motors. Squirrel-cage motors with two-speed magnetic
controls provide satisfactory results with slow
bridge and trolley speeds, and should be specified
when short travel distances are involved and where
fine positioning is not required. For faster bridge
and trolley speeds or finer positioning
requirements, specify electronic controls.
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[Provide two-speed magnetic controls for the bridge drive, trolley drive,
and hoist motors. Ensure that an energized drive motor initially rotates
only in the direction selected by the operator by depressing the
corresponding pushbutton; i.e., is not overhauled. Feed control circuits
from a single phase, air cooled, double wound transformer with a grounded
metal screen between the primary and secondary windings of the transformer.
a.
Bridge and Trolley Control
Provide bridge drive and trolley drive motor control systems with
resistive or reactive reduced voltage starting, acceleration, and
deceleration utilizing, for each, separate banks of voltage
reducing resistors or reactors and timing relays. On
deceleration, insert the resistors or reactors into the motor's
high speed leads prior to de-energization of the high speed
contactor. Ensure smooth acceleration and deceleration. Provide
bridge drive and trolley drive motor control systems with a drift
point between OFF and the first speed control point in each
direction.
b.
Plugging Protection
Provide plugging protection for the bridge and trolley drives.
Provide time delay relays, which will prevent the plugging of
bridge and trolley drive motors, in the control circuit; arrange
the controls to set the associated drive's brake if attempts are
made to plug.]
[Provide two-speed magnetic controls for the hoist motor; provide
four-speed adjustable frequency controls for the bridge and trolley drive
motors. Ensure that an energized drive motor initially rotates only in the
direction selected by the operator by depressing the corresponding
pushbutton; i.e., is not overhauled.
a.
Bridge and Trolley Control
Provide static reversing, dynamic braking, adjustable frequency
(achieved by sinusoidal pulse width modulation), constant torque
controllers for the bridge and trolley drive motors. Size each of
the controllers so as to provide sufficient starting torque to
initiate motion of that crane drive from standstill with rated
load under the hook. The acceleration time and the deceleration
time shall be independently adjustable from 2 to 20 seconds;
initially, set acceleration at 5 seconds and deceleration at 2
seconds. In each direction, provide a drift point between OFF and
the first running speed point; provide four running speed points,
namely 10, 33, 67, and 100 percent of rated speed. The motor
shall run smoothly, without torque pulsations at the lowest speed,
and shall be energized at a frequency not exceeding 60 Hz at the
highest speed. Include with the controller a full wave rectifier
SECTION 14636N
Page 14
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