Two Phase - Three Phase - Whats the scoop?

[DaVinci]

I am a little embarassed asking this, as I was an aircraft electrician at one time, but have since forgotten the answer. :bugeye:

When dealing with house electricity, you commonly hear the terms two phase and three phase. Now I understand this to be that the 'waves' are 'out of phase' by a certain degree.

But why do we have them? Why multiple phases? Simple so we can tie two together to get 220V for certain appliances? Is it simply a matter of convience that we can bring in more on the same line?

[Averagesupernova]

There really is no such thing as two phase. Have a look at the earthing alternatives thread and you will find some good info.

[DaVinci]

This is actually the info I was looking for....

http://ths.gardenweb.com/forums/load/wiring/msg112302526261.html

[brewnog]

I know he was corrected, but in that thread one guy is bleating on about 2 phase systems. Ignore him!

[Dual Op Amp]

...........

[pack_rat2]

There IS such a thing as two-phase AC. It's used with servo motors, where the phase angle determines position. But it's not used for AC power.

[Ivan Seeking]

... In Germany and Switzerland, where three-phase power was originated and developed, it is known as Drehstrom, "rotating current" for this property of constant power. Ordinary AC is called Wechselstrom, or "change current." Nikola Tesla, the discoverer of polyphase currents and inventor of the induction motor, employed two-phase current, where the phase difference is 90°. This also can be used to create a rotating magnetic field, and is more efficient than single-phase, but is not quite as advantageous as three-phase. Two-phase power was once rather common in the United States, where Tesla was important in the introduction of AC, but has now gone completely out of use.

Two-phase can be supplied over three wires, but there is no true neutral, since the phases are not symmetrical. However, it is always easy to double the number of phases in a transformer secondary by making two secondary windings and connecting them in opposing phases. Four-phase does have a neutral, like three-phase, but requires four wires. In fact, three-phase is more economical than any other number of phases. For applications like rectifiers and synchronous converters where DC is produced, it is most efficient to use six-phase AC input, which is easily produced from three-phase in a transformer. [continued]
http://www.du.edu/~jcalvert/tech/threeph.htm

[Astronuc]

Domestic or house current in US is single phase (line-to-neutral) 120 V, 60 Hz.

IIRC, 240 volt comes from 2 line to neutral phases.

T&D systems use 3 phase, each 120° out of phase to the others. This also helps to reduce inductive line losses.

[exequor]

i was the one that started the "Earthing alternatives" thread and i now realise that what peope are referring to as 3 phase and 2 phase (which i was referring to myself), is the misuse of a term as i understood from Averagesupernova. What they are referring to as the different phases is just the difference in the voltage (and current) but as i learnt it is much more than that.

[wire2]

>Domestic or house current in US is single phase (line-to-neutral) 120 V, 60 Hz.
>IIRC, 240 volt comes from 2 line to neutral phases.

Actually domestic 240 volt is single phase with a center tap typically grounded and identified as neutral.
The confusion stems from the assumption that 3 live lines equals 3 phases, so 2 live lines are 2 phases. Not so.
If we label the 3 lines A, B, and C, we create the phase relationships of AB, AC, and BC. If one fuse blows, say line B, we lose AB *and* BC, so we're down to single phase of AC only.