Everything I read on the Internet says that AC drive systems on locomotives is so much better than DC systems in every way. Why then when you go to the EMD factory web site do they advertise their locomotives in DC versions? Why do customers want DC locomotives?
You might want to have a look at two of the major manufacturers before you ask this question. General Electric Locomotive Division and Electromotive Division of General Motors Corporation, (AKA: EMD) The newer model locomotives, 1990 and newer use an alternator in their system to generate the DC Power needed for the main traction motors. General Electric with their new Dash 9 AC4400CW are a Hybrid. Infact all Diesel Electric Locomotives since 1930 have been Hybrids! Bet you did not know that. The only ones that were not were the Krauss Maffei Locomotives. These were Diesel Hydraulic just like your automatic transmission in your car. EMD's Line-Up are the SD70MAC, SD75MAC and now their new SD90MAC-T Because they are using AC to DC power they generate more than 4000 horsepower. The other reason for using DC at the Traction Motors is a device called a Dynamic Brake. This device allows the polarity to be reversed in the traction motors, and act as a brake during down hill operations. This almost elminates the need for physical braking to control train speeds down hill. I hope that answers most of your question!
The only advantage i see to AC units is the ability to cut out a traction motor and still have dynamic brakes on the unit.And the dynamic on an AC unit will work virtually til you stop, whereas a DC unit fades out just before you stop.And i've never had to do it but on an AC unit you can reverse them and use them to hold your train on a hill if need be.I think the AC units with all the reliance on computers are going to be a pain after they get some age on them.A railroad environment is harsh and only time will tell if they can stand up to it.
Probably for the same reason we do not use electric cars with fuel cells. There are a ton of good old DC motors hauling out there. The simplicity of the system makes them work and easy to diagonose and fix. The biggest draw(funny huh) back to DC is the damage that is done to the switching gear turning them on and off due to the arc that is pulled on DC comared to AC. The other trick is that generators by default create AC, which is then turned into DC, which has some loss in the process.
DC traction motors can still be ordered on new locomotives, depending on the the customers request. Most light rail systems, and subway trains will always use DC traction motors.
Well . . . . . you sort of answered your own question there, they still make DC motors becuase they have not been proven to be as inferior to AC as promised. I was on a brand new ES44DC last trip on a coal train. It was a great engine, pulled hard, excellent wheel slip control, terrific dynamic brakes. I see no real advantage to AC, in fact from the engineer's seat I prefer DC. When AC power was first brought on the property we were told by everyone what a superior system it was, I have yet to be convinced. Maybe from a maintenance standpoint, I have no idea but the fact that RRs are still purchasing DC makes me think not.
