Why 3-Phase?

Status
Not open for further replies.
Transformers and motors get lighter as you go to higher frequencies.

The higher the frequency, the more volts per turn that you get for the same peak flux in the magnetic core. This means fewer turns to get the same voltage, and an over-all smaller device.

-Jon
 
Besoeker said:
APUs for example.
Click to expand...
Never got to see that the generating end of it. I noticed on some of the panels and smaller transformers in spaces that used electronic equipment (radars, computers, weapons systems, etc) that they do use 400 Hz as well as 60Hz. :rolleyes:
 
philly said:
I understand most of the math behind three phase systems but wasn't sure if there was some underlying issue why 3-phase was an advantange of invening or adopting some other multi-phase system as mentioned.
Click to expand...
In a way, 3-phase was an adaption of single phase. Many early 3-phase services, which were high-leg open Deltas, were a modification of existing 1-phase 120/240v services.

Often, the 3-phase was first added for a single piece of machinery. String one more primary conductor and you can now install them new-fangled 3-wire contractions!
 
Greeting's all. Ron here, an electrician who just passed his licensing exam and it's me first post here. :)

Three questions in this thread I'd like to consolidate into one coherent post.

Q1) Why three phase and not more?
A1) Because adding more phases doesn't appreciably add more efficiency to a motor. The original reason why three phases was adopted in the first place. Adding more phases would add more cost of wire, however.

Q2) Why is residential called single phase instead of a two phase system?
A2) Years ago there was a two phase system in existence. The phases were 90 degrees out of phase with each other for running motors, etc.. It's obsolete now. It's called "single phase" because most of the loads are actually single phase and unlike the ~original~ two phase sytem, single phase systems are 180 degrees out of phase with each other. Perhaps if the system were perfect to begin with then it would have been called a two phase system, because the *other* two phase system wouldn't have ever come to be. That phrase was already used so the new name is the one we have now and the new nomenclature stuck. It's as accurate a phrase as calling it as an 'opposing asynchronous bifurcated phasing system'.

Q3) Why 60Hz to begin with and Europe uses 50Hz instead of our 60Hz?
A3) Using 60Hz utilizes power most efficiently for, shall I say, 'man's use'. 50Hz is just a bastardizaion of what Nikola Tesla figured what was right all along. The Europeans didn't understand why changing the frequency by 10Hz would make much difference. Perhaps this is myth, but I heard that the Europeans changed the Hz to 50 because it fit better with the metric system. Who knows, but the fact remains they changed it and their power supply system isn't as efficient as the American 60Hz system.
Someday I'd like to figure out just how less efficient that 10Hz difference really is. Maybe do some calculations to see how much energy they've wasted for being so "smart".

**

Nikola Tesla figured all about AC/Transformers/Hz/three phase systems and many other things by himself! We could have been stuck with a vastly inferior system still in use today if we had started off wrong. What a genius he was and our text books still credit Edison for our modern way of life. Even when Edison was resoundedly proven wrong he still held on to his ideas that DC was better than AC, even going so far as to invent the electric chair to prove how much more dangerous AC was.

Edison really hated learning and using his mind to uncover things, rather than just endless experimentation. He was more of a showman IMHO. That, unfortunately, is much more celebrated in America than anything resembling the art of inventiveness.

One thing is for sure: without Tesla our carbon footprint -- ahum, energy usage -- would have been much greater than it is today!

P.S.
I've no references, just from what I have in me memory from school and all.
 
3 phase systems

3 phase systems

Nice bit of comments about Tesla vs. Edison. I believe that Edison was more of the business type, and Tesla was the scientist. After all, are there any units of measurement called the Edison? There is a unit called the Tesla :)

Three phase systems are great since they set up a rotating magnetic field in the air gap of an induction machine, and that field rotates at the same frequency of the system to which it is connected. This is the reason why the power produced is not pulsating, it is seen to be constant. This is not the case with two phase, and adding more phases seems to be less efficient.

A transformer and a motor/generator can be modeled by the same circuit pretty much, except the motor/generator has rotating parts, i.e. the aptly named rotor :)
 
Just one minor correction about Tesla. He didn't "invent" 3 phase power, he invented poly-phase power. His first systems were in fact 2 phase; the generators in Niagara built in 1895. In fact some of the big pumps from the turn of the century that failed and contributed to the flooding of New Orleans during Katrina were still using 2 phase Westinghouse built motors from when Tesla still worked there.

