It is intended for portal crane travel drives, as above, but for locations with
high wind gusts.
(DC compound-wound motors are not used on hoist drives; therefore no
motor branch circuit is shown for that application.)
AC Motor Branch Circuits. Figure 39 illustrates the simplicity of the
squirrel cage motor branch circuits, which in large part accounts for the drives'
the wound-rotor motor branch circuit.
Figure 49 also highlights the role of the eddy-current brake in a hoist
drive. It applies a major retarding torque to limit the speed in the first two
hoisting speed points and prevents overhauling in the first four lowering speed
Speed-Torque Curves. The motor speed-torque (or hook speed-load) curves,
which define the crane drives' behavior at various loads and operator's control
inputs, are constructed using the characteristic curves and the motor branch
Figures 42 through 47 are representative motor speed-torque curves of
motors and their control circuits in typical crane drives, and are correlated with
the quadrants of operation. Figures 48 and 49 are hook speed-load curves for
hoist drives with AC wound-rotor motors and supplemental brakes; that is,
mechanical load brake or an eddy-current brake. The presence of these brakes
shields the motors from the full effect of the hook load, and restricts the drive
motors to operation only in Quadrants I and IV; the motors are never subjected to
the conditions normally associated with Quadrants II and III. Therefore, the
drives' behavior is depicted by the relationship of hook speed and hook load,
rather than motor speed and motor torque.
It is desirable for the motor speed-torque curves and hook speed-load
curves to be approximately equidistant in all quadrants. Once the required high
and low speeds are established by the selection of the electrical circuit design,
the intermediate speed points are obtained by the proper selection of resistances,
or other braking means.
Sizing and Selection of Components. Following are the criteria to be
utilized for the sizing and selection of certain crane components. All electrical
components are required to be the industrial or marine grade products of
manufacturers specializing in the production of this type of equipment.
Diesel Engine-Generator Sets. Diesel engine-generator sets on Navy
cranes are small industrial power plants, used whenever outside electric power is
not convenient. They account for most of the operating costs of the crane and
most of the outages for repair and overhaul. Therefore, reliability, long life,
and low maintenance cost are the desirable characteristics.