smaller and lighter for a given power rating. Many radars and such in the military are designed for 400Hz power. It makes the 50-100k watt transformers much smaller and lighter and the generators are the same size. Very valuable overall for mobil equipment.
Lower frequency (than 50-60 Hz) would require much larger transformers all through the power grid, and would be more expensive.
If we started over designing the power grid today, with what we know now, I suspect that the system would be designed around 120 - 200 Hz power. But, the old standards work well enough that it would much more expensive to abandon/replace them than would be saved by the change.
This is probably mixing cause and effect but Televisions use 30fps interlaced which they can conveniently sync with the AC power to give them a reliable timer.
Higher frequency AC makes for much cheaper DC power supply design. This might have somethng to do with why the military likes 400Hz.
Thanks for the link Squink, but the writer never explains why 50 or 60Hz is the best - no mathematical explanation. Besides the grammar is terrible too.
Scotth, I’ve seen 400Hz on commercial airlines too. But I suspect this is due to the high speed alternators speed which may not be economical to decrease(I maybe wrong on this). Or maybe they want high speed motors. Would’nt transformers have a higher hysteris loss at higher frequency ?
Some things in perspective -
The above formula can be reversely used to find the speed of the alternator to generate a given frquency.
Also, although higher frequency is good, higher frquencies give rise to more losses in transmission due to the skin-effect. The skin effect in conductors occur at high frequencies where the current instead of flowing through the cross section of the conductor flows primarily through the skin of it.
I still think it is pretty arbitrary to have that 60Hz and wait for more clarifications.
Thanks all for the posts.
In the US there were many different systems at first. Some ran on 50 Hz, some on 60 Hz, a few on some oddball frequencies, and there were some that were even DC. Eventually 120V 60 Hz became the most popular and most of the other things went away. Some other systems do still exist, and DC power transmission is used in a few rare places.
When I worked in a power plant, one of the old engineers there told me that the original demonstration system for the power system ran at 50 Hz, but they couldn’t get enough power out of it. So, they cranked up the generators a bit, and they got a bit more power, which was just enough for the test system. Then all of the people who bought it of course said we want it just like that, and so they all ran at 60 Hz. The engineer who told me this was a fountain of old trivia like this, and I was never able to find a flaw in anything he said, but then I’ve never heard this exact story from another source either. It seems like every other time I’ve seen someone explain where 60 Hz comes from they all have a different story. I vaguely recall the original system being at Niagra Falls, but I’m not very confident of that memory.
The numbers quoted by Squink seem a little high to me. The transmission losses are mostly the resistive losses in the wire, which depend on the current, not the frequency. It is true that lower frequencies require larger transformers, but a 30 percent larger core for about a 20 percent change in frequency seems a little high.
Sounds reasonable ecg, but why did’nt they crank it up further ? Why stop at 60Hz then ?
Transmission losses depend on frequency too. Read Skin-Effect, above.
I’m not sure, but I’ll make an educated guess: While boosting the frequency increases the electrical efficiency, it places more mechanical stress on the generator assembly, since everything must now be spinning faster.
I learned in electronics school that aircraft use 400 Hz because the transformers can be so much lighter.
A lot of Ontario used 25 Hz until sometime before the Second World War… the first Canadian plant at Niagara was 25 Hz. There was apparently a government program to replace all old 25-Hz appliances (mostly the motors). This would have been easier then than now, because most home equipment (lights, toaster, etc) was resistive in nature.
I don’t know if increasing the speed increases electrical efficiency or for that matter what electrical efficiency is. I thought what ECG said was when you increase the RPM of the generator you get more power out of it, which I agree to.
Also agree Crafter that higher RPM means higher frequency. But you don’t have to have higher RPM to increase the frquency, Doubling the number of poles on the alternator will double the frequency at the same RPM.
Even, if we consider your argument of higher RPM resulting in wear tear, why select 60 Hz why not 55 Hz ? I mean is there an objective optimization for the 60 Hz ?
This is probably not true, but I reading reading someplace that 60Hz was chosen so that highly accurate electric clocks would be easy & cheap to produce. Sort of like using the national power grid as a way to keep the whole country in sync as well.
BTW, the frame rate on TVs is not 30 frames per second but something like 29.97. Does anyone have any idea why it is not 30?
Part of Japan has 60 cycle and part has 50. All at 100 volts. Maybe because it is safer. While 50 cycle may be less efficient, 220 volts means smaller wires between the transformer and the house and in the house. So there are all kinds of tradeoffs.
Can anyone explain how my laptop works at either 50 or 60 cycles and anywhere from 100 to 240 volts? The charger, I mean.
The charger converts AC to DC, so it doesn’t care if it’s 50 Hz or 60 Hz. The same is true of a lot of devices. Most electronics designed for 60 Hz will run on 50 Hz as long as the voltage is the same.
Your charger also has a fairly decent voltage regulator, which accepts a wide range of input voltage and regulates it down to what the laptop needs. My guess is that it’s a fairly simple switch mode regulator.
Early broadcast standards were 60Hz and 30FPS. RCA ruined it for us when they had the wacky idea of broadcasting a color signal. Read all about it here.
Nikola Tesla personally chose 60Hz when he was working for Westinghouse. The company engineers were already building large AC generators before Tesla arrived, and the company standard was 133Hz. Tesla had to step on toes and pull rank to get this changed to 60Hz.
I’ve never seen a detailed explanation for his choice of 60Hz, but there’s one eerie coincidence which was discovered decades later. One of the Shumann overtones (earth-ionosphere cavity resonances) is 59.9Hz. Electromagnetic leakage from the national power grid even drives this resonance a bit. Tesla was planning to broadcast electrical energy as VLF waves circling the Earth, so Earth’s 60Hz resonance probably was part of his plan. At the time, contemporary physicists ignored Tesla’s claim that the Earth had such resonances, and they weren’t redisocovered until the late 1950s. In the 1980s an engineering analysis of Tesla’s “broadcast electricity” showed that it probably would have worked.
But Tesla was also hot on centrally-synchronized clocks, so as Hail Ants says, his choice of 60Hz might have had something to do with clock motors. (But why not 120Hz then?)
Another possibility: human eyes can detect flickering light. Our peak sensitivity is around 25hz, and sensitivity drops off sharply above 55hZ. Tesla had been experimenting with flourescent tube lighting for years. Perhaps 60Hz was the lowest frequency which would still give totally “smooth” flicker-free lighting if the lamp only turns on for one half of a cycle. (All modern lamps turn on twice per cycle, flashing at 120Hz, so maybe the flicker problem had nothing to do with 60Hz after all.)
I thought transmission losses were alsp due to capacitance between the cable and ground. This would increase with frequency over large distances. Can anyone confirm this bit of info? I heard it in school, but didn’t read it anywhere.
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bbeaty has a nice Tesla page at http://www.eskimo.com/~billb/tesla/tesla.html
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When Tesla went to work at Westinghouse, he already had motors designed to operate a 60 Hz. Supposedly they could not be adapted to the 133 Hz Westinghouse system. Apparently it was simpler to redesign the generators than the motors ? From here.
Sunspace mentioned 25Hz that was, at one time, prevalent in Canada. If i’m not mistaken, he may have been speaking of the railroad ‘signal’ lines. Before the Rural Electrification Program here in the States, the only rural residents with power were those close enough to be fed from the railroad and they were fed at 25Hz. Flicker is very apparent at this frequency.