Why Is there 3-Phase?

What are the top 10 (or 2 or 3) reasons for 3-phase power?
What advantages are there over single phase?

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There is basically only one reason:
A more even load is placed on the prime mover and more even power supplied to the load.
Chuck P.
The fact that 3 phase motors are simpler is just a side benefit.

Economics as Salmon Egg indicates: a) cheaper transmission-systems and lower losses per Kw delivered b) better use of iron in generators, motors and transformers c)Better operation of motors as well as being simpler than single phase motors (single phase motors are less efficient than 3 phase motors and are not inherently self starting). d) Although not initially recognised, lower electric and magnetic fields from transmission lines than would be the case with single phase. e)The ability to supply single phase loads as well as 3 phase loads. f)The ability to use 3 phase, 6 phase or 12 phase rectifiers to get smoother DC without filtering where needed.
Shall I go on?
Sure the load on the prime mover etc. is more even but that is actually an insignificant factor considering the mechanical time constants involved.

Don Kelly wrote:
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I'll pick what is behind door number c.!
Back at the end of the 19th century when Tesla was inventing polyphase electric systems, the main interest at the time was largely industrial because of the scale of steam power plants required to generate electricity as well as it's novelty which made it expensive. The genius of Tesla who basically invented the 20th century, revolved around several key patents.
One was the use of alternating current. The genius of this was that it allowed transformers to adjust voltage levels along the distribution network. The DC Edison system had no such easy methods and in spite of a hard-fought political battle and Occam's Razor which says the simplest system is always the "correct" one, Edison's failure to solve this distribution problem led to demise of DC distribution until quite modern times when DC-DC conversion advanced beyond the motor-generator set level.
The second was Tesla's invention of the induction motor (which he actually invented back in Europe before coming to the USA). And induction motor is really sort of a variation on a transformer and also requires alternating current for that reason. It eliminated all the commutators, sparking and other deficiencies associated with DC motors. The principle of the 3 phase induction motor is the rotating magnetic field generated by sets of coils driven by AC phases 120 degrees apart. The genius here is that if you use 3 phases, you find that the fields add in such a manner that the rotating field vector is of constant amplitude! A 2 phase motor isn't that way and a greater number of phases than 3 just adds complexity for no reason unless you have a special application of some kind (say low ripple in a AC-DC rectifier).
For you sitting here in the 21st century it's hard to understand the huge advance the Tesla polyphase induction motor made on industry. Your view of some "shop" is all these rows of machines sitting on the factory floor with hoards of happy workers turning out parts. In the 19th century it wasn't like that. There were rows of machines alright, but overhead in the factory were all these axles and pulleys running down to other pulleys on each machine which was powered by a flat leather belt. To "start" your machine you had to flip the flat belt onto the ceiling pulley with a long pole. The whole room was a maze of dangerous flapping flat belts. Outside the factory "stationary engineers" fired and maintained the steam engine that ran all the overhead axles, pulleys, and belts. The 20th century norm of a shop where each machine was individual and self-powered by electricity came from Tesla and his patents.
As Don pointed out, one huge advantage of a 3 phase induction motor over a single phase motor is that the three phase motor is simple, efficient and self-starting. Single phase motors take special efforts to start. Also by replacing a local steam engine with a HUGE generating plant steam engine you reap the economics of scale and the cost of the stationary operating engineers is spread among many users. Thus, industrially, the whole concept of electric-powered beltless machine tools must have seemed like "Star Trek" to mechanics of the 19th century. It was such genius and so obviously "correct" that the standard for whole next century was based upon these Tesla patents and ideas!
And behind door e. we also have the bonus that the industrial polyphase electric source also allowed single phase uses as well for lighting and heating applications!
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Not necessary. You already said it!
I will note however, that in spite of agreements granting rights to all this technology to Westinghouse, eventually the Westinghouse investors reneged on the deal cheating Tesla out of most of the revenue for his patents. There are vague rumors around that Tesla was tapped by the government before or during WWII to work on some things and that at the time he was waving around a check from Westinghouse made out to him for one million dollars, but these rumors or his participation in any WWII projects have never been confirmed. The standard story is he died destitute in a NYC hotel in 1943.
Benj (Those who do not study history are doomed to repeat it!)

