Answer:
A GCU (Generator Control Unit) typically switches the starter to gen mode once the generator has reached the correct speed and voltage to generate electricity. This is usually monitored by sensors that detect the speed and voltage of the generator, and once these parameters reach the required levels, the GCU will automatically switch the starter to gen mode.
The exact timing of the switch from starter to gen mode can vary depending on the specific design and configuration of the generator system, as well as the specific application and load requirements. However, in general, the switch from starter to gen mode is typically made as soon as possible after the generator has reached the required speed and voltage to ensure that the generator is producing power efficiently and reliably.