what is 1-phase or 3-phase ?

After I've done lots of searching online, here is the best explanation, I think,  about what phase means in electricity physics. Other explanations are either misleading or simply just wrong, which could lead to bad consequences if you think the way they think and try to do something with that thinking.  

Simply put, it's the number of times you combine any two hot lines. For instance, if you have two phases( or two hot lines ), you only get one combination therefore your system is one-phase system. If you have three phases ( or three hot lines ), you get three different combinations therefore your system is three-phase system.

Link goes here: http://www.justanswer.com/home-improvement/0u6yf-exactly-does-phase-mean-wired-differently.html

Expert:  Inactive replied 6 years ago.

Three phase just means that you have 3 "hot" phases. Each one of those phases is 120degrees out of phase with each other. When wiring motors in three phase, you will use 4 wires, 3 hots and a ground. Which is why you will only get 120v from phase to ground and 208v from phase to phase. It really is in the way the power company set up the electricity to your facility with the configuration of the transformers.

Single phase is 180degrees out of phase with each other. When wiring motors such as these, you will use either 2 hots and a ground or 1 hot-neutral-ground. Which means that you will get 120v phase to ground and 240v phase to phase.

1.  1-Phase, 3-Wire System
The most common supply system used in residential applications today is a 1-phase, 3-wire supply system. This system provides 240 volts from the A phase connection to the B phase connection and 1 20 volts between either the A or B phase connection and neutral.



2. 3-Phase, 3-Wire, 240 Volt
Load centers can also be used in commercial applications that may require 3-phase power. Two of the more common approaches for providing 3-phase power in these applications are shown below.

 3. 3-Phase, 4-Wire, 208Y/120V
There are multiple approaches for 3-phase, 4-wire services. The following illustration shows the utility transformer secondary windings for one of the more common approaches, a 3-phase, 4-wire, 1 20/208 volt system. This system uses a wye transformer configuration with a grounded neutral (N). This system provides 1 20 volts between any phase connection and neutral and 208 volts between any two phases.