Apprentice path · Stage 3: The theory you cannot fake later · Lesson 12

Three-phase, gently

Three-phase is three AC supplies doing the same thing at the same speed, but each starting a third of a cycle after the last.

Why this matters on the job

It is most of the power in commercial and industrial work, and it is how a motor turns without anything mechanical telling it which way to go.

How to think about it

  1. All three phases run at the same frequency in a balanced supply.
  2. Each one is shifted by one third of a cycle, which is 120 degrees.
  3. Together they can create a rotating magnetic field in a motor.

Worked through

A full cycle is 360 degrees. Split it three ways and each share is 120 degrees. That is why the three phases are drawn 120 degrees apart, and why 120 + 120 + 120 comes back to 360 -- the three of them fill one complete cycle between them, with none left over and none overlapping.

Check yourself: Why are the three phases spaced 120 degrees apart rather than some other angle?

Answer: Because a complete cycle is 360 degrees, and three equal shares of 360 is 120 each.

Equal spacing is what makes the supply balanced. Three equal shares of one cycle is 360 / 3 = 120 degrees, and that even spacing is what produces a smoothly rotating field instead of a lurching one.

Watch out for: The colours on a phase diagram are there to tell the three phases apart on paper. They are not wiring colour instructions, and using them as such is how people get conductors wrong.
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Written from the owner's poster for this lesson. Nothing added beyond what the poster states.

Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.