The starter-generator automatically transitions from start mode to generator mode when engine N1 speed exceeds -__________%?

• A. 40
• B. 60
• C. 20
• D. 80

Answer: (A)

The starter-generator switches from starting to generating when the engine reaches a speed where it can run reliably on its own, not just when it’s being started. This switch is set at about 40% N1. At that speed, the rotor is turning fast enough that the clutch can disengage the starter mechanism and the machine can start producing electrical power into the system without dragging the engine down or causing instability.

Choosing 40% ensures a smooth handoff: below this speed, the starter needs to provide torque to bring the engine up; above it, the engine has enough momentum to sustain operation while the generator supplies electricity. The other thresholds would either switch too early or too late, risking inefficient operation or excessive load on the engine.