two phase power

if they were 180 deg apart, then it wouldn't be different from single phase

i think two-phase systems were created to provide revolving field for motor starting

| And also how would you decompose these "balanced" phasors into their | symetrical components? |

Va = V /_0 Vb = V /_90 Vc = 0

You can think of "two phase" power as two phases of a four-phase system.

Bill

-- Ferme le Bush

in this case sum of two line voltages ain't zero. (should it be zero always?)

If Va=V @0 andVb= V @90 then you only have one component. if the Va and Vb are unbalanced in phase or magnitude then they can be resolved into two two phase balanced sets in a manner similar to that for 3 phase. One is forward rotating and the other backward. This is done for modelling of a single phase motor as an unbalanced two phase device.

That was the problem with (3 wire) two phase systems. The neutral carried current. You could transfer more power by making the third wire another phase of a 3 phase system. That's why modern power is 3 phase.

(There can be 4 wire and 5 wire 2 phase systems. The 4 wire system is

2 unconnected single phase systems at 90 degrees, the 5 wire is essentially two Edison systems at 90 degrees connected at their neutrals)

would it be disadvantageous if we were to make power transmission in 4 phases or 5 phases? or is it only the cost factor which prevents us from doing so?

Certain mine duty power systems were more than three phases in order to smooth out the torques. There are lots of research going on in academia on making more than three phase motors. Hamid Toliyat at Texas A & M has a lot of work fgoing on with five phase machines. This becomes feasible because we can use a power electronic interface to convert three phases to four or five phases, whereas we would have to change the entire distribution net works for the world.

Six-phase is easy to obtain by phase shifting using Y & Delta windings. That is the principle of 12-pulse converters. Six are from the original voltage, and six are phase shifted. It eliminates 5th and 7th harmonics, making the current cleaner and easier to filter.

Ben Miller

I don't think there is any advantage, and the disadvantage is we have more things to break (4 or 5 hot conductors, 4 or 5 transformers instead of 3 each time we need to step the voltage down or up)

Having said that, polyphase has been done. Somewhere on the net is a photo of two bizarre transmission line towers. One carries 6 phase, the other 12 phase. The conductors are arranged in a circle.

I do not see any advantage in going to more than three phases for power transmission. It is true, however, that if you need to double up on the transmission line, going to six phase (two three-phase systems shifted by

60° from one another is not a big deal.

The advantage of more phases arises when rectifiers are used. More phases will usually end up with less ripple and conductor currents that are closer to sine waveforms.

Such systems are popular for pulsing particle accelerator magnets. Kinetic energy is stored in a rotating machine. Upon thyristor rectification, the voltage is applied across a magnet winding. Current rises linearly with time. After the peak of the magnetic field is reached, the thyristors are triggered to provide inversion so that the inductive energy goes back into replacing energy extracted from the rotating machine.

Bill

-- Ferme le Bush

The gain from the additional phases drops significantly after three phases. It takes two wires to carry a single phase, three to carry two, three for three phase, and above that the number of wires/phase is constant, IIRC. Thus, there is no advantage to distributing more phases. Certainly there may be reason to have more than three phases, but cost isn't it. ...at least AIUI.

There have been 6 phase distribution systems built. The advantage is that the line is more compact than it would be for 3 phase. phase to phase insulation is the same as phase to ground. I know it was done in some urban areas but I don't know whether it is still being done.