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 MIL-HDBK-1038
For DC systems, the drive power equipment normally operates on 240 or 500
volts (VDC) and the control equipment is limited to 240 VDC. Also, 12 or 24 VDC
power would be available from the diesel engine starting batteries on-board portal
and floating cranes.
For AC systems, the drive power equipment and control equipment are
almost always 480 volts (VAC) and 120 VAC, respectively.
The voltage of the various ancillary systems is determined by the ratings
of the available equipment for the required service and the power that is most
readily available from the primary power source; typically it is 240 VDC or 120,
208, or 240 VAC. The power for ancillary systems may be obtained either by means
of step-down transformers or dedicated generators or alternators.
4.5.2
DC Drive Motors. DC motors are categorized according to their armature
and field winding arrangements series-wound, shunt-wound, and compound-wound.
Each has its unique operating characteristics and all three have fitting
applications on cranes.
4.5.2.1
Series-Wound Motor Drives. Series-wound motors have the armature and
field winding connected in series. The operating speed of these motors is
inversely proportional to the imposed load. This characteristic makes them
desirable in applications such as hoist drives (where a lightly loaded hook can be
raised quickly) and on some travel drives (where a high starting torque is
required to overcome a large inertia). However, to limit the maximum speed in the
first one or two hoisting speed points, it is necessary to install a resistance in
parallel with the armature. This feature, known as armature shunting, increases
the field strength to a higher level than would be obtained with the armature
current alone.
Another operating characteristic of series-wound motor drives is the
excessive speed of the drive caused by overhauling loads, due either to the
lowering hook load on a hoist or a tail wind on an outdoor traveling crane. On
hoist drives, overhauling during lowering is avoided by circuitry which
automatically reconfigures motor armature and field connections into a shunt-wound
(constant-speed) design. On travel drives, the overhauling condition is less
severe and develops gradually, so that it is readily controlled by the operator.
The controls for series-wound motors are of the "constant potential"
type, usually with five speed points in each direction. These controls are
referred to as constant potential because the line-to-line voltage remains
unchanged during motor operation. The controls apply a potential (voltage) to the
motor armature at each speed point by shunting out preset segments of resistance
in series with the motor. However, the motor speed is also related to field
strength which is dependent on the load (due to voltage drop across the series
resistance). A weaker field results in faster motor speed. When high values of
series resistance are used to restrict motor speed in slower speed points, a
heavily loaded motion may not begin to move until the control is placed into
second or third speed point.
Series-wound motors have a unique application on travel drives of cranes
that must negotiate curves. When individual motors in same positions (opposite
each other) on the two rails are connected in series, they develop the same torque
but
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