electric load brake housing to excite load brake if power supply
and holding brakes fail.
d.
Hoist motor control shall provide five-speed dc magnetic control
of ac wound rotor motor with a mechanical load brake. First point
for hoisting shall provide not more than 40 percent of full-load
motor torque. First point lowering speed with any load up to
rated load shall not exceed more than 80 percent of full-load
hoisting speed. Second point lowering shall cause 75 percent of
rated load to lower at not more than 50 percent of full-load
hoisting speed.
e.
Hoist motor control shall provide five-speed, countertorque
control for wound rotor motors. When the control handle is in the
fourth and fifth point lowering position there shall be a positive
drive down of the hook or load to not less than synchronous speed
nor more than 25 percent of synchronous speed with full-rated
load. This fourth and fifth point for lowering shall provide
regenerative braking. All other lowering points shall provide
speed retardation by the application of countertorque.
Countertorque shall increase as the control handle is moved toward
the NEUTRAL or OFF position. Countertorque secondary-resistance
control shall provide for not less than five manually held and one
automatic speed point in each direction of motion.
f.
Hoist motor control shall provide ac static stepless secondary
saturable reactor control. Control shall provide
continuously-adjustable speeds throughout the range from minimum
speed to maximum speed. Eddy-current braking shall provide a
retarding torque for control of light loads in the hoisting
direction and all loads in the lower direction of subsynchronous
speed. To reduce holding brake wear, control shall be arranged so
that the electric load brake is effective in slowing the motion
when the control is in OFF position. Minimum hoist position of
control shall not allow hook to lower with full-rated load on the
hook. Minimum lowering speed at rated hook-load shall not exceed
15 percent of synchronous motor speed. Minimum speed hoisting
with an empty hook shall not exceed 20 percent of synchronous
motor speed. Loads up to 100 percent rated capacity shall raise
on minimum speed point of master. A self-excited alternator shall
be mounted on electric load brake housing to excite the load
brake, if power supply and holding brakes were to fail.
g.
Hoist control system shall provide reversing, constant potential
dc, five-speed, dynamic lowering, variable-resistance, dc magnetic
control of dc series wound hoist drive motors. Full-load lowering
speed shall not exceed the following percentages of rated
full-load hoisting speeds: 30 percent on first point; 205 percent
on fifth speed point. First point hoisting shall provide not more
than 30 percent of rated-motor speed at no-load motor torque, and
zero speed (plus 5 percent, minus 0 percent of rated speed) at not
less than 60 percent of rated-motor torque. Emergency dynamic
speed at no-load motor torque. Emergency dynamic braking circuits
shall be established when the motion control switch is in the OFF
position and when power supply is disrupted.
h.
[Hoist motor speed control shall provide dc stepless, speed
regulated, adjustable-voltage control of dc shunt-wound motors.
Control shall provide continuously adjustable speed from minimum
SECTION 14630A
Page 36
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