Originally posted by Jim Stewart
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Come on folks. This isn't rocket science. Alternating current has a time-dependent voltage. That time dependent voltage is regular and oscillatory, meaning that it can be represented by a single sine or cosine wave at a fixed frequency and amplitude (neglecting small variations due to generation). Mathematically, one way to express that is A*sin(t+p) where t is the time and p is normally the Greek letter Phi, known as the phase angle. This is an arbitrary angle that is only useful when comparing multiple signals; it essentially defines a common starting point for the timer used to clock the signal. So three phase power consists of three time dependent voltages that is represented as A*sin(t), A*sin(t+120*), A*sin(t+240*). Split-phase power is represented as A*sin(t) and A*sin(t+180*). In mathematics, physics, and electrical engineering (and mechanical if we were talking about waves in some kind of solid or liquid medium) we would call each of these a "phase" because they are identical in amplitude and frequency - only their phase angle is different.
Just like phase, voltage is never an absolute quantity. It always needs to be referenced against something for it to have any meaning at all. If you only have a single channel on your oscilloscope, you will always only get one sine wave. The amplitude and offset change based on what you are referencing but it can never display more than one sine wave (on noisy signals, it may look like it does because of persistence on the screen), regardless of whether you are on a single phase, split phase, 90* two phase, three phase, 6 phase, 5 phase, whateverthehell number of phase system you're on.
The REASON electricians don't refer to split phase as two phase is to distinguish it from the three phases being provided by the utility company (each separated by 120*) and older 90* 2-phase systems (which evidently still exist in some places?). Mathematically, physically, scientifically, there are two phases in a split phase system, each separated by a phase angle of 180*.
Doozer, that's a great suggestion. I will use whompey-jawed from now on. If you catch me using "run-out" please correct me.
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