FAA Powerplant Written Practice Test

What is the purpose of the undercurrent relay in a starter-generator system?

1. To regulate fuel flow

2. To disconnect power from the starter-generator

3. To maintain battery charge

4. To increase engine speed

The correct answer is: To disconnect power from the starter-generator

The undercurrent relay plays a critical role in the operation of a starter-generator system by serving as a protective device that disconnects power from the starter-generator when it detects that the current falls below a certain threshold. This is particularly important because a drop in current can signify that the engine has started successfully and the starter-generator is no longer needed for cranking the engine. By disconnecting the power at this point, the undercurrent relay helps to prevent potential damage to the starter-generator from overheating or excessive wear resulting from continuous operation during the starting phase. Maintaining safe operating conditions is essential in aviation systems, and the undercurrent relay contributes by ensuring that once the engine is running, the starter-generator transitions properly from a starting role to a generating role without risking harm to the equipment.