Non-patent document 1 discloses a conventional inverter modulation method for driving an electric locomotive. In the inverter modulation method disclosed in Non-patent document 1 multi-pulse asynchronous modulation is used when inverter frequency is low, three-pulse synchronous modulation is used when the inverter frequency increases to a predetermined level, and single-pulse synchronous modulation is used when or after the output voltage reaches a termination voltage (rated output: 100%).
Non-patent document 1:“Instruction to inverter-controlled electric locomotive” Hideki Iida, Atsushi Kaga, Denkisyakenkyukai