Of all the possible setups, why did the US choose 120 V at 60 Hz? Globally, we are in the minority. Denmark uses 230 V (!) at 50 Hz, for example, and the UK seems to like that setup too. I’m not even going to mention the strange plug shapes that exist in other places in the world.
So, what possessed the US to choose 120 V at 60 Hz?
Others can perhaps provide details, but my understanding is that the U.S. electric industry standardized earlier than most of the rest of the world. By the time the other countries did so they had figured out that slightly higher voltages (e.g., 220 volts) were more efficient.
I seem to recall Cecil discussing this – I’ll see if I can find it online.
Okay, so we have the 120 V part nailed down. But why 60 Hz? Why not 50 Hz? Why not 70 Hz? Why not 100 Hz?
According to this page the reason is that :
> Originally it was all 50Hz (or 50 cycles, as it would have been
>> stated then), a nice round number. But then the grid got over-
>> loaded in the U.S. and the, then young, utility companies were faced
>> with a great expense for all new transmission equipment. However,
>> a way was found. A given transformer can handle more VAs at higher
>> frequencies (within limits), so the utilities jumped the frequency
>> to 60 cycles and saved big bucks - and the public had to buy all
>> new clocks (AC electric clocks are controled by the accuracy of the
>> frequency provided by the utility, which is adjusted daily to keep
>> clocks on time). This same logic is why aircraft use 400 Hz. By
>> using a high frequency they can pump plenty of VAs through relatively
>> light transformers. A similar phenomenon: musicians oficially
>> started at 400 cycles/sec., but kept cheating a little to for a
>> “briliant sound” until the standard drifted up to A=440 cycles.
I wish I’d read the rest of that article before quoting from it… ignore my quote.
An authoritive source at the bottom of that page covers the topic in much more depth.
Ships also use 400Hz last I heard.
Some more generic info:
Losses in transmission lines will occurr at two places. One is the transformer itself, and here we want to get a highish frequency to avoid these losses. The other place is to the ground. With the earth and the wires being the conductive plates, and air being a dieletric, we find that along the entire transmission lines we have one monster capacitor(as well as between multiple power lines)! Well, anyway, too high of a frequency and we lose more to this capacitance (since the effective resistance of a capcitance approaches zero as frequency increases).
Now, the question on MY mind…
In the US our “neutral” and “earth ground” are at the same potential. In fact, I just took a meter and read the resistance: 0 ohms! My boss informs me that this isn’t the case in England (and europe too I think he said). There, their 0 volt potential is only in relation to the other transmission lines: earth ground is not a part of their electrical system (they are floating!) This seems like a dangerous way to do things. Anyone know the idea behind this?
[sub]oh yeah, and the UK has fuses on their plugs instead of fuseboxes![/sub]
Europe did not start out with 220V but have moved up. The change requires effort but also savings in the long run. The US is lees willing to put effort today into improving things tomorrow.
Yes, 400 HZ transformers would be a great savings in the future everywhere if we could figure out how to do the transition without major pain. Changing frequencies is much harder than changing voltages.
Regarding ground: European systems are grounded just like the US systems are grounded.
I may add all these things have been extensively discussed in previous threads so i won’t go into more detail.
Right.
If you were to focus on the transfer of power via the conductors only, DC is more efficient than AC. This is true for two reasons: 1) With DC, you don’t have to worry about reactive losses, 2) With DC, you don’t have to worry about losses due to the skin effect. It’s when we look at the “big picture” that AC becomes the winner. (Because of the high cost and prohibitive size of heavy-gage copper conductors, the ability to increase/decrease voltage is a necessity when implementing a large-scale power distribution system. Unfortunately, there is no efficient method for increasing/decreasing voltage in a DC system.)
In the UK we have a 3-phase transmission system which most of our industry uses at 415Volts.
One phase measured down to neutral gives 220 Volts so it is very convenient to ensure that domestic systems operate at this voltage rather than put in tranformers to bring it down further.
We do use 110Volts in certain installations though, in temporary installations such as building sites especially, what is more this is centre-tapped to earth so that the maximum shock that can be delivered through a person to ground is 55Volts, it is possible to get a shock between two lives of 110Volts but this is a much less likely ocurrance.
