Definition
A compound-wound DC generator in which the series field winding is designed so that output voltage decreases as electrical load increases. The shunt field provides the base voltage, and the series field adds only a small amount of additional excitation under load — not enough to fully offset the voltage drop caused by internal resistance and armature reaction.
Plain English
A type of generator that produces slightly less voltage when more electrical equipment is switched on. It has two sets of field windings working together, but the second set isn't strong enough to keep the voltage steady as the load grows.
Context Anchor
Seen in aircraft electrical system maintenance, especially when checking generator output voltage under different electrical loads.
Derivation
Compound' comes from Latin 'componere', meaning 'to put together' — referring to the combination of two field windings (shunt and series). 'Under' simply means the series winding's contribution is below what's needed to hold voltage constant under load.
Why Pilots Care
Helps explain why some aircraft generators require external voltage regulators to keep power steady under changing loads.
Grounding Statement
When more electrical equipment is switched on, an undercompounded generator tends to sag in voltage instead of staying steady.
Intuition Check
Do not read “undercompounded” as meaning the generator is simply weak or poorly built. It specifically means its voltage drops as electrical load increases because the added field winding does not compensate enough.
Example Sentence 1
The aircraft's older DC system uses an under-compounded generator, so bus voltage decreases slightly when the landing lights and pitot heat are switched on together.
Example Sentence 2
In the electrical theory class the instructor showed how an undercompounded generator behaves differently from a properly compounded one under increasing load.