PRR Man posted:
A question for the trainmen here:
Watching the Horseshoe Curve webcam the other day, I got to wondering what is happening when diesel motive power has the engine running at full power but actual train speed is quite slow climbing the grade. How does the engine/generator and its traction motors handle the great amount of current being sent to the motors without burning up because they're turning so slowly?
First of all, there is LOTS of forced air for cooling, being blown into each and every traction motor, from the Traction Motor Blower/Blowers.
I imagine its a function of motor design to cope with high current even when it's RPM is low?
Yes, pretty much. For example, the EMD 60 and 70 Series units, which are DC Traction, have a continuous TM current rating of 1050 amps, per motor. Thus, they can safely handle 1050 amps, for many, MANY hours without doing damage to anything. Naturally as the speed increases, the voltage increases as the current decreases.
For AC Traction units, voltage and current is NOT the factor as with DC Traction. For example, the huge stranded copper cables required to deliver all that current from the main generator, through the controlling switch gear, and then down to the traction motors on DC Traction units, are over 2" in diameter. Conversely, the supply cables on AC Traction units are less than 1 ". With DC Series Wound traction motors, it is pretty much all about current (with its corresponding high heat), while with AC 3 phase induction traction motors, it is all about frequency (the high heat is thus generated in the big Traction Inverters on board the unit).