Definition
A type of alternating current (AC) electrical power that uses three separate current waveforms offset from each other by 120 degrees, producing smooth, continuous power delivery. In gyroscopic instruments, three-phase AC is used to drive electric motors that spin the gyro rotor at high, stable speeds.
Plain English
Electrical power delivered in three overlapping streams instead of one. The three streams take turns peaking, so the motor receives steady power and spins very smoothly.
Context Anchor
Seen in discussions of electrically driven gyroscopic instruments, especially where the handbook explains how an electric gyro motor is powered.
Derivation
From Greek 'phasis' meaning 'appearance' or 'stage,' used in physics to describe the timing position of a wave. 'Three-phase' simply means three waves staged at different points in their cycle. Knowing this helps the term feel less mysterious: it is just three timed-out streams of electricity working together.
Why Pilots Care
Gyro instruments need a very steady, high rotor speed to give accurate readings. Three-phase power provides that smooth, consistent drive, so the instrument behaves predictably in flight.
Analogy
Think of three people pushing a merry-go-round at evenly spaced moments. Each push arrives just after the last one, so the motion stays smoother than it would with only one person pushing.
Intuition Check
Three-phase does not mean a three-step process. Here, it means three separate alternating electrical waves timed evenly apart.
Example Sentence 1
The attitude indicator's gyro is spun by a small three-phase electric motor powered from the aircraft's AC bus.
Example Sentence 2
During preflight the pilot confirms the alternator is producing three-phase output before relying on the electric gyros.