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 MIL-HDBK-1038
4.5.5
AC Drives Electrical Braking/Speed Modulating. The electrical
(frictionless) braking for speed control of AC drives is provided by dynamic
braking, inherent regenerative braking, or eddy-current braking. Regenerative and
dynamic modes of braking transfer the overhauling energy to the electric power
utility or resistors, respectively. Eddy-current braking involves an additional
electromotive rotating device which applies a retarding torque to the drive motor
or against the overhauling torque.
4.5.5.1
Plugging. Plugging, as described in paragraph 4.5.3.1 above, is used on
travel drives with wound-rotor motors and on hoist drives with squirrel cage NEMA
Design D motors; however, it is prohibited on travel drives with a squirrel cage
NEMA Design B motors due to the harshness of the resulting torque reversal. NCC
policy is to require electro-mechanical devices to preclude unintentional
plugging.
4.5.5.2
Emergency Dynamic Braking. Emergency control of the hoist load by means
of dynamic braking, as described in paragraph 4.5.3.2 above, is used on hoist
drives with wound-rotor motors. However, since the AC motors cannot be re-
circuited into a generator (alternator) configuration, an eddy-current brake is
used to provide emergency dynamic braking. If the speed control eddy-current
brake is to be used in this secondary function, an alternator must be provided to
energize it when all electrical power is lost. In either case, the voltage across
the eddy-current brake is established for a maximum terminal velocity of 40
percent of the rated hoisting speed with rated load on the hook.
4.5.5.3
Reduced Voltage Operating Modes. In many crane applications, the sudden
changes in the drive torque of squirrel cage motors are detrimental to crane
operations. In order to soften the behavior of such drives, the applied voltage
is momentarily reduced when starting and during transitions between speed points.
The voltage is reduced either by momentarily inserting resistance in series with
the motor winding or by electronically reducing the voltage being applied to the
motor. In the latter case, the rates (ramping) of voltage reduction and increase
are adjustable. The selected ramping values are set on the control equipment and
are automatically applied during crane operation.
4.5.6
Ancillary Systems. Ancillary equipment on portal cranes consists of
lighting, heating, ventilation, air conditioning, pumps, compressors, battery
chargers, communications equipment, and portable tools plugged into receptacles.
Most of this equipment operates on voltages lower than 480 VAC and is supplied
from panelboards fed from a step-down transformer. Use of a dedicated panelboard
for equipment heater and battery charger circuits provides a convenient way to
assure that this equipment is energized when the crane is not operational. It
would also be appropriate for aircraft warning lights to be supplied from this
panelboard. Receptacles are ground-fault circuit-interrupter protected type or
are supplied from circuit breakers providing ground-fault circuit-interrupter
protection for personnel.
On bridge cranes, ancillary equipment generally consists of lighting and
air conditioning. A step-down transformer and a panelboard are used for this
equipment.
4.5.7
Facility Power Sources. All new Navy facilities provide 480 VAC power in
buildings and at dockside outlets. Virtually all OET, underrunning, and
cantilever cranes have drives designed to utilize this power source. Some gantry
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