MIL-HDBK-1038
direction will not allow a hook block or boom to lower under rated load. When
shunt-wound DC motors are connected in parallel, as in the rotate drive on a
portal or floating crane, resistance is required to be connected in series with
each armature for load equalization.
NEMA utilizes Class Numbers to designate the current ratings of
resistors. The Class 90 rating is the continuous rating of a resistor. Other
Class Numbers used on industrial (non-Navy) cranes are 150, 160, and 170. The
current rating of each of these other Class Numbers is based upon the resistor
being "on" for 15 seconds and being "off" for 45 seconds, 30 seconds, and 15
seconds, respectively. NEMA Standards Publication No. ICS 9, Part 2 addresses
resistors. It is NCC policy to require that the continuous rating of a resistor
must equal or exceed the full load amperes of the associated motor. When armature
shunting is used to limit the hoisting speed of a lightly loaded motor, armature
shunt resistor selection is required to be verified for across-the-line current
because it could initially draw an equivalent current during plugging. With a
shunt-wound DC motor, the resistors in the generator's field circuit are selected
to provide evenly spaced speed points; for a hoist motor, the resistance in series
with the generator field is different in each speed point for hoisting and
lowering directions.
When a diesel engine is not equipped with electronic speed and fuel
controls, a load bank is required to be used with the set to maintain a minimum
loading on the diesel engine (typically 40 percent) to obtain complete combustion
of the fuel. The output from the generator is sensed and when it falls below 40
percent, the load bank is energized. When the output exceeds 80 percent, the load
bank is disconnected. Load banks are required to establish the maximum
regenerative load level for diesel engine-generator sets. Load banks are sized
based upon the maximum amount of regenerative power available from predetermined
combinations of simultaneous deceleration of travel or rotate drives and lowering
of a fully loaded hook hoist and/or the boom. Limiting regenerative power
prevents overspeeding of the diesel engine due to "back-driving" by the generator.
When regenerative power exceeding 50 percent of the diesel engine's friction power
rating is sensed, the load bank is connected across the generator to absorb the
regenerative power. When regenerative power being sensed is less than 50 percent
of that rating, the load bank is disconnected.
Resistors are required to be fabricated of stainless steel or other
corrosion resistant metal; the use of "wirewound" type resistors is prohibited for
segments of 8 ohms or less. Resistors are required to be mounted in substantial
enclosures designed to permit the free flow of air sufficient to cool the
resistors within their current rating by natural convection. When the resistor
enclosure is mounted outdoors, it is required to be fabricated from stainless
steel.
5.5.5.11 Eddy-Current Brakes. Eddy-current brakes are required to be rated to
produce not less than the following percentages of rated hoist motor torque: 75
percent at 1/8 full speed; and 112.5 percent at 1/4 full speed.
5.5.5.12 Limit Switches. On bridge cranes, there are required to be two hoist
upper limit switches. The first one is required to be an adjustable geared
control circuit type limit switch. On a DC powered crane, the second one is
required to be a block actuated power circuit type limit switch requiring
resetting of the crane control circuit by a keyed bypass switch prior to resuming
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