field, power taken from the engine can be regulated: increasing the field current increases the power
delivered by the locomotive; decreasing the current decreases it. Connecting the motors in parallel is
one of the two main methods of transition in the diesel-electric locomotive; the other is shunting off part
of the current drawn into the circuit, forcing more current to be drawn from the generator to replace it.
Transition, then, is accomplished by changing traction motors from series connection to series-
parallel or parallel connection, or by closing field shunting circuits to weaken the traction motor fields.
When motors are connected in series, the same current flows through each motor and the
generator voltage is impressed on the entire group of motors, resulting in a comparatively low voltage
for each motor. If the connections are changed to put motors in series-parallel or parallel with each
other, higher voltage is impressed on each motor resulting in a corresponding increase in power output
of the generator. When the fields of a series motor are shunted, only a portion of the current continues
to flow through the fields. This weakens the field and the counter-voltage, or back emf. This is
equivalent to lowering the resistance of the circuit and results in an increased flow of current. Increased
current in every instance increases the torque, the tractive effort, and the power drawn from the
generator; and a balanced condition for voltage, current, and tractive effort is reached at progressively
Hand-operated transition controls are in use but transition is usually automatic. When original
connections are made in the opposite sequence as speed decreases, the process is called backward
1.39. DYNAMIC BRAKING
Braking with brakeshoes puts wear on both the shoes and the wheels, and the friction of the shoe
pressing against the wheel causes extra heat. In dynamic braking, these disadvantages are eliminated.
While the locomotive is coasting to a stop, the engine-man can use the traction motors as generators to
slow the locomotive's speed without using the brakes. During dynamic braking, traction motor
connections are changed; the motor acts as a generator sending its current to resistors that dissipate the
energy into the air. The amount of braking is regulated by changing the field excitation of these traction
motors; controls are built into the throttle and reverse levers.