Definition
The order in which the three voltage peaks of a three-phase AC electrical system reach their maximum values, typically labeled A-B-C or 1-2-3. Phase rotation determines the direction in which a three-phase motor will turn and must match the equipment's design rotation for correct operation.
Plain English
In a three-phase electrical system, each of the three wires carries power that peaks at a slightly different time. Phase rotation is the sequence in which those peaks arrive. That sequence decides which way a three-phase motor spins.
Context Anchor
Seen in aircraft electrical power and maintenance discussions, especially when checking generators, external power, or three-phase equipment.
Derivation
Phase comes from the Greek phasis, meaning appearance or stage of a cycle. Rotation refers to the order in which those stages appear. Together the term describes the rotating sequence of voltage peaks in a three-phase system.
Why Pilots Care
Incorrect phase rotation will make motors run backward, causing hydraulic pumps, fans, or other systems to operate in the wrong direction.
Grounding Statement
In a three-phase electrical system, three voltage waves rise and fall in sequence; phase rotation names that sequence.
Intuition Check
Do not read “rotation” as a part physically spinning. In this term, rotation means the order of repeating electrical peaks.
Example Sentence 1
Before connecting external power to the aircraft's three-phase bus, the technician verified correct phase rotation to prevent the hydraulic pump from running in reverse.
Example Sentence 2
Swapping two leads reversed the phase rotation and made the cooling fan run backward.