MIL-HDBK-1038
the voltage is generally 120 VAC, single phase. For circuits supplying ancillary
equipment, the voltage is either 120, 208, or 240 VAC.
4.5.18.2 Isolation Transformers. Isolation transformers do not generally change
the power system voltage. They are designed with a 1:1 turns ratio. Three phase
transformers have their primary windings connected in the delta
( ) configuration and their secondary windings connected in the wye (Y)
configuration.
4.5.19
Eddy-Current Brakes. Eddy-current brakes are frequently used with wound-
rotor motors in hoisting applications. The eddy-current brake provides a
supplemental load on the motor and causes the motor to rotate at a slower speed
than it would due only to the load under the hook. The retarding torque is
produced by the rotor turning through the magnetic field produced by direct
current being applied to the eddy-current brake's stator. The eddy-current brake
is usually energized in the first two hoisting speed points and in the first four
lowering speed points.
The eddy-current brake is also energized when the controls are in the OFF
position. This provides the emergency dynamic braking function. An alternator
and rectifier can be outfitted to energize the eddy-current brake if there is a
loss of power while the load is being lowered. This also provides the emergency
dynamic braking function. In both cases, the voltage applied to the eddy-current
brake is selected to limit motor speed to a maximum of 40 percent of rated
hoisting speed.
4.5.20
Electrification Systems. Bridge cranes require both runway and bridge-
to-trolley electrification systems. The three constructions which are most
commonly used are the rigid conductor type, the festooned conductor type, and the
cable reel type.
4.5.20.1 Rigid Conductor Systems. The rigid conductor type of electrification
system consists, for a runway system, of conductors supported from the building
structure and collectors supported from the crane bridge. A bridge-to-trolley
system consists of conductors supported from the crane bridge and collectors
supported from the trolley.
Conductors fabricated from copper are available with ampacities ranging
from 60 to 500 and, depending on ampacity, are available in maximum continuous
lengths of 600 to 2,000 feet. Conductors should be continuous, that is having no
splices, because it had been found in the past that the splice assemblies were the
most troublesome part of the system due to loosening of connectors and corrosion
increasing impedance. NCC policy is to require, for all new cranes, that rigid
electrification conductors be continuous, that is have no splices or mechanical
connections. The conductors are insulated in either rigid PVC housings or
flexible neoprene covers, with special covers being available for high and low
temperature environments.
Redundant, full capacity, sintered copper-alloy/graphite collectors are
to be used. This is to ensure that there are no breaks in the conduction path
during crane operation.
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