Service Training
Section 6
Page 2
RP01 / Chapter 1
115 804 2401.1000
6.1.1.1 SPEED & DIRECTION CONTROL
In general terms the speed of the truck is proportional to the average voltage applied to the motor armature,
and inversely proportional to the current flowing through the motor field. The direction of travel is dependent
on the direction of current flow through the motor field relative to the motor armature.
6.1.1.2 FIELD CONTROL
A bridge of 4 mosfets within the power module control the direction and magnitude of the traction motor field
current. When the truck is at rest the field is alternatively pulsed in one direction then the other to ensure
that no significant remnant magnetism is induced into the yoke of the motor.
6.1.1.3 ARMATURE CONTROL
Two banks of mosfets within the power module are in series with the positive and negative ends of the
motor armature connections. Under normal driving, one would be in full conduction and the other pulsed
to vary the average voltage applied to the armature in accordance with driver demand and load conditions
imposed. At maximum speed both armature mosfet banks are in full conduction and the field current will
be at minimum level.
On initial depression of the accelerator, the field current and armature voltage will both progressively
increase from zero, and will achieve maximum levels at about a third of maximum truck speed. Further
depression of the accelerator will result in the field current weakening, and the truck will continue to
accelerate to maximum speed.
The controller monitors the armature current, and in order to ensure that this does not become excessive,
will automatically override the accelerator demand and increase the field current to reduce truck speed and
thus armature current as required. Under slow speed conditions when high torque is also required, for
example when climbing a gradient, the field current will also be increased automatically into the shaded
area of the graph.
115_06-4
MAX
MAX
0
0
Speed
Field current
Armature voltage
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