PRR 5841 posted:I was wondering about the mixing of AC and DC locomotives in a consist. During prolonged slow-speed/high-current operations, and assuming the DC unit(s) are trailing, how does the engineer in the lead AC unit know and respect the traction motor limitations of the DC unit(s) which cannot endure anywhere near what an AC unit can.
Every DC locomotive has a continuous rating for traction motor current, which is usually marked on the ammeter. As locomotive development increased horsepower through the 1940's through the EMD Dash-2 and the GE Dash-9 era, the electrical components became more durable and the continuous rating increased. An F7 could run all day at 750 amps and an SD40-2 could do the same at 1050 amps. Depending on the gear ratio, locomotives would hit the continuous rating at a particular speed. So, when speed dropped on a hard pull, amperage increased until the continuous rating was reached. On a freight F7 that would be around 13 MPH, but it would be 18 MPH on a passenger F7 and 11 MPH on an Alco-GE RSD-5. An E8 would have a minimum continuous speed of around 24 MPH.
The railroad furnished the minimum continuous speed for each class of its diesel locomotives to employees in the timetable Special Instructions, and the Engineer was governed by the continuous speed for the weakest locomotive, regardless of its position in the consist.
It works the same way today. And it's a good thing, too, since DC locomotives have a gauge showing amperage, whereas AC locomotives show kilowatts.
