Definition
An alternating current electrical system in which two or more separate AC voltages are produced together, each offset in time (phase) from the others by a fixed amount. The most common form in aircraft is three-phase AC, where three voltages are produced 120 electrical degrees apart and delivered on three conductors.
Plain English
Instead of one AC voltage rising and falling on its own, polyphase AC is several AC voltages running side by side, each one timed to peak a little after the last. In aircraft, this is usually three voltages working together on three wires.
Context Anchor
Seen in aircraft electrical system discussions, especially when describing alternators, generators, power distribution, and AC motors.
Derivation
Poly comes from the Greek 'polus,' meaning 'many.' Phase refers to the timing position of a wave. So 'polyphase' simply means 'many phases' — several AC waves running together, each offset in timing.
Why Pilots Care
Three-phase AC drives most of the heavy electrical loads on transport aircraft — pumps, motors, and large transformers. If one phase is lost, those systems can run poorly or trip offline, which is why electrical panels show separate indications for each phase.
Analogy
Think of several people pushing a merry-go-round at evenly spaced points. Each push happens at a different time, so the motion stays smoother than it would with only one person pushing.
Intuition Check
Polyphase does not mean “extra strong” current. It means more than one AC wave is being produced or used, with the waves timed apart from each other.
Example Sentence 1
The aircraft's main generators supply 115-volt, 400-hertz three-phase AC, which is a polyphase alternating current system.
Example Sentence 2
Technicians check that polyphase alternating current remains balanced across all phases during engine run-up.