Because there are three positive charged conductors and there is electrical pressure in between.
Your question is rather vague. If you are asking what do you call a group of single-phase transformers, connected to supply three phase, then the answer is a 'three-phase transformer bank'.
How much current equipments are drawing from three phases (combined or individual) is called three phase load and how much supply we are getting from source is called three phase supply. Ex. In INDIA single phase supply is 220 volt ac with neutral. So in three phase supply all RYB phases should be 220 volts ac with neutral individually. If we calculate with phase to phase than it shoulb be 415 volts ac.
changes of state are called phase changes.
single phase, double phase & three phase
This point is call the "triple point" and this is where all three lines meet together on the phase diagram.
I think you mean 'single-phase supply', rather than 'phase supply'. All high-voltage a.c. transmission and distribution systems are three-phase systems. This is because, for a given load, a three-phase system uses less copper than a single-phase system. Three-phase generators produce three 'phase voltages', each displaced, by 120 electrical degrees. These voltages are produced in three windings which are electrically connected in what is called a 'delta' configuration, with each 'corner' of the delta connected to the transmission system by 'line' conductors. Three-phase systems are either 'three-wire' or 'four-wire' systems. Generally, three-phase, three-wire, systems are used for high-voltage transmission and distribution, whereas three-phase, four-wire, systems are typically (but not always) used for low-voltage distribution. Three-wire systems comprise three conductors called 'line conductors'; four-wire systems comprise three 'line conductors' and a 'neutral conductor'. Depending the voltage standards used in the country in which you live, a single-phase supply is obtained either by connecting single-phase load between any two line conductors, or between any one line conductor and the neutral conductor.
There is no such thing as a two phase instrument. There is only single phase and three phase. You can only have: single phase/ one pole single phase/ two pole three phase/ three pole
Yes, there is a difference between single phase and three phase circuits.
Three phase is ac
You don't. A three phase motor will not start unless it is connected to a three phase supply.
Yes, you can use three single phase transformers on three phase applications and interconnect them to serve as a single transformer or you could use one single phase transformer fed from two of the three lines in a step down application.
Three phase or two phase? Three phase requires three large wires for the current needed