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Re: Locomotive electrical systems
Author: Dr Zarkoff
Date: 08-01-2013 - 09:59
Yes, because it increases the input voltage to the traction motors.
The faster a traction motor turns, the more difficult it becomes to force the electricity through it (to accelerate), which requires increases in the input voltage. Transition is from the electric railway industry (a diesel is merely an electric with a self-contained, portable diesel generator set), and was developed as a means of increasing current at a certain point during acceleration because of the limitations dictated by the constant voltage of the trolley wire and the somewhat primitive electrical insulation materials available then. These designs were the basis for the "first generation diesels" (DC/DC drive).
How transition is accomplished depends on whether you use open transition (everything disconnected, connections rearranged, then everything reconnected), shunt transition (½ of the motors bypassed during the reconnection process), or bridging transition (nothing bypassed). For the most part, diesels have used shunt transition because open jerks too much upon reconnection (gotta watch out for them drawbars), and if bridging isn't done at precisely the right point in the acceleration-current curve, there can be excessive arcking and burning at the contactor contacts (and besides locomotive loads are too great to make it practical).
Main generator output voltage is controlled by the excitation of the main generator. The problem with DC generators is that as they are redesigned to increase the maximum desired HP output, because of the currents involved, the required diameter of the generator frame eventually becomes too large to fit in the carbody (Alco's largest DC-DC diesels suffered from this), not to mention that copper costs start becoming prohibitive.
With alternators, HP output can be increased by raising the voltage pretty much to to any value desired, based on the constraints dictated by electrical heat dissipation and quality of insulation, without materially increasing the diameter of the alternator frame. What really opened the door to their use in locomotives was the development of solid state, semiconductor devices (VGN's E-33s and PRR's E-44s were also part of this process), which then lead to the elimination of transition.
The early model AC-DC drive EMD diesels had transition (like the SP's 8800/8900s and 3200s), but once they figured that it wasn't really necessary when you used thyristor/GTO diodes/etc, they did away with it (field shunting too). Doing away with transition also reduced costs because of the elimination of the transition switchgear.
I've often wondered whether this commute switch in the 3200s played a part in all this, and I suspect it did. The reason for the switch is that without it, the 3200s couldn't even come close to matching the acceleration of the Trainmasters.
The faster a traction motor turns, the more difficult it becomes to force the electricity through it (to accelerate), which requires increases in the input voltage. Transition is from the electric railway industry (a diesel is merely an electric with a self-contained, portable diesel generator set), and was developed as a means of increasing current at a certain point during acceleration because of the limitations dictated by the constant voltage of the trolley wire and the somewhat primitive electrical insulation materials available then. These designs were the basis for the "first generation diesels" (DC/DC drive).
How transition is accomplished depends on whether you use open transition (everything disconnected, connections rearranged, then everything reconnected), shunt transition (½ of the motors bypassed during the reconnection process), or bridging transition (nothing bypassed). For the most part, diesels have used shunt transition because open jerks too much upon reconnection (gotta watch out for them drawbars), and if bridging isn't done at precisely the right point in the acceleration-current curve, there can be excessive arcking and burning at the contactor contacts (and besides locomotive loads are too great to make it practical).
Main generator output voltage is controlled by the excitation of the main generator. The problem with DC generators is that as they are redesigned to increase the maximum desired HP output, because of the currents involved, the required diameter of the generator frame eventually becomes too large to fit in the carbody (Alco's largest DC-DC diesels suffered from this), not to mention that copper costs start becoming prohibitive.
With alternators, HP output can be increased by raising the voltage pretty much to to any value desired, based on the constraints dictated by electrical heat dissipation and quality of insulation, without materially increasing the diameter of the alternator frame. What really opened the door to their use in locomotives was the development of solid state, semiconductor devices (VGN's E-33s and PRR's E-44s were also part of this process), which then lead to the elimination of transition.
The early model AC-DC drive EMD diesels had transition (like the SP's 8800/8900s and 3200s), but once they figured that it wasn't really necessary when you used thyristor/GTO diodes/etc, they did away with it (field shunting too). Doing away with transition also reduced costs because of the elimination of the transition switchgear.
I've often wondered whether this commute switch in the 3200s played a part in all this, and I suspect it did. The reason for the switch is that without it, the 3200s couldn't even come close to matching the acceleration of the Trainmasters.