On 5/11/07 10:41 PM, in article snipped-for-privacy@l77g2000hsb.googlegroups.com, "Benj"
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My nit picking comment is that two-phase rotating field is just as constant amplitude and speed as three-phase. You still need three wires because neutral current is not zero even with a balanced load. If you go four-phase, the equivalent of center-tapping the two phases of two-phase you can avoid using a neutral but then you need four conductors for the lines. You get no advantage in terms of winding motors.
Bill -- Fermez le Bush--about two years to go.

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A 2 phase system is that way. Remember math where the sum of the square of the sine and the square of the cosine is 1 everywhere? Replace the cosine with the sine shifted 90 degrees, and you have the phase relationship for a 2 phase system. Two phase systems were fairly common in the early days of electricity. The problem with 2 phase systems is the 3 wire version has a heavy neutral current, while a balanced 3+ phase system will have zero neutral current. A 4 or 5 wire 2 phase system doesn't have this problem, but takes more wires to transfer the power than 3 phase (4 hots vs. 3 hots) In other words, 3 phase systems can transfer more power with the same amount of copper than 2 phase could.
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I once worked in a building complex that once had that system. 99.9% of the pulley system was long gone, but there were occasional large pulleys still up in the ceiling, or remains of slots in the ceilings for belts etc.

On 5/12/07 8:44 PM, in article f261j6$ier$ snipped-for-privacy@pcls6.std.com, "Michael Moroney"
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As a kid, I remember shoe repair shops often had various machines powered off of a main shaft by using pulleys, belts, and clutches. Circa 1943.
Bill -- Fermez le Bush--about two years to go.

wrote:
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There is a bar in downtown Denver that has ceiling fans that are all operated by one motor and a leather belt that runs all around the bar to each fan. It's over by the new ball park IIRC.

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Yes. Examining a full wave rectified single phase sine wave has a LOT of time below the RMS value.
Looking at the three peaks a three phase probing yields show that there is a lot of time when the sine value is up near of above the RMS value for a single sine wave.
Far more efficient at the load end. It's like only needing 60 cycle transformers but getting the value of a 180Hz wave.
I saw an explanation of the DC intertie once that stated that the feed was no more than a rectified 3 phase AC set-up.
A series of "bumps"... all at a good voltage level, never going to zero.

On 5/10/07 2:12 PM, in article p8Sdnfh16N46F97bnZ2dnUVZ snipped-for-privacy@centurytel.net, "ecarecar"
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Economics.
You save money on generation and transmission because materials are used more effectively. But there are other advantages as well. All these advantages are achievable in other ways at higher cost.
The sheer amount of electrical power used overwhelms the advantages of other sources. For example, high voltage dc would be of great advantage for x-ray production but it is not economically feasible to drive x-ray tubes from a suitable dc source.
Bill -- Fermez le Bush--about two years to go.

I'm not trying to pull an Edison. Specifically, what makes 3-phase more efficient than single phase?
Is it that you can kinda, sort of have a complete circuit with only one wire? Or, that you can have three complete circuits with only three wires instead of six?
Remember, I am pre-admitting ignorance, so you don't have to tell me how ignorant I am.
Salmon Egg wrote:
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ecarecar wrote:
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as was explained to me in collage it's 3 times the transmitted power with one third the copper... but then a lot of things they said in collage turned out to be wrong.
for a much more elaborate explination see: http://en.wikipedia.org/wiki/Three-phase_electric_power

wrote:
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Perhaps like the spelling of the word COLLEGE.
You are a collage of insanity! Bwuahahahah!

