Definition
The small voltage produced by a generator or alternator due to residual magnetism remaining in the field poles when no external excitation current is applied. This leftover magnetism allows the machine to begin generating output as it starts to rotate, before the field is fully energized.
Plain English
A small amount of electricity a generator can produce on its own at startup, because its iron parts still hold a little magnetism from the last time it ran. That tiny bit of magnetism is enough to get the generator going.
Context Anchor
Seen in aircraft electrical-system maintenance when checking whether a generator can start building normal output voltage.
Derivation
Residual comes from the Latin residuus, meaning 'remaining' or 'left behind.' Here it points to the magnetism — and therefore the voltage — that is left behind in the generator's iron core after it has been shut down.
Why Pilots Care
Low or absent residual voltage prevents the generator from building normal output, which can leave an aircraft without electrical power after engine start.
Intuition Check
Residual voltage does not mean normal usable generator power. It means a small remaining voltage that can help the generator begin producing normal output.
Example Sentence 1
When the generator was tested on the bench, the technician measured a small residual voltage at the output terminals as soon as the rotor began to turn.
Example Sentence 2
If residual voltage measures below specification, the generator may fail to produce charging current once the engine is running.