layman's question about power generation 2

[chichuck]

All,

I'm a structural engineer, and have worked in power generation industry for about 15 years now. I've asked around many times, about a point I'm just curious about. I've not yet found and "sparky" that knows the answers.

I've learned that although we in the US use 120V/60Hz power, in many other parts of the world, they use different voltages, eg 240V, and also different frequencies, eg 50Hz.


Why is the US standard for voltage and frequencies different than in other parts of the world?


I'm just curious, and also surprised that I've yet to find an elec. eng. who knows about this.

Thanks,

regards,


chichuck

 

[bonzoboy]

Good luck. I've been looking for that answer for 25 years. I have a couple of theories.

1) The higher frequency (US) is slightly more efficient (in a broad sense). A generator turning at 60 hz puts out more power than the same generator at a slower speed, although obviously the frequency goes up. I don't why it isn't faster, unless there is some fundamental EE issue about moving power at higher frequencies.

2) I've also heard that the amount of power from a 60 Hz generator is a "maximum for the amount of copper and iron in the generator". This may be part of the efficiency side of the argument.

3) 60 Hz is typically stepped to very high voltages, and it is difficult to hijack power off the system. In India, a 50 Hz country, I hear that as much as 20% of the electricity is tapped off by non-paying users.

4) My favorite.....Nikolai Tesla, the father of the 60 Hz system, had a fetish for the number 3. He could only stay on a 3rd, 6th, or 9th floor of a hotel, used three spoons, three forks, etc. 60 is divisible by 3; 50 isn't.

Anybody else???

 

[dpc]

thread238-3663

You should try searching this forum - this question has come up many times.

Bottom line - there is a range of frequencies that works well for power generation and distribution. There are trade-offs between 50 Hz and 60 Hz, but the reason for the two standards is largely historical. There are no compelling technical issues for one frequency over the other.

 

[jraef]

This subject has been covered extensively in this forum for years, there are plenty of discussions available by doing a Keyword search on the terms "50Hz 60Hz", because most of the time the voltage issue is covered in the same discussion.

Capsulated version however is that Tesla and Westinghouse were geared up to compete using their new AC system against Edison's DC system in New York City, hich was already 110VDC. So they began providing 110VAC, and everything else stemmed from that. Appliance manufacturers were already making household goods based on 110VDC, so their system allowed that to continue without forcing manufacturers to make too versions of everything. Eventually AC won out.

Because other countries electrified later, many took advantage of lower distribution costs by distributing higher voltages in simpler configurations. here in the US, the legacy voltages essentially held us back from doing that for residential services.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[itsmoked]

That and another big reason; 60Hz enjoys 20% smaller magnetic structures over 50Hz. This starts adding up when you think about millions of distribution transformers and billions of motors.

 

[rbulsara]

Also see thread249-101972

 

[bonzoboy]

itsmoked....thanks for that in. I think that was the story that was related to me some years back. Not exactly sure what smaller magnetic structures implies, but if its helpful...so be it.

BB

 

[itsmoked]

Oh hey! A smaller structure means just that!

A 20kW transformer for 60Hz will be about twenty percent smaller then the size of one for 50Hz.

I will now give a glossed over description of why;

The iron core is smaller because as each power cycle occurs, (you know the sine wave), the magnetic field builds up in the iron core.

As the current reaches maximum with the voltage the core approaches saturation. This where if more magnetic field is jammed in the iron will not support it. This is a very bad event in power land.

Because this is a time thing also, the shorter the cycle time the less time is available for this saturation effect to occur... Hence the core can carry the same power while being smaller in size.

 

[rmw]

I assume that is the reason why AC systems on aircraft where weight is paramont is 400 hz?

I am kind of a "if a little is good, a lot is better" type of person. If the first paragraph is the case, why stop at 400 hz? Why not 4000?

rmw

 

[itsmoked]

Yes that is exactly why aircraft are 400Hz. Well heck you say! Why didn't we make our whole grid 400hz?!?! Imagine a 50hp motor you can carry!

