Traction motor
See also: Electric vehicle and Electric motor
Atraction motor is an electric motor used for propul-
sion of a vehicle, such as an electriclocomotive or electric
roadway vehicle.
Traction motorsare used in electrically poweredrail vehi-
cles such as electric multiple units and other electric vehi-
cles such as electric milk floats,elevators,conveyors, and
trolleybuses, as well as vehicles with electrical transmis-
sion systems such as diesel-electric, electric hybrid vehi-
cles, and battery electric vehicles.
1 Motor types and control
Direct-current motors with series field windings were the
oldest type of traction motors. These provided a speed-
torque characteristic useful for propulsion, providing high
torque at lower speeds for acceleration of the vehicle, and
declining torque as speed increased. By arranging the
field winding with multiple taps, the speed characteris-
tic could be varied, allowing relatively smooth operator
control of acceleration. A further measure of control was
provided by using pairs of motors on a vehicle; for slow
operation or heavy loads, two motors could be run in se-
ries off the directcurrent supply. Where higherspeed was
desired, the motors could be operated in parallel, making
a higher voltage available at each and so allowing higher
speeds. Parts of a rail system might use different volt-
ages, with higher voltages in long runs between stations
and lower voltage near stations where slower operation
would be useful.
A variant of the DC system was the AC operated se-
ries motor, which is essentially the same device but oper-
ated on alternating current. Since both the armature and
field current reverse at the same time, the behavior of the
motor is similar to that when energized with direct cur-
rent. To achieve better operating conditions, AC railways
were often supplied with current at a lower frequency
than the commercial supply used for general lighting and
power; special traction current power stations were used,
or rotary converters used to convert 50 or 60 Hz com-
mercial power to the 16 2/3 Hz frequency used for AC
traction motors. The AC system allowed efficient distri-
bution of power down the length of a rail line, and also
permitted speed control with switchgear on the vehicle.
AC induction motors and synchronous motors are simple
and low maintenance, but are awkward to apply for trac-
tion motors because of their fixed speed characteristic.
An AC induction motor only generates useful amounts
of power over a narrow speed range determined by its
construction and the frequency of the AC power supply.
The advent of power semiconductors has made it possi-
ble to fit a variable frequency drive on a locomotive; this
allows a wide range of speeds, AC power transmission,
and rugged induction motors without wearing parts like
brushes and commutators. [1]
2 Transportation applications
2.1 Road vehicles
See also: Hybrid electric vehicle and battery electric
vehicle
Traditionally road vehicles (cars, buses and trucks) have
used diesel and petrol engines with a mechanical or hy-
draulic transmission system. In the latter part of the
20th century, vehicles with electrical transmission sys-
tems (powered from internal combustion engines, batter-
ies or fuel cells) began to be developed—one advantage
of using electric motors is that specific types can regen-
erate energy (i.e. act as a regenerative brake)—providing
braking as well as increasing overall efficiency.
2.2 Railways
Swiss Rhaetian Railway Ge 6/6 I Krokodil locomotive, with a sin-
gle large traction motor above each bogie, with drive by coupling
rods.
Traditionally, these were series-wound brushed DC mo-
tors, usually running on approximately 600 volts. The
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