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.
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 and a three-phase inverter.
Select diodes and power transistors so that their current ratings
are not less than 200 percent of full load motor current. Provide
motor overload protection utilizing a thermal sensitive device
embedded in its windings.
Provide dynamic braking. Connect resistors to the controller's
DC bus whenever motor regeneration causes the DC voltage to rise
to a predetermined unacceptable level. Provide resistors
continuously rated at a minimum of 125 percent of the full load
motor current.
b.
Hoist Control: Feed control circuit from a single phase, air
cooled, double wound transformer with a grounded metal screen
between the primary and secondary windings of the transformer.
Hoist shall be equiped with a mechanical load brake.
]2.4.5
Protection
Not less than that required by NEMA ICS 3, and NFPA 70. Provide enclosed
type circuit breaker for crane disconnect. The main line contactor shall
be the floor operated disconnect.
2.4.6
Resistors
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NOTE: Include "125 percent of" only if electronic
controls have been selected previously.
**************************************************************************
Provide resistors rated for continuous duty operation based upon [125
percent of] the motor nameplate amperes and fabricated of corrosion
resistant metal; the use of "wire wound" type resistors is prohibited for
segments of 8 ohms or less. Mount resistors in substantial, ventilated
enclosures constructed entirely of non-combustible materials. Provide
resistors with terminals fitted in the coolest position in the enclosure.
2.4.8
Limit Switches
Provide upper and lower limit switches which de-energize the hoist motor.
SECTION 14637N
Page 17
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