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 MIL-HDBK-1038
- motors
- resistors
- conductors
In cases of non-compliance with these criteria, sometimes discovered on
older cranes, the equipment or components may be permitted to remain in places
with the approval of NCC.
5.5.1
Motor Operating Regimes. All motors on all crane drives are subjected to
four distinct operating conditions, or "quadrants" of operation. For travel and
rotate drives, these operating conditions are as follows:
Quadrant I motor torque acts in the direction of motion. The speed of
the motion is either maintained (against resisting forces due to friction, wind,
or slope/gravity) or is being accelerated against these forces and inertia.
Quadrant II motor torque acts opposite the direction of motion.
The
speed of motion is being decelerated. This condition is called "plugging."
Quadrant III motor torque acts in the direction of motion, but both are
opposite (in reverse) of those of Quadrant I.
Quadrant IV motor torque acts opposite the direction of motion, but
both are opposite of Quadrant II. The effect is identical to that of Quadrant II,
but in reverse.
These four quadrants are depicted in Part A of Figure 29.
Motor operating conditions of hoist drives are similar, but the dominant
influence of the hook load distorts the operating symmetry between the quadrants.
Quadrant I motor torque acts in the hoisting direction of motion.
The
hook load is hoisted at a constant speed or is being accelerated.
Quadrant II motor torque acts opposite the hoisting motion. This
condition develops only when a lightly loaded but relatively fast hoisting motion
is being stopped, and motor torque is required to counteract the inertia of the
drive's moving parts quicker than would the force from the hook load.
Quadrant III motor torque acts in the lowering direction. The motor
torque is accelerating the drive's moving parts and the hook load accelerates
downward more quickly than it would if it had to overcome the drive's inertia
without the motor torque. If the hoist drive motor does not have "stiff" (non-
load sensitive) torque characteristics that prevent overhauling, an external
supplementary brake torque is required to control the lowering of the hook load.
An eddy-current brake or mechanical load brake is normally used for this purpose.
When combined with the torque produced by the eddy-current brake, the motor torque
is adequate to control the lowering of the hook load. In the case of a hoist
drive with a mechanical load brake, motor torque is required to continuously
relieve the pressure between the wedging friction elements, allowing them to slip
on each other and thereby control the lowering of the hook load.
163
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