How are main generators different from the APU generator?

• A. APU generator is liquid cooled.
• B. Main generators are air cooled.
• C. Main generators are liquid cooled.
• D. APU generator is air cooled.

Answer: (C)

Main generators are liquid cooled because they deliver a high and continuous electrical load, which produces a lot of heat. To keep temperature under control, their heat is carried away through a liquid cooling loop connected to the engine’s cooling system, using a heat exchanger to remove heat efficiently. The APU generator, by contrast, is a smaller auxiliary unit and does not generate as much heat or run as long, so it is cooled by air (with a smaller oil cooler in some setups). That difference in cooling method—liquid cooling for the large, engine-driven generators versus air cooling for the smaller APU generator—explains why main generators are liquid cooled.