Well there are some limits. As the frequency gets higher the current travels farther out from the center of wires making them a little less effective. This is called skin effect.

Also as the frequency goes up the wavelength gets shorter. As the dimensions of an electrical system get within a tenth of the length of the wavelength the system starts to act in a distributed manner which causes lots of reflections and transmission problems. 60Hz is a wavelength of about USA west coast to east coast which means these effects only need to be dealt with in a few cases. If we were running 400Hz it would be a constant requirement.

 

[MacRae]

I have looked into this for quite some time and,while the answers vary, here is the best that I have come up with.
As was mentioned before Edison was the first on to the market in the States and he used 110V DC so when Tesla was trying to come in to the market as competition he had to use the same voltage to lure existing customers. I have read that Tesla actually wanted to use 240V because of the lower current (smaller wires, less voltage drop) but due to the existing 110V had to go with it. As for 60 Hz he calculated that as the best trade off between being able to transmit power for long distances (due to reactance loss problems, I think 400Hz is limited to around 2 miles max)and being a high enough frequency where flicker is not noticeable in light bulbs. In Europe there was no competition with Edison so 240V won out (there were many competing voltages and frequencies at one time in Europe early on) As for 50Hz that was the frequency that AEG chose because of the metric systems convention of numbering 1,2,5. Being that AEG had a virtual monopoly at the time the 50Hz standard spread throughout Europe. This is the best that I can come up with, or at least the most believeable anyway

 

[cuky2000]

Although higher frequency help to reduce the size of inductive equipment, this also undesirable since increase the transmission line reactance close to 20% (60/50), requiring larger towers (electrical clearances) to obtain equivalent values.

Market protection was another driving force in addition to historical development, local preferences and technical reasons. Having a different system parameters including frequency, voltage, measuring, standard, etc., discouraged outsider to compete in the ANSI marketplace controlled primarily by GE and Westinghouse for almost a century.

[sub][blue]Historical Note: In the pass two decades, GE & Westinghouse were more interested in the financial, TV (ABC CBS)/ entertainment and other business. This open the doors to foreign companies such as ABB (BBC &ASEA) and Siemens to purchasing the remain power systems assess from Westinghouse, GE and others. Unfortunately, they decided later to close, sold or reduce the operation to a single “mail-box” to import products from other plant abroad.[/blue][/sub]

On the other hand, 400 Hz was primarily adopted as standard in the military environment to reduce the weight of electrical inductive apparatus and provide less interference with electronic equipment such as radar and sensitive instrumentation.

Here is additional historical information in this topic

http://www.ieee.org/organizations/pes/public/2003/sep/peshistory.html

 

[fsmyth]

Could it be that Europeans just wanted to be onery (i.e.,
"not invented here")? <g>
I like the one about metric roots. More plausible. Not
enough folks with 6 fingers when counting the blinks
between cycles. <G>
I wonder what the dominant freq. was/is on Mars. With all
that loose Fe, seems that the answer would be definitive.

<als>

 

[rmw]

Somehow I lost track of this thread but I would also add that there is a lot of interesting commentary on 25 Hz systems related to discussions of Hurricane Katrina and the old S&WB pumps in NOLA.

itsmoked, thanks for you answer. This clears it up for me. I had once heard it said that the wavelength of 60 Hz power was approximately a coast to coast dimension, but I had never verified it, and regarding that, I rarely take uncorroborated information at face value, even on Eng-tips.

I'll give you a star for your effort, because it was valuable to me.

rmw

 

[MacRae]

Here is another source for this info:

http://www.epanorama.net/links/wire_mains.html

 

[MacRae]

and another:

http://users.pandora.be/worldstandards/electricity.htm

 

[alehman]

Other reasons 400hz (or other high frequency) isn't used universally:

1. High iron losses. The higher the frequency, the more energy that is wasted in cores of transformer, motors, etc.
2. Motors for low RPM are more difficult to build because more poles are required.