As for using 50Hz of 60Hz or even 400Hz up to the 1100Hz is have seen used in gyro compasses, well there are some trade-offs to be made.
A tranformer designed to carry a certain power(not exactly the right term but near enough)can be made smaller if the frequency is increased, but doing this also increases the reactive element too which can be a serious limitation in high current systems.
When we talk about power systems we look the the useful work that can be accomplished and we call this ‘true power’.We can do straightforward calculations on this and they would show that the power which appears to be there ‘apparent power’ is larger than the ‘true power’.
This differance between the two is called the ‘reactive power’.This power is 90 degrees out of phase with the true power.
The actual power that the cable ‘see’ is a combination of these elements and is greater than either, Pythagoran triangles are useful to visualise this but more sophisticated methods are used in practice.
By increasing the mains frequency we increase that reactive power, but without increasing the useful true power, hence the cables, circuit breakers and the rest of the distribution system ‘sees’ a greater current, this limits the amount of useful work available in a network.
In certain industries, most notably aviation and space, weight is a very important consideration and increasing the circuit frequency allow the use of physically smaller, and thus lighter, transformers and motors.
erislover
UK supplies - we have differant ways of connecting depending upon the local circumstances, a consumer with its own high voltage feeders, such as a large farm or factory will often have an incoming earth from the supply plus this will be linked to earth electrodes driven into the ground.
In such cases neutral and earth are at virtually the same potential but for a few, important at times, tenths of and Ohm.
Most domestic premises operate from one phase of a 3-phase transformer which is star connected on the secondary, the star point is earth connected direct to ground and this is where the neutral returns to.The neutral cable has impedance from the consumer to the transformer star and this combined with the current being consumed is responsible for our neutral being a fraction of a volt above earth.
As you are probably aware the star point of a balanced transformer is at earth anyway so tying that point to earth is a way of fircing it to stay there when coping with out of balance currents, particularly under fault conditions.
If you are a a generator of power, large enough to supply the grid network, it is vital to have a certain amount of resistance between neutral and earth because the potential fault currents are absolutely immense, you will always find a large tank full of a very mildly conducting solution, such as bicarbonate of soda dissolved in water, acting as a low value resistance with massive power handling characteristics, and this is called the neutral-earth resistor. This resistor can be switched in and out of circuit dependant on the network requirements to deal with either out of balance currents or third harmonic currents.
When it is out of circuit the neutral is referanced to earth through the network lines.
There is provision for completely floating supplies in the wiring regulations but I have never seen them in the UK though aparrently there are some consumers in Europe who are supplied via long lines on poles who are.
Nearest I have seen to power lines not referanced to earth in any way is in very specialist lab conditions in very carefully controlled earth-free zones and I believe it is done to some extent in the anodising/electroplating industry but at very low volatages indeed, around maybe 5Volts - don’t quote me on the latter though as I could be wrong
The German Railways use seventeen and two thirds Hz for their electrified network. Don’t ask me were an earth that came from.
Originally, in Edison’s labs at the end of the 19th Century, they used 477 Hz due to its arithmetic convenience. (“omega equals two pi eff…”)
At that time, the few electric power systems were DC. Nobody trusted AC, partly because it was hard to keep “inside the wires.” (AC at the right frequency will treat a gap the right size as just a good conductor as a solid copper wire.) Thomas Edison and the Edison Electric Company were fanatically devoted to selling the world on DC municipal power. But it could only be transmitted a couple miles. Edison and the company selling AC municipal power, Westinghouse, had an infamous campaign of marketing warfare that raged for more than a decade.
AC, it turned out, could be transmitted for leagues and leagues. Nicola Tesla, inventor of the AC induction motor (something everyone in that day said couldn’t be made), knew this, and early in his career, he worked for Edison. Surely he and Edison discussed the tradeoffs between AC and DC. Tesla was intimatly familiar with AC, since he literally played with it, and IN it. So surely, he also discussed with Edison the benefits and dangers of the various frequencies of AC.
