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1.3 Diesel-Electric
A diesel–electric locomotive's power output is independent of road speed, as long as
the unit's generator current and voltage limits are not exceeded. Therefore, the unit's ability to
develop tractive effort (also referred to as drawbar pull or tractive force, which is what
actually propels the train) will tend to inversely vary with speed within these limits. (See
power curve below). Maintaining acceptable operating parameters was one of the principal
design considerations that had to be solved in early diesel–electric locomotive development
and, ultimately, led to the complex control systems in place on modern units.
Originally, the traction motors and generator were DC machines. Following the
development of high-capacity silicon rectifiers in the 1960s, the DC generator was replaced
by an alternator using a diode bridge to convert its output to DC. This advance greatly
improved locomotive reliability and decreased generator maintenance costs by elimination of
the commutator and brushes in the generator. Elimination of the brushes and commutator, in
turn, disposed of the possibility of a particularly destructive type of event referred to as
a flashover, which could result in immediate generator failure and, in some cases, start an
engine room fire.
The important components of diesel–electric propulsion are the diesel engine (also
known as the prime mover), the main generator/alternator-rectifier, traction motors (usually
with four or six axles), and a control system consisting of the engine governor and electrical
and/or electronic components, including switchgear, rectifiers and other components, which
control or modify the electrical supply to the traction motors. In the most elementary case, the
generator may be directly connected to the motors with only very simple switchgear.
In a diesel–electric locomotive, the diesel engine drives either an electrical DC
generator (generally, less than 3,000 horsepower (2,200 kW) net for traction), or an
electrical AC alternator-rectifier (generally 3,000 horsepower (2,200 kW) net or more for
traction), the output of which provides power to the traction motors that drive the locomotive.
There is no mechanical connection between the diesel engine and the wheels.
Besides steam- and diesel-powered locomotives, many trains operate solely on
electrical power. They get the electricity from a third rail, or electrical line, along the track.
Transformers transfer the voltage from the lines, and the electrical current drives the motors
(AC or DC) on the wheels.