Tesla postulated that poly-phase was more efficient than single phase, which is correct, but there wasn't much improvement from 2 to 3 phases, just 1 fewer wire of smaller gauge! In fact, given that measuring tools were not yet as good as was necessary for a poly-phase system, 2 phase was easier to troubleshoot than 3 phase at first. The decision to build 3 phase generators and motors was that of George Westinghouse in around 1918 and it was based on reducing the cost of long distance distribution systems.
 
ron_o said:
Q3) Why 60Hz to begin with and Europe uses 50Hz instead of our 60Hz?
A3) Using 60Hz utilizes power most efficiently for, shall I say, 'man's use'. 50Hz is just a bastardizaion of what Nikola Tesla figured what was right all along. The Europeans didn't understand why changing the frequency by 10Hz would make much difference. Perhaps this is myth, but I heard that the Europeans changed the Hz to 50 because it fit better with the metric system. Who knows, but the fact remains they changed it and their power supply system isn't as efficient as the American 60Hz system.
Someday I'd like to figure out just how less efficient that 10Hz difference really is. Maybe do some calculations to see how much energy they've wasted for being so "smart".

.
Click to expand...
What makes you think that 60Hz is more efficient than 50Hz?
 
learning a lot, thanks! Why so we not use 400Hz distribution systems ?
 
augie47 said:
learning a lot, thanks! Why so we not use 400Hz distribution systems ?
Click to expand...
Good question. The greater the frequency, the greater the losses within the wires themselves. That is because the inductance and capacitance of wires (and other loads, for that matter) varies with frequency. So it would not be practical to use for residential or commercial applications. Also, it is not necessary. As long as the frequency is high enough to avoid the visible flicker, there is no need to make it higher.

Regarding flicker, imagine if the frequency we used was much lower, say one cycle per second (instead of 60). I should think that the overhead lights would be growing bright and then dim at the rate of once per second. How very annoying that would be. I suppose that 50 HZ is fast enough, or else it would not be used in many countries throughout the world.
 
Besoeker said:
What makes you think that 60Hz is more efficient than 50Hz?
Click to expand...

I learned it in school that Tesla figured it out mathematically, but I can't find the exact reference in the few books I have around handy. I'll keep looking for a better reference than I'm about to give. For now, I've found a few sites that mention 60Hz is more efficient:


**
http://users.telenet.be/worldstandards/electricity.htm#voltage

"When the German company AEG built the first European generating facility, its engineers decided to fix the frequency at 50 Hz, because the number 60 didn't 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. A mistake, however.

"Not only is 50 Hz 20% less effective in generation, it is 10-15% less efficient in transmission, it requires up to 30% larger windings and magnetic core materials in transformer construction. Electric motors are much less efficient at the lower frequency, and must also be made more robust to handle the electrical losses and the extra heat generated. Today, only a handful of countries (Antigua, Guyana, Peru, the Philippines, South Korea and the Leeward Islands) follow Tesla’s advice and use the 60 Hz frequency together with a voltage of 220-240 V."


**
http://www.school-for-champions.com/science/ac_world_volt_freq.htm

by Ron Kurtus
" ... [Tesla] had calculated that 60 cycles per second or 60Hz was the most effective frequency. Tesla later compromised to reduce the voltage to 110 volts for safety reasons.

Europe goes to 50Hz

"With the backing of the Westinghouse Company, Tesla's AC system became the standard in the United States. Meanwhile, the German company AEG started generating electricity and became a virtual monopoly in Europe. They decided to use 50Hz instead of 60Hz to better fit their metric standards, but they kept the voltage at 110V.

"Unfortunately, 50Hz AC has greater losses and is not as efficient as 60HZ. Due to the slower speed 50Hz electrical generators are 20% less effective than 60Hz generators. Electrical transmission at 50Hz is about 10-15% less efficient. 50Hz transformers require larger windings and 50Hz electric motors are less efficient than those meant to run at 60Hz. They are more costly to make to handle the electrical losses and the extra heat generated at the lower frequency."
 
I read something similar, although I am too lazy to look for it. To paraphrase...

Tesla's first systems in Niagara were also 25Hz 2 phase because he had to deal with existing turbine speed designs. But when applied to Arc lights and Edison's new incandescent light bulbs, the 25Hz made a detectable flicker. So Westinghouse then told Tesla to undertake investigating the best frequency to run at as a standard. He determined that 50Hz was the lowest at which the flicker was undetectable to humans (which may have leaked to AEG and became their standard), but later decided that 60Hz offered the advantage of being easily divisible into 360 degrees of rotation, which made rotor design a bit easier (which to me makes sense in light of the level of precision machining available at that time).
 
Status
Not open for further replies.
Top