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I'm reading Wikipedia on GE Evolution Series diesel-electric engines. The series includes ES44AC and ES44DC models. Looks like they are very similar but one uses AC traction and another uses DC traction. They are being built in parallel - it's not like one version is "old and boring" and the other is "newer and brighter".

These are not catenary powered engines - they don't need to be compatible with railroad electrification parameters. They just burn diesel fuel and run the onboard generators and use generated electric power to run the traction motors.

AFAIK it's usually "what combination of generator, controller and traction motor can be made most maintainable and cheap" consideration that affects the choice of generator-motors combination. However this time the manufacturer decided to build both versions at the same time.

Why build both versions in the same series of diesel-electric engines?

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    \$\begingroup\$ Don't underestimate how far companies and especially publicly regulated railroad operators will go to avoid having multiple different devices to maintain. Also, if you sell someone say 800 locomotives to be delivered distributed over 15 years, you don't switch to the new model in the middle without your customer paying for that upgrade – after all. \$\endgroup\$ Aug 22, 2018 at 7:10
  • \$\begingroup\$ I would hazard a guess at low speed pull, as it seems to be mining companies buying them rather than general railways. Mining would involve trains that are always at maximum load, and no great benefit to speed. \$\endgroup\$
    – Henry Crun
    Aug 22, 2018 at 7:55
  • \$\begingroup\$ @HenryCrun for the mines it is most likely due to reducing / preventing sparks... \$\endgroup\$
    – Solar Mike
    Aug 22, 2018 at 8:30
  • \$\begingroup\$ @HenryCrun true , so there must be something - perhaps it is just the DC motor characteristics then as you say. \$\endgroup\$
    – Solar Mike
    Aug 22, 2018 at 8:42
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    \$\begingroup\$ @HenryCrun Very few AC motors today actually use slip rings. \$\endgroup\$
    – R Drast
    Aug 22, 2018 at 9:37

1 Answer 1

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DC Motors with series field windings have good characteristics for traction and a long history of usage on railways, but the maintenance problem of having to replace the carbon brushes. AC induction motors can only really be used for traction with variable frequency drives, so this is a trade off of simpler mechanics (no brushes) for more complex power electronics.

The rail industry is very conservative, risk averse one because you have to maintain a high level of service 24/7/365 over changing environmental conditions. Although variable frequency drives have been in use on railways since the early 90's, this is still seen as new technology.

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