Main engine start is usually from the APU auxiliary starter generators (ASG).
The ASGs supply 235v ac. If the ASGs are not available, 115v ac external
power sources supply the power.
The power goes to the autotransformer rectifier units (ATRU) on the P700/800 rack. The ATRUs supply 270v dc to common motor start controllers (CMSC) also on the P700/800 rack.
The CMSCs then supply the 235v ac to the variable frequency starter generators (VFSG) for engine rotation for start or engine motoring.
For external power (EP), it is preferred to use all 3 EP receptacles for engine
start.
The EP carts are 90 KVA each. This provides the most power for the
airplane when the APU is not available and there will be less load shedding. It is recommended to start the right engine first to use the aft EP source.
When you use the 3 EP sources, the aft EP can only provide power to start the RIGHT engine. If 3 EP sources are used and the left engine is started first, the aft EP source is not used. The system reconfigures as if the airplane has 2 forward EP sources for engine start.
Author – Deniz Turan – Technical Instructor (Theoretical/Practical) B1/C Licensed Aircraft Maintenance Engineer ( A320, B737, B777, B787) Turkish Technic