Full power current draw

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).  

Thank you Jack.

Then the reason why modern high horsepower diesels are now designed with AC traction motors, that are variable frequency?

PRR Man posted:

Thank you Jack.

Then the reason why modern high horsepower diesels are now designed with AC traction motors, that are variable frequency?

There are many, MANY reasons for the development & use of AC Traction locomotives, one of the main reasons being the drastic reduction in maintenance costs of the AC 3 phase induction motor (which has only one moving component, i.e. the rotor), over the DC series wound motor, as well was the great increase in torque (tractive effort).

PRR Man posted:

Thank you Jack.

Then the reason why modern high horsepower diesels are now designed with AC traction motors, that are variable frequency?

What Jack said is of course correct.  I might add that the AC armature attempts to stays in sync with the rotating field in the stator.  The more the armature lags behind the HARDER it gets pulled.  The control system for AC is far more complicated than that of DC which of course is reflected in the significantly higher initial purchase price.  The Diesel/Alternator set produces AC (Single phase I think) which is then rectified to DC (DC link as EMD calls it) which is then converted via inverters back to 3-phase AC for the traction motors.  This process (AC to DC then back to AC) enables the Diesel/Alternator set to operate independently at Volts/Frequencies that are most efficient for it while traction motors operate at Volts/Frequencies that are most efficient for them, all under computer control.  While AC traction offers huge advantages in getting heavy trains moving, it also offers previously unattainable dynamic braking capability that ALONE (Without air) can bring a train to a stop.  

WARNING:  DO NOT ever touch ANYTHING inside any electrical cabinets on any locomotive but especially an AC unit.  Even if the AC locomotive is shut-down, AC units have capacitors that can produce LETHAL shocks if they haven’t been properly discharged.  

Question, what frequencies/voltages are applied to the motors in modern AC locomotives? I understand that they both vary, but what is "full speed" voltage and "full-torque" current? A related question; how many poles do the AC traction motors have?

Thanks again,

There are many advantages that AC traction motors have over DC.  About one third of a DC traction motor's volume was taken up by that "electro-mechanical switch", better known as a commutator.  Commuter DC traction motors had a lot of issues, including carbon brush life, unstable commutators that became oval or irregularly shaped with age and abuse, raised bars, and a tendency to flashovers. 

With AC, ALL of that "available" commutator volume was used for active core material and the function of the commutator was basically moved "upstairs" to the inverter(s).  AC traction motors are also larger in diameter, and the old rule of "d squared L", applies, where an increase in diameter is much more effective in improving rating than an increase in core length, with its increased temperature gradient (that requires cooling).  The "standard" wheel diameter of a USA freight diesel equipped with a DC traction system was/is 40-inches.  The most common wheel diameter of an AC equipped diesel is 42-45 inches, and this does permit a nominal increase in rotor diameter.

The tendency of  an AC traction motor to run at the specific frequency instructed by the control system has resulted in adhesion improvements that in the DC age were literally laboratory numbers.  And that is one reason why modern AC traction locomotives of North American design are rated at a starting tractive effort north of 180,000 lb.  I believe (from memory) that MCB type couplers were rated at a yield of 560,000 lb, so you can see that three modern AC locomotives are a good match for a train equipped with these couplers.  The corollary here is that with three AC's on the head end, the train WILL MOVE, either as a train or the crew might find out where the weak link is!