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Re: Locomotive electrical systems
Author: Dr Zarkoff
Date: 08-01-2013 - 10:44
Current, measured in amperes, is the quantity of current. One amp = one coulomb per second. Voltage is the electrical pressure driving the current.
The resistance of a traction motor varies with temperature, not speed. It becomes more difficult to force electricity through it the faster it rotates because of "back-emf", also called "counter emf", which can be thought of as an increase in motor resistance, although strictly speaking it isn't. "CEMF" is generated internally in the armature by the coils in the armature's windings cutting the magnetic lines of force created by the field coils, and it's opposite in polarity to the impressed current causing the motor to rotate. It rises as the motor's rotational speed increases. Acceleration is accomplished by increasing the impressed voltage sufficiently to overcome CEMF.
>Early EMD switchers start in full series, and then change to series-parallel.
>"Transition" varies with locomotive design. Early EMD switchers start in full series, and then change to series-parallel.
EMD had 300v traction motors? If so, never heard of it before.
>On the SDP45 being discussed, my guess is that the light trains should not have taxed the system too bad and still allowed decent acceleration without the normal loss of drawbar pull during the few seconds needed to affect transition.
Well, it was precisely this which was the issue: the loss of output (acceleration) during transition.
>Starting in full parallel would not have been the fastest unless there were some additional excitation changes made. I wonder if another modification allowed the full AR10 output of 4200 amps (700 amps per motor max) instead of the normal 3200 amps while starting in series-parallel?
As I recall, it did. It's also to be kept in mind that the periods of high current draw were relatively brief because of the lighter nature of the commute trains. They didn't do too badly in "commute" when used in freight service.
All DC drive diesels on all railroads are limited to about 300-400 amps max when in run 1 in order to avoid frying (annealing) the traction motor commutators because once annealed, the copper bars tend to fly out of the commutator.
The resistance of a traction motor varies with temperature, not speed. It becomes more difficult to force electricity through it the faster it rotates because of "back-emf", also called "counter emf", which can be thought of as an increase in motor resistance, although strictly speaking it isn't. "CEMF" is generated internally in the armature by the coils in the armature's windings cutting the magnetic lines of force created by the field coils, and it's opposite in polarity to the impressed current causing the motor to rotate. It rises as the motor's rotational speed increases. Acceleration is accomplished by increasing the impressed voltage sufficiently to overcome CEMF.
>Early EMD switchers start in full series, and then change to series-parallel.
>"Transition" varies with locomotive design. Early EMD switchers start in full series, and then change to series-parallel.
EMD had 300v traction motors? If so, never heard of it before.
>On the SDP45 being discussed, my guess is that the light trains should not have taxed the system too bad and still allowed decent acceleration without the normal loss of drawbar pull during the few seconds needed to affect transition.
Well, it was precisely this which was the issue: the loss of output (acceleration) during transition.
>Starting in full parallel would not have been the fastest unless there were some additional excitation changes made. I wonder if another modification allowed the full AR10 output of 4200 amps (700 amps per motor max) instead of the normal 3200 amps while starting in series-parallel?
As I recall, it did. It's also to be kept in mind that the periods of high current draw were relatively brief because of the lighter nature of the commute trains. They didn't do too badly in "commute" when used in freight service.
All DC drive diesels on all railroads are limited to about 300-400 amps max when in run 1 in order to avoid frying (annealing) the traction motor commutators because once annealed, the copper bars tend to fly out of the commutator.
