Engine Electrical Systems
Generation and distribution
Engine electrical systems cover generation, distribution, and circuits tied to the engine.
DC generators use residual magnetism and a commutator; alternators rectify AC with diodes and require field current to begin producing. Both need a voltage regulator; generators also need a reverse-current relay (cutout) so the battery cannot motor the generator. Alternators typically use an overvoltage relay that opens the field circuit.
Know the starter circuit: battery to master relay to starter relay (closed by the start switch) to the starter. High current demands short, heavy cables and clean, tight terminals; voltage drop across a connection should be essentially zero.
Ammeters show current into or out of the battery, not system voltage; a shunt-type ammeter measures the small voltage drop across a shunt. Voltmeters show bus voltage. Know the difference between a circuit breaker and a fuse, and why wire size and terminal condition matter.
The magneto trap
Magnetos are independent of the aircraft electrical system and are shut off by grounding the P-lead through the ignition switch. If the P-lead breaks, the switch cannot ground the magneto and the engine may keep running with the switch OFF. Never turn a propeller with magneto leads disconnected.
Rule of thumb
Never disconnect the battery cable while an alternator is running - the field collapses and the voltage spike can destroy diodes and avionics. When jump starting, make the final ground connection to the airframe, not the battery.