FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
A DC generator on a twin-engine aircraft fails in flight, causing generator output voltage to drop below battery voltage. Which component automatically opens the circuit between the generator and the bus to prevent the battery from discharging backward through the generator armature?
- ACurrent limiter
- BCarbon pile voltage regulator
- COvervoltage relay
- DReverse-current relay (cutout relay)
Show answer & explanationAnswer & explanation
Correct answer: D. Reverse-current relay (cutout relay)
The reverse-current relay (cutout relay) senses when generator voltage falls below bus voltage and opens its contacts, disconnecting the generator from the bus so the battery cannot feed current backward into the failed generator.
Why the other options are wrong
- A. A current limiter restricts maximum generator output current, it does not prevent reverse current.
- B. The voltage regulator controls field current to maintain output voltage, it does not disconnect the generator.
- C. An overvoltage relay protects against excessive voltage, not reverse current flow.
Reverse-Current Relay
A protective relay that disconnects a DC generator from the bus when generator voltage drops below bus/battery voltage, preventing battery discharge back through the generator.
- Also called a cutout relay
- Senses reverse current flow direction
- Automatically opens generator-to-bus connection on generator failure
Memory trick: Reverse relay = the bouncer that kicks the dead generator out before it drains the battery.