"What is a phase?" I can remember asking that question as my mind tried to relate phase to electricity. When I started building welders a few years back, the need to know intensified. Improperly aligned phases will buck or oppose each other and has the potential to destroy, devastatingly. (Please understand safety proceedures and protocols when it comes to electricity and follow them faithfully. A mistake or error in judgement can kill you unmercifully) but, before we get into the subject of phase, let's take a brief look at how AC power is produced.
A generator is a device that converts a source of power (fossil fuels, wind, solar, geothermal, nuclear or hydro) into electricity.
The process involves spinning a magnet inside a coil of wire at a predetermined speed. In the case of mains power production, a magnet spins at a rate of 3600 rpm, rounds-per-minute, producing AC voltage and current at 60 Hertz or 60 cycles per second. One single cycle is a complete 360 degree revolution of the magnet.
When we refer to a generator in terms of poles, we are describing the number of magnets and the speed the generator will need to spin. A 2-pole generator has one magnet with a North pole and a South pole, hence, 2-poles. This generator needs to spin at 3600 rpm ( rounds per minute ) to achieve the 60 Hertz target : 3600 rpm / 60 seconds = 60 cycles per second. A 4-pole generator has two magnets and needs to spin at 1800 rpm because with 4-poles, one complete 360 degree revolution delivers 2 cycles: N,S,N,S.
For the sake of clarity and to minimize confusion, the remainder of this
text will focus on the coil arrangement of a 2-pole generator.
A cycle is one complete revolution of the generator armature (a 2-pole
magnet) where the magnetic flux (field) of the magnet cuts through the
windings of the 2 coils inducing a voltage and current. This process is
represented by the sine wave:
This is single phase.
My dictionary defines phase as: "A particular point in the time of a cycle; measured from some arbitrary zero and expressed as an angle." To understand this definition, let's complete the generator.
A cycle begins at zero degrees and zero volts. The magnet begins inducing
a voltage and current in the first coil set. This is the first phase of the
cycle.
After 120 degrees of rotation, and starting at zero volts, the magnet begins
to induce a voltage and current in the second coil set. This is phase 2 of
the cycle.
Another 120 degrees of rotation, and starting at zero volts, the magnet
begins to induce a voltage and current in the third coil set. This is phase 3
of the cycle. This can be represented as follows:
Each phase is 120 volts rms (root mean square) and 120 degrees out of phase with the next.
I trust that this description of 3 Phase does not remain arbitrary. If You discover a grey area, let me know. I am faithful to correct it.