Re: Amps to HP?

You're getting things confused here.

As pointed out, horsepower is converted to watts (or kilowatts if you prefer). While 746 watts do equal 1 hp, for diesel-electric locomotives the number of 700 watts = 1 hp is more typically used to account for transmission losses.

And yes, amps x volts = watts. BUT - the 600 volt "rating" is strictly a nominal figure. Due to ohms law, a DC traction motor at low speeds has very little resistance, therefor little resistance which results in a high current flow (amps). Since the diesel engine is generating a fixed amount of horsepower at a specific throttle setting to input to the generator (hp or watts), the high current rating results in very ow voltage. As speed increases, the lines of magnetism between the field poles and rotating armature cross each other at an increasing rate. This results in in increased counter electro-magnetic force (cemf) increasing, which results in a higher resistance in the traction motors. The increased resistance results in a lower flow of current (amps) and a corresponding increase in voltage, assuming the input hp or watts remains the same.

Small locomotives may top out at only 200 volts, larger ones at 1300+ volts. Without having specific engineering information on the traction motors used, knowing the connection and transition arrangement, and considering the speed - it would only be a wild guess as to what the amps would be at any one speed. Another common trick is to limit the electrical excitation at low speeds, so regardless of the diesel engine being in full throttle, the locomotive electrical system limits itself to a lower setting, typically to save the traction motors from excessive current and overheating (aka melting down), and to limit excessive wheel slip. In the 1980s, EMD typically limited their standard gage D-77 traction motors to 500 hp at low speeds (under 10 mph). Current DC designs will try to push more current at low speeds, and electronically offer adhesion control and protect the traction motors from abuse.

Some builders have pushed the concept of using a DC chopper circuit to increase tractive effort. Since tractive effort is in direct proportion to speed, this extra tractive effort is usually only applicable at low speeds and poor rail conditions. If you plan on running a train at 10 mph or slower, a potential advantage. Depending on the design, most locomotives are capable of using their full horsepower at 15-20 mph with little adhesion or overheating issues.

The NRE flyer apparently is referencing narrow gage traction motors they have bought on the open market and rebuilt. The 761 and 764 are GE designs dating back to the 1950s. The D29 and D31 are EMD designations. Note that the EMD designs (meter gage+) have far less capacity than the GE designs. By comparison, the gold standard standard gage traction motors of the mid 1960s to mid 1980s were the EMD D77 good for 950 amps continuous, and the GE752 at 1050 amps continuous. Gear ratio and the amount of cfm of traction motor cooling are used to determine the continuous traction motor rating, also specified as the minimum continuous speed, though as stated most modern systems adjust the maximum excitation allowed based on diesel engine speed (affects amount of cooling air), ambient temperature, current flow and elapsed time. this can be done through an algorithm, or actual heat sensors on the traction motors.

My guess is that they will likely be using a B+B-B+B arrangement. A typical C-C locomotive weighs 180-220 tons, but they can make lighter weight frames. Assuming a weight of maybe 140 tons (limited by rail size and bridge strength) and modern adhesion system, the locomotive might have a maximum tractive effort of 80,000 to 100,000 pounds, but this is a short time rating. A 1975 GE rating of the GE764 is for 26,400 pounds continuous tractive effort with 4 motors and 1000 hp. Improvements in insulation will result in a much higher modern rating. I wouldn't be surprised if an 8 axle/motor had a continuous rating of around 70,000+ pounds of tractive effort, requiring a minimum continuous speed of around 13 mph and requiring 25% adhesion. GE does now make a narrow gage AC traction motor but to my knowledge only for meter+ gage.

It is somewhat amusing that both White Pass and Durango are repeating the same errors made when railroads first dieselized - trying to use one huge locomotive to do everything.



The above picture is a former EMD SD40T-2 at 3000 hp that was retired, purchased by Brazil and converted to B+B-B+B for meter gage. I know those are a GE based truck design, so I presume they have GE761 traction motors rather than the EMD D31. Notice the paint and nose gyralight - this is a former Rio Grande unit. They also had to modify the roof on this to get the height down, as I recall the dynamic brakes are now in the radiator section.