Not a very well-informed brain re locomotives. But I have been wondering:
1. I know the advantage of diesel-electric locomotives over a straight coupling (via a transmission say in a caterpillar earth mover) of a diesel engine to driving wheels is the full torque even at low speeds. But what is the actual physical mechanism of that? As the traction motor receives "juice" from the ramping up diesel via the generator, what factor is at full play at all speeds....is it amperage, voltage or what? You can tell I am not conversant, but curious.
2. What was the limitation to the DC traction motors? I heard that the actual physical wiring, coil size etc. has limitations in DC, such that the higher power required means unmanageable sizes of physcial wiring etc. Is it that the AC current is only energizing the coils half the time that give the edge to AC over DC in efficiency so that they can be of useful size even carrying huge currents?
3. In loco information where is sometimes is said that various types of trucks are swapped for another, say in a rebuild do the traction motors always mate in some universal way? So if the old AAR trucks say, are swapped out for Blombergs or HT-C trucks, are the traction motors kept or do they have to be matched to the type of truck coming in, or do they come in a set of trucks with traction motors?
Just wondering.
Hello,
On #1, I'm not sure.
On #2, the limitation on DC motors is that they overheat if run at slow speeds for long periods of time. AC motors can stand much hotter temperatures, as well as other advantages, such as less complexity and better dynamic braking.
On #3, I believe that the same traction motor will fit on HT-Cs, Blombergs, and AAR-Bs and AAR-As.
Voltage and current go opposite of each other. Current "does the work", voltage is "the pressure" behind the current. Think of it as a garden hose. With no nozzle, maximum water flows, but the stream doesn't go very far. Put a nozzle on, not as much water flows but it flows more forceful and a lot farther. At starting you need maximum power. Thus maximum current goes to the motors. As speed increases, less force is needed to do the work, so voltage increases and current decreases. A generator has a maximum load, lets say 120 watts for example. it can provide 12 volts / 10 amps or 120volts at 1 amp. Either way it's 120 watts. It can't go more than that.
Matching the right combination of voltage and current to the particular load on the locomotive and particular RPM - throttle setting of the diesel prime mover is the job of the load regulator equipment, which includes the governor that controls fuel supply and these days by electronic control the field current to AC-generator ("alternator"). These days it is all done by "smart electronics" computer control. This equpment also includes the speed of the invertor that delivers ac to the field coils of the hysterisis-non-syncronous or induciton motor. With modern ac motors and ac generator it is possible to match what is delivered to the traction motors for even better than 90% efficiency of diesel prime mover HP to the rails. In DC days, much was done with relays and with some loss is callibration resistors, and the efficiencies were between 85% and 90% depending on type of service. DC motors require commutators and brushes, which are serious maintenance items. The commutator windings that can overheat on a dc motor are replaced by rotating aluminum or copper bars in an AC motor, and these can stand far, far greater heat. So the only windings that can overheat on an ac traction motor are the field windings, and because they do not rotate, they can be much more robust. They will overheat of course, long before the rotating bars could be deformed by heat, so there still is a limit on current for ac motors but it can be as great as five times that for dc motors. In general, ac diesel electrics are limited in tractive effort only by their wieght and not by the limitations of the ac motors.
Overheating occurs when the current in a coil is so great that the resistance of the wire produces so much heat the insulation starts to fail. With the oldest diesels, with cotton insulation, it woujd actually burn. Modern plastic insulation will melt, and the short circuit will cause brreaker to trip and the locomotive will simply shut down and not restart.
In general, today's GE motors will fit only GE trucks and EMD EMD trucks. Originally, this was not the case, since both GE amd Westinghous provided motors for EMD, Alco, Baldwin, and Lima, but early on EMD started manufacturing its own motors but only for EMD locomotives.
DC traction motors can and do “stall”…in that they can no longer overcome the physical forces of gravity and mass…when that happens, the brushes begin to arc to the commutator (the part that switches the polarity) and can burn a section of it, ruining the motor…and as stated, the windings and feed cables can over heat due to the high resistance created.
AC motors have no brushes, and even if you stall them, they will not overheat in the same manner as a DC motor..(think ceiling fan)…AC motors in simplest form simply switch magnetic fields, with no electric exchange between a brush and commutator, therefor under stall conditions, no arching.
Because AC current is easier to transmit than DC current, and creates less resistance (heat) than DC, the cabling and wires are not as big.
Funny thing, even though it is listed as a generator, all modern locomotives, since about the mid-1960s, use an alternator to create the electricity….DC traction motor systems just use half the current produced, it is run through a rectifier and turned into DC current.
By the way, the alternator on a locomotive is almost exactly like the one in your automobile, only much much larger…your car makes 15 volts at 75 to 100 amps, the locomotives make 600 volts at 1900 plus amps.
Up until just recently, most DC traction motors were interchangeable, there was pretty much a standard in the way they mounted, but with the advent of both AC traction and steerable trucks, like the HT-C you mention, the traction motors have become pretty much proprietary to each manufacturer.
You could make GE motors fit an EMD, and vice versa, but you would have to change all of them on the locomotive, and the necessary adaptions would be expensive, with the end result not very satisfactory.
Last, not all of the construction equipment, (your Cats) use direct drive and transmissions, those huge dump trucks you see in mine pits and dam sites, and most big earth movers, like bulldozers and the big shovles use the same concept as a locomotive, a diesel prime mover powers an alternator that powers electric motors on the axles, some use a shift able gearing system, some a simple bull and pinion gear like a locomotive.
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