MIL-HDBK-1038
allow the speeds on the inner and outer rails to conform to the severity of the
curve and drive the crane without skidding on the rails.
4.5.2.2
Shunt-Wound Motor Drives. Shunt-wound motors have the armature and field
winding connected independently. The operating speed of these motors is virtually
insensitive to the imposed load due to the constant field current. Their speed is
proportional to the voltage that is applied to the armature, while the field
voltage is fixed. These motors are desirable on drives whose motion needs to be
precisely controlled regardless of the load on them, or when overhauling cannot be
tolerated.
In an adjustable voltage system, the line-to-line voltage is varied
between zero volts and the motor's rated voltage, in both positive and negative
polarity. This is accomplished by varying the output of the DC power supply.
Older drives adjust the armature voltage (speed) by means of fixed speed points
(usually five in number) selected by activation (operation) of electro-mechanical
field relays in the drive's generator field circuit or by manual operation of a
rheostat in the drive's generator field circuit. Newer drives use electronic
controls to provide either fixed speed points or variable speed throughout the
full operating range.
In DC drives where a single, relatively constant, speed is required
such as a shore powered motor-generator set or a micro-drive a constant
potential is applied to the armature. The constant speed characteristic of the
motors is similar to that of the single speed AC "squirrel cage" induction motor,
and their higher cost and complexity can only be justified by the unavailability
of an AC power source.
4.5.2.3
Compound-Wound Motor Drives.
Compound-wound motors incorporate two
field windings one in series with the armature and one independent of the
armature. These motors exhibit some of the characteristics of both the series and
shunt-wound motors. The degree to which the compound-wound motor resembles either
of the other motors is determined by the strength of the series winding relative
to the shunt winding. Speed is controlled by varying the applied voltage to the
armature and the resultant strength of the series field.
Compound-wound motors may be used for rotate drives on portal and
floating cranes. In these applications they provide high starting torque, smooth
transition between speed points, and control the overhauling effects due to wind
or list and trim. The relative strengths of the field winding should be selected
in consultation with NCC.
4.5.3
DC Drive Electrical Braking/Speed Limiting. Electrical (frictionless)
braking for speed control of DC drives is provided either by dynamic or
regenerative braking circuits. In dynamic braking, resistors are used to absorb
the overhauling energy. The dynamic braking is used when the power supply is
obtained from rectifiers or diesel-generator sets. In regenerative braking, the
overhauling energy is transferred to the electric power utility.
4.5.3.1
Plugging. Plugging is a short-duration form of braking to bring the
drive to a stop. It is a condition where the motor armature continues to rotate
in the previous direction while electrically the direction of the current flow has
been reversed. The level of the sudden, electrically imposed, counter-torque is
determined by current limiting resistance inserted in series with the armature.
Plugging is not used on drives with shunt-wound motors.
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