On 5/10/07 8:19 PM, in article 4643e0c0$0$1391$ snipped-for-privacy@roadrunner.com,
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Just to nit pick, it is three times the power of a single pair for the 3/2 amount of copper for a single pair. Consider three single phase systems. These deliver 3 times the power that can be delivered by a single pair. They use 6 conductors. If these are combined into one three-phase line, the common neutral can be eliminated for a balanced load. Thus, only three wires are required and that is 3/2 the copper for a single pair. Thus, to deliver a given amount of power, you can do with half the copper.
One can argue that you now need more insulation to insulate for the higher line to line voltage compared to line to neutral. Even if that were a problem, the insulation is much cheaper than the copper. Have you ever heard of wire being stolen for the insulation?
Bill -- Fermez le Bush--about two years to go.

Salmon Egg wrote:
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that makes much more sense. i probably misremembered the lecture. speculation: with the cost of copper (and coopers) rising may we see more 3 phase appliances in residential?
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around here they will steal darned near anything.... but the insulation is kinda hard to get off by itself :)

ecarecar wrote:
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Um, it's FAR better at killing small animals especially dogs and cats which is why it's always the current of choice for electric chairs! :-)
Benj (Oh, yeah, I lied about that, but everything is fair in love and electric wars!)

| I'm not trying to pull an Edison. Specifically, what makes 3-phase more | efficient | than single phase?
We have to consider 2 reasons to use three phase power. One is because there are demands/needs for three phase power, such as very large motors. That may or may not be more efficient. But you can't serve those needs unless you have the phases. You can get single phase out of three phase easily, but not the other way around. The other is because of efficiency even if the loads are all single phase (evenly balanced on three phases).
But let's consider a case of efficiency.
Comparing 3-phase power to a 2-pole single phase system (which looks like it could be called "2-phase" but that isn't standard terminology): If you have a need for 600 kW of power, you could get that by having 200 kW on each of 3 line wires, or 300 kW on each of 2 line wires (pick your own voltage to figure the amps). For the line wires alone, it works out basically even, if cross sectional area of wires equates to cost, and in large scale systems it does. However, unless all your loads are only line-to-line loads, you need a neutral wire, too. Potentially it could see the full current a line wire could see, in the most unbalanced case (before assumping harmonics or differential power factors). So that neutral will need to have the same size as the line wire, which will be 50% larger in the single phase case.
Consider another case.
For a given line-to-neutral/ground voltage, the line-to-line voltage will be higher in single phase, and in certain circumstances require a higher level of insulation. Or considered in reverse, for a given level of insulation that handles line-to-line voltages, three phase lets you have a 15.47% higher voltage (and thus get more power with the same amps) over what you can do in single phase. This would apply even if the load does not need a neutral and it is not carried over the wiring.
I don't know enough about motors, yet, to tell you how they might be more efficient in three phase than in single phase. But I know that as the motor power goes up, the probability a given installation uses three phase over single phase also goes up. But this could be due to issues that are similar to the cases for transmitting and distributing power.
| Is it that you can kinda, sort of have a complete circuit with only one | wire?
No.
| Or, that you can have three complete circuits with only three wires | instead of six?
You can have 3 "circuits" with 4 wires, vs. 2 "circuits" with 3 wires.
| Remember, I am pre-admitting ignorance, so you don't have to tell me how | ignorant I am.
I'll refrain. We all started there at some point.

On 13 May 2007 04:19:04 GMT, snipped-for-privacy@ipal.net wrote:
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Excellent reply, Phil.

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What? Nearly ALL X-ray tubes are driven by DC!
I have made several hundred supplies for such devices.
One was +90kV and -90kV for a 180kV DC electron beam splashing onto a Palladium target in a $900 German made X-ray tube which was/is used at every airport in the country!
The cleaner the DC E-beam, the cleaner the X-ray flux.The cleaner the X-ray flux, the better the contrast ratio of the imagery.
Where did you ever get the idea that X-ray tubes were AC driven?
PURE DC.
Even the 50kV supply that we sent to LANL to look at nukes was ALL DC ALL THE TIME.