Why 3-phase power?

[vaughgc]

Ok all you sparkies. Here's a chance to help out a lowly chemical engineer. I think I have wrapped my head around 3-phase power as far as what it is, but I have a question (probably a stupid one). Why is power transmitted in 3-phase vs say 4-phases separated by pi/2 radians or 2 phases separated by pi radians. I'm just trying to learn for my own gratification.

War Eagle!

 

[davidbeach]

The amount of power transmitted per unit of conductor is maximized at 3 phase. Three phase also is the lowest number of phases that allows continuous power transmission, as opposed to pulsed power in two-phase (quadrature - very rare) or single phase.

 

[waross]

2 and 4 phase were common 100 years ago acording to the old text books. Three phase has the advantage that the instanteneous sum of sin waves seperated by 120 electrical degrees sum to zero. Thus a delta transformer connection may be used. This connection would be a short circuit on a 2 or 4 phase connection.
Three phase allows more power to be transmitted over the same amount of copper than single phase, two phase, or four phase.
From three phase we can readily derive six phase and occasionally twelve phase. The higher number of phases have the advantage of less ripple in rectifier circuits for large DC supplies and large motor drives.
As a guess, I would expect three phases to generate more starting torque than two phases.
Note. Four phase was derived from two phase by connecting the center taps of the transformers together. It may have had a wire count advantage for lighting circuits, but heavy loads and multi phase loads saw the same power on four phase systems as on two phase systems.
respectfully

 

[jraef]

vaughgc,
davidbeach hit it. (nothing wrong with what you said waross, but I seriously doubt a Chem.E. understands a delta xmr. ;-) )
1 phase = discontinuous power but only 2 wires
2 phase = more continuous power but requires 4 wires
3 phase = continuous power, only 3 wires
4 phase = continuous power, 8 wires
etc. etc.

What we mean by the term "continuous" power is that in 1 phase supplies, there is a moment in each AC sine wave cycle where the power flowing through the system, i.e. the work being performed, is zero. When you have a polyphase system, i.e. 2 or more phases, when one phase is crossing zero, power is still flowing through another one. The more the merrier in that case, but when you get above 3 phases, the more wires you need to carry that power. 30 phase would be fantastic in that regard, but running 60 wires would be ridiculous. 3 is a magic number compromise offering the fewest number of wires with the most continuity of power flow.

 

[itsmoked]

Nicely sumarized jraef.

Once more vaughgc; The torque required to drive the generators, and torque developed ultimately by motors will be in a pulsing form in single phase but is smoothly delivered in a three phase system. This is usually unnoticed in small machines but in large machines...

Picture the bolts holding down a 5,000HP motor running on single phase with pulsating torque verse no pulsation 3 phase.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ters]

I would just add a bit of history. Three phase AC system was invented by Nikola Tesla, probably the biggest and at the same time most ignored electricity genius of all times, a miracle man but with very a poor business attitude... His patents were bought by Westinghouse and some other businessmen and that is how it started some 110 years ago. Tesla’s initial idea was to develop AC induction motor, where it appeared that rotating magnetic field would be the most optimal (in terms of efficiency, torque, cost and complexity) with three phases. This led him to invention of 3 phase generators and transformers and here we are 100 years later with billions of 3 phase machines around the globe which in essence did not change at all since Tesla’s time….

 

[cuky2000]

Here are factors that favor the use of three-phase over other phase configurations:
a) The ratio of losses over number of conductors for a transmission line is less than other systems for the same amount of power transmitted for the same distance.
b) The winding of motors and generators for the same capacity produce one of the smaller sizings of rotating machine.
c) Phase to phase voltage are symmetrical requiring symmetrical clearances.
d) O&M is easier with less human error supplying single phase power.


Paraphrasing Yogi Berra, engineering is 90% technical and the other half is administrative.

 

[vaughgc]

Thanks guys. The quick and dirty that I gather from this is "more bang for the buck." i.e. The most juice per unit conductor.

War Eagle!

 

[Skogsgurra]

A short addition: Tesla actually started with a two-phase system. Using a sine and a cosine voltage and a common zero (neutral).

