30-10-2012, 06:03 PM
Railway Technical Web Pages
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The DC Traction Motor: How it Drives the Axle
The traditional DC (Direct Current) electric motor driving a train or locomotive is a
simple machine consisting of a case containing a fixed electrical part, the stator (called the stator because it is
static and comprising what is called the field coils) and a moving electrical part, the rotor (because it rotates)
or armature as it is often called. As the rotor turns, it turns a pinion which drives a gearwheel. The gearwheel
is shrunk onto the axle and thus drives the wheels as shown in the diagram above.
The motion of the motor is created by the interaction of the magnetism caused by the currents flowing the the
stator and the rotor. This interaction causes the rotor to turn and provide the drive.
AC and DC Motors
Both AC (Alternating Current) and DC motors have the same basic structure but there are differences and,
for various reasons, the DC motor was originally the preferred form of motor for railway applications and
most systems used it. Nowadays, modern power electronics has allowed the use of AC motors and, for most
new equipments built today, the AC motor is the type used.
Often, people ask about the differences between AC and DC motors as used in locomotives and multipleunits.
In the early days of electric traction at the beginning of this century both types were tried. The limits of
the technology at the time favoured the DC motor. It provided the right torque characteristic for railway
operation and was reasonably simple to control.
By the early 1980s, power electronics had progressed to the stage where the 3-phase AC motor became a
serious and more efficient alternative to the DC motor because:
1. They are simpler to construct, they require no mechanical contacts to work (such as brushes) and they are
lighter than DC motors for equivalent power.
Nose Suspended Motor
The following diagram shows the layout of the traditional DC motor mounted in a bogie as a "nose suspended
motor".In electric trains or locomotives, the DC motor was traditionally mounted in the bogie
frame supported partially by the axle which it drove and partially by the bogie frame. The motor case was
provided with a "nose" which rested on a bracket fixed to the transom of the bogie. It was called a "nose
suspended motor" (see diagram above) and is still common around the world. Its main disadvantage is that
part of the weight rests on the axle and is therefore unsprung. This leads to greater wear on bogie and track.
Nowadays, designers try to ensure all the motor weight is sprung by ensuring it is carried entirely by the bogie
frame - a frame mounted motor.
Monomotor Bogie
As its name implies, the monomotor bogie has a single motor which drives both axles. Click
on the thumbnail to see a photo with the part names.
The design is much favoured in France, where it was introduced in the 1950s for the rubber tyred train
concept. The motor is mounted longitudinally in the centre of the bogie and drives each axle through a
differential gearbox, similar to a road vehicle. The differential gears are required to compensate for the
operation of the rubber tyres round curves. It requires a special bogie frame construction to accommodate
the motor.
Another version of the monomotor bogie has also been applied to a number of French locomotive designs but
here the arrangement is more conventional. Each bogie has a single motor mounted transversely over the
centre as shown in the diagram left.