Locomotives

 I have seen pictures of AC and DC current locomotives running together.  I understand that the traction motors in AC locomotives can take more current at lower speeds for much longer periods of time than the traction motors in a DC locomotive.  Is there any danger of cooking a DC traction motor under certain conditions with an AC-DC locomotive consist?  Do the MU controls keep this from happening?

 From everything I've read here and elsewhere, each locomotive is responsible on its own for preventing wheelslip and traction motor overheating.  If the DC locomotive could not keep providing full tractive effort, then it should automatically reduce the throttle and power. 

This means that in an AC-DC locomotive pair, the AC locomotive might keep on pumping electricity to the traction motors, while the DC locomotive hits its load limit, or temperature limit, and starts reducing the power its providing to the traction motors.  Basically, if the DC locomotive can't keep up with the AC locomotive at the given throttle setting, the computer onboard should reduce the power output to something it can sustain before anything gets cooked.  In an AC-DC pair, your 3,000HP DC locomotive might only be able to perform as a 1,500HP locomotive and lots of added dead weight.

Without the computer, things would get cooked.  The power reduction is also a reason why railroads look at the applications of their locomotives when they decide what to buy - a coal drag might benefit from AC power because of its ability to run longer and slower at higher power. 

 Newer DC motored locomotives have computers that will protect the motors from overheating(SD60s, Dash8s, and newer). It is up to the Engineer to protect old locomotives by being aware of power setting and train speed. It is rare to see an older locomotive in heavy bulk service, except on railroads that have nothing newer.

I can promise you that the MU jumpers do not care what they are plugged into. A C45ACCTE will MU just fine with a GP-7. However, as stated earlier the locomotive's onboard controls should prevent motors from burning up or wheels slipping. the DC motors do not like to be static loaded (current running through the motor without movement.) they will just melt down the insulation. Where a AC motor can hold itself just fine in a static load condition.

I think that you are right the computer's take care of all of this. Also on the older models, won't the TMB's (traction motor blower's) take care of the heat.

The Blowers can help a lot. but the amount of current going into the motors at low speeds and high loads can exceed 1600 amps easily.  The insulation just cannot take that kind of current.

bubbajustin:

I think that you are right the computer's take care of all of this. Also on the older models, won't the TMB's (traction motor blower's) take care of the heat.

 When the first AC's were tested and the railroad purchased them, they were said to be used only with other AC units so they could lug coal or heavy loads at a much reduced speed and not burn up the traction motors.  When the AC units are mixed with DC units, the full capacity of the AC is not used and is probably restained by the computer system.  The Z trains are high speed trains and the Union Pacific seems to use one or two AC's with at least one DC model for most of those runs also.  Maybe it is just the fact they have so many new AC locomotives and they are used almost any place any time.   

I have included a picture of the 1988 on a Z train with two SD70M's as its extra units.  It seems like the Union Pacific mixes and matches at will, but if I remember correctly, most of the coal trains still use AC's for the most part.  

bubbajustin:

I think that you are right the computer's take care of all of this. Also on the older models, won't the TMB's (traction motor blower's) take care of the heat.

 When the first AC's were tested and the railroad purchased them, they were said to be used only with other AC units so they could lug coal or heavy loads at a much reduced speed and not burn up the traction motors.  When the AC units are mixed with DC units, the full capacity of the AC is not used and is probably restained by the computer system.  The Z trains are high speed trains and the Union Pacific seems to use one or two AC's with at least one DC model for most of those runs also.  Maybe it is just the fact they have so many new AC locomotives and they are used almost any place any time.   

I have included a picture of the 1988 (02/16/2009) on a Z train with two SD70M's as its extra units.  It seems like the Union Pacific mixes and matches AC and DC at will, but if I remember correctly, most of the coal trains still use AC's for the most part.  

As I understand it on CP when the new GE AC units arrived they were sent west out of Calgary in pairs over the Kicking Horse and Rogers Passes, but they had some serious near accidents and they had to include one SD40-2 at the front to provide the requisite Dynamic Braking power to get the trains safely downhill. The part I can't confirm 100% though, is that I believe the DC unit was cut out on the hill climbs to not cause any loss of AC pulling power.  Apparently an AC unit can pull more weight uphill than it can safely control going down a 2.2% grade. For a time they solved this problem by reducing the size of the train to where the two AC units could handle the braking chores and the reduced fuel consumption cost made it work. But last year when the fuel costs shot up you began to see a return to DPU 2+1 on the rear with even longer trains than the old DC units could pull, 3+2 mid-train. But like other readers have mentioned on different threads, just over 3 weeks ago I saw an AC DPU of 2+1 mid-train on a double stack train. The mathematics of switching costs to cut in a slave unit plus the fuel costs verses cost per ton mile of longer vs shorter trains is getting pretty severe.

AgentKid

Thank's for the pic. It is very good! I really enjoy the SD70m's.