I believe I read when these units first came out they where causing problems with the older units if they where trailing. Example AC4400 leading, SD40-2 trailing. I believe it said that the older units would not be able to handle the load being put on it by the newer units. I see newer units with older units trailing now all the time. Anyone know how this was corrected or am just misinformed?
In the example you mentioned, the problem is that the AC4400 is an AC traction motor locomotive, and the SD40-2 is a DC traction motor locomotive. The minimum continuous speed for the SD40-2 is typically around 7-8 mph. The AC4400 can drag at 4 mph all day long and not burn up the AC traction motors. Meanwhile the trailing SD40-2 is cooking the electrical system as it cannot get rid of all the heat.
The solution can be:
- Do not run AC and DC traction motor engines together
- Have the SD40-2 lead so that the engineer is aware of the engine getting in the ‘red zone’
- Keep the consist running above the minimum continuous speed of the SD40-2
Jim
The problem is that DC traction motors at high amperages have a short time rating before they overheat, especially significant when struggling up a grade at slow speed. The elegant solution is to upgrade the control electronics in the SD40-2 so that it knows to derate when necessary.
The upgrade also provides better wheel slip control allowing the same unit to handle heavier tonnage up grades. Trains that may have needed 5 SDs now can be handled by only 4, the freeing up of units more than justifying the cost of the modification.
Trains are assigned units based on either haulage capacity up the ruling grade, or horsepower to give speed. Unit coal trains typically use the first method, time sensitive intermodals use the second.
John
So lets say we have a heavy train with all DC power that needs to pull up a steep grade.
The RR would have to put enough power on that train to keep it moving at a suffeciant speed, as to not melt the motors out from under the locomotives?
-Justin
You hit the nail on the head. Considering that AC traction motors are relatively recent, that has been standard procedure since the FT’s hit the road in the 1940’s.
DC gets the train there quicker, AC gets it there cheaper.
That is a totally absurd statement.
On a particular territory I had responsibility over there was a route that handled a high number of coal trains over a rolling territory that had 4 grades that had the ability to over-match the standard of two Dash-8 6-axles and the standard 90 car train on about 3 out of 10 trips when the weather was dry and a guaranteed stall if the weather was wet. Standing instructions were add another unit to the train at the crew change point North of this territory if it was raining.
When two AC units were introduced on these runs, the stalling on these grades stopped - dry weather or wet weather - it made no difference; the trains continued to roll over the territory; and at higher overall speeds than the Dash-8’s were able to maintain. Subsequently, the standard train size has been increased to 95 cars and the trains are still handled without problems over the grades.
The minimum continuous speed for DC locomotives with standard main line gearing that limits maximum speed to 70 MPH or slightly less (depending upon manufacturer) is on the order of 11 MPH. A 10 MPH speed restriction on a segment of a ruling grade can be a real train killer as 10 MPH is less than minimum continuous speed and will stress the locomotives into their short time ratings