It just so happens that when all was said and done, the AC power frequency chosen for the standard was the one that was the most dangerous to the human body. The 50 and 60 Hz frequencies are best transmitted by myelinated nerves, meaning that if you complete a circuit with your body, the current will produce deep tissue burns as it runs along your major nerves, cooking you from the inside. Not only that, but nerve damage is often irreversible, and can interfere with breathing, heartbeat, organ function, and even the brain… Was it a coincidence?
No one knows how 50/60 Hz was chosen, or who made the decision. Some people claim it was Tesla’s doing, because he loved to pass electricity through his body, and at very low currents, 60Hz can be fun, in a twisted sort of way. Other people think it was Edison, deliberately making AC as dangerous as possible in order to make it unattractive and establish DC as the standard for municipal power.
For you see, the latter argument goes, if DC were the standard, cities would have to install Edison’s generators and power repeaters every few blocks… creating a huge market for Edison products.
It’s just another example of Hanlon’s Razor: “The perversity of the universe tends to a maximum.”
The frequency is optimized for different considerations in different settings. Higher frequencies allow smaller transformers, but they also cause greater losses in steel cores in transformers and motors. You may notice transformers and motors are usually made with laminations - thin layers of steel laminated together to build up the part. This is to reduce the losses associated with currents circulating around inside the part. Making the laminations thinner and more numerous improves this, but at a higher construction cost. In the US, railroads use 20 Hz - I think this is to reduce these losses.
Crafter_Man wrote:
The skin effect doesn’t come into play for frequencies that low. It’s not even an issue for audio speaker cables at 20 kHz, despite what some audiophiles claim. In fact, if an audiophile makes a technical statement, you’ll be right more often if you assume what they say is not true.
CurtC
You also get skin effects at very high currents too.CrafterMan is absolutely correct in this, but it’s his point so if he wants to prove it he can.
bughunter: “Thomas Edison and the Edison Electric Company were fanatically devoted to selling the world on DC municipal power. But it could only be transmitted a couple miles.”
In fact, not only can DC be transmitted long distances just fine, it is actually much less subject to loss during transmission than is AC (for reasons alluded to in previous posts in this thread). A web site that describes the development of High Voltage DC transmission technology http://www.ece.umr.edu/courses/ee353/HVDC/l114.htm mentions, among others, the Cahora Bassa (Mozambique - South Africa), Skagerrak (Norway - Denmark), Inga-Shaba (Zaire), CU Project (USA) and Nelson River 2 (Canada) HVDC lines.
However, it is true that it is terribly difficult (read expensive) to convert HVDC to low voltage AC usable by consumers. An example of this was the Inga-Shaba power line that stretched 1700 km from close to Matadi in western DR Congo to Lubumbashi in southeastern DRC. It passed only a few kilometers from Kananga, DRC’s second largest city, but Kananga was nearly dark at night because there was no practical way to tap into the DC line.
casdave, I have to disagree with you.
From Fields and Waves in Communciations Electronics, (although my copy is the second edition), the current cross-section in a circular wire is proportional to
J[sub]0/sub,
where r is the radius from the wire center, and delta is the skin depth. The skin depth depends on frequency and conductivity. Nothing depends on the current amplitude.
Crudely, the skin depth comes in to play when the wire radius is larger than the skin depth. At 10 kHz, for copper, this occurs at a radius of about 0.7 mm. At 60 Hz this occurs at a radius of about 9 mm. The effect is more important as the wire gets larger, so it’s the monster cables where the skin depth is important, not the cheap thin wires.
Zenbeam say it ain’t so… but 'tis true, you are right, being involved for the past ten years with heavy power and earthing I took the skin effect caused by lightening to be caused by the massive currents but it turns out to be the very high rate of change which is part of the lightening strike, and this corresponds to a high frequency.
There is another effect, with which I never had any reason to concern myself, where conductors carring very large currents running very close to each other can ‘push’ the current in the other cable to one side - apparently it’s called the proximity effect, however all my problems in this regard are more down to inter-cable capacitance and induction.
eg “Is that circuit isolated”
“Yup!”
Touch >>>>ZAP!!!<<<< “Owww!”
“Ok, knock the whole switchboard off and put the earth straps on” ::::sigh::::