Diesel locomotive propulsion system operation
As previously explained, the diesel locomotive control system is designed so that the main generator electric power output is matched to any given engine speed.
Given the innate characteristics of traction motors, as well as the way in which the motors are connected to the main generator, the generator will produce high current and low voltage at low locomotive speeds, gradually changing to low current and high voltage as the locomotive accelerates.
Therefore, the net power produced by the locomotive will remain constant for any given throttle setting
As the load on the engine changes, its rotational speed will also change. This is detected by the governor through a change in the engine speed feedback signal.
The net effect is to adjust both the fuel rate and the load regulator position so that engine RPM and torque/power output will remain constant for any given throttle setting, regardless of actual road speed.
In newer designs controlled by a traction computer, each engine speed step is allotted an appropriate power output. The computer compares this value with actual main generator power output, calculated from traction motor current and main generator voltage feedback values.
The computer adjusts the feedback value to match the reference value by controlling the excitation of the main generator, as described above. The governor still has control of engine speed, but the load regulator no longer plays a central role in this type of control system.
At standstill, main generator output is initially low voltage/high current, often in excess of 1000 amperes per motor at full power. When the locomotive is at or near standstill, current flow will be limited only by the DC resistance of the motor windings and interconnecting circuitry, as well as the capacity of the main generator itself.
Torque in a series-wound motor is approximately proportional to the square of the current. Hence, the traction motors will produce their highest torque, causing the locomotive to develop maximum tractive effort, enabling it to pull the train.
As the diesel locomotive accelerates, the now-rotating motor armatures will start to generate a counter-electromotive force back EMF, meaning the motors are also trying to act as generators, which will oppose the output of the main generator and cause traction motor current to decrease.
Main generator voltage will correspondingly increase in an attempt to maintain motor power, but will eventually reach a plateau. At this point, the diesel locomotive will essentially cease to accelerate, unless on a downgrade.
Since this plateau will usually be reached at a speed substantially less than the maximum that may be desired, something must be done to change the drive characteristics to allow continued acceleration. This change is referred to as transition, a process that is similar to shifting gears in an automobile.
In older diesel locomotives, it was necessary for the engine driver to manually execute transition by use of a separate control. As an aid to performing transition at the right time.
The load meter an indicator that informs the engine driver on how much current is being drawn by the traction motors was calibrated to indicate at which points forward or backward transition should take place.
Automatic transition was subsequently developed to produce better operating efficiency, and to protect the main generator and traction motors from overloading from improper transition.
Continue reading about the diesel locomotive