His inspiration was the equation for the circle, r^2 = x^2 + y^2 or, on complex form r/alpha = r*[cos(alpha) + isin(alpha)]. He realised that that equation represented a rotating vector and also that that rotating vector could drag a cylindric object with it to produce a rotating motion.

That's how he built his first brushless motor. Two voltages 90 degrees apart. I am not even sure if it was Tesla that introduced the three-phase system. I think someone else did that (Steinmetz? Westinghouse?)

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

Tesla conceived of polyphase in 1888 and extolled the virtues of 3 phase, but the first actual implementation of 3 phase was in 1891 in Germany by Michail von Dolivo-Dobrowolsky and Oskar von Miller. Westinghouse, using Tesla's patents, built the first AC distribution system at Niagara Falls in the US in 1895 (4 years after the Germans) and it was 2 phase. But because he was the first to conceive of it (on paper anyway), Tesla is considered the "father" of 3 phase power.

 

[ters]

Tesla suffered of some obsessive disorder and was obsesses by number 3, maybe that is why 3 phases...

Tesla was, in a similarly fuzzy way, father of many other things inducing radio and radar. He had about 700 patents, some of which never materialized and have been subject to various conspiracies and controversies until today (his work related to teleforce weapons, wireless energy transmission, etc). He filed his patent for radio in 1897, 3 years before Marconi did the same, and Tesla’s patent application was approved by the US patent office. Then in 1904 it was suddenly reversed and radio patent rights were given to Marconi... Tesla sued... and after some 30 years, just after he died in 1943, the court decided that Tesla was actually father of radio not only on the paper...

 

[jraef]

Tesla probably suffered from OCD, but I have never heard the number 3 issue. I did read that he would only make left turns when walking, so to go right he had to make 3 consecutive lefts. It is also written that he could not eat a meal without first calculating the volume of material on the plate. I question some of these anecdotes however, only because they are anecdotes and I know for a fact that people tend to exaggerate eccentricities they observe in others. He was undoubtedly quirky, as many geniuses are, but probably not quite as bizarre as people like to believe.

I knew a very brilliant nuclear physicist once who worked at Lawrence Livermore Lab. He had OCD that manifested in his constantly checking to see if he still had his car keys, even when he was driving his car. It was so distracting for him that he often couldn't remember to tie his shoes and shave every day. But to hear other people talk about him you would think he was a complete loon who fouled himself and ate only oatmeal cookies wrapped in foil (actual anecdotes I heard people say about him). None of those other things were true, he was just always disheveled looking, I think mostly because it was just so difficult for him to get out of the house and off to work every day.

 

[ters]

The list of Tesla's phobias is rather long
Columbiphilia (pigeon-love), kakiphobia (fear of dirt), scotophilia (love of the dark), pathophobia (fear of germs), spherophobia (fear of round objects), triphilia (obsession with the number 3), and visual and auditory hallucinations.

http://sprott.physics.wisc.edu/PICKOVER/strange.htm

The other one, for which I don’t happen to know a correct English name is some kind of “women-phobia” - never wanted to be close to any women...

Being such an eccentric, Tesla was misunderstood most of the time. His achievements were also constantly (for decades) downplayed by Edison, who was mightily and wealthy at that time but for some reason kept desperately pushing his DC current system regardless of obvious advantages of AC poli-phase one.

It is hard to understand how a human brain could be so different from one individual to another. Tesla was and still is a mystery. When he died, FBI took possession of anything left behind, probably because of his work on cosmic rays, teleforce weapons, etc, and it was only about 5 years later when his relatives managed to get back most (but reportedly not all) of it. Today, it is exhibited in his museum in Belgrade – an unbelievable collection of 160 000 original documents.

"Were we to seize and eliminate from our industrial world the results of Mr. Tesla's work, the wheels of industry would cease to turn, our electric cars and trains would stop, our towns would be dark, our mills would be dead and idle. Yes, so far reaching is his work that it has become the warp and woof of industry." (B. A. Behrend)