PRR Man posted:
Thank you Jack.
Then the reason why modern high horsepower diesels are now designed with AC traction motors, that are variable frequency?
What Jack said is of course correct. I might add that the AC armature attempts to stays in sync with the rotating field in the stator. The more the armature lags behind the HARDER it gets pulled. The control system for AC is far more complicated than that of DC which of course is reflected in the significantly higher initial purchase price. The Diesel/Alternator set produces AC (Single phase I think) which is then rectified to DC (DC link as EMD calls it) which is then converted via inverters back to 3-phase AC for the traction motors. This process (AC to DC then back to AC) enables the Diesel/Alternator set to operate independently at Volts/Frequencies that are most efficient for it while traction motors operate at Volts/Frequencies that are most efficient for them, all under computer control. While AC traction offers huge advantages in getting heavy trains moving, it also offers previously unattainable dynamic braking capability that ALONE (Without air) can bring a train to a stop.
WARNING: DO NOT ever touch ANYTHING inside any electrical cabinets on any locomotive but especially an AC unit. Even if the AC locomotive is shut-down, AC units have capacitors that can produce LETHAL shocks if they haven’t been properly discharged.