MIL-HDBK-1038
4.5.3.2
Emergency Dynamic Braking. Emergency control of the hoist load is
provided by electrical re-circuiting of the drive motor into a self-excited
generator driven by the descending hook load. The emergency dynamic braking
circuits are automatically established whenever the controls are in the OFF
position. The load cannot be brought to a complete stop, but its terminal
velocity can be limited.
For drives with series-wound motors, the emergency dynamic braking
circuit establishes a loop with the armature, field, and resistance connected in
series. The terminal velocity is determined by the amount of the series
resistance, and is normally selected for a maximum terminal velocity of 40 percent
of the rated hoisting speed with the rated load on the hook.
For drives with shunt-wound motors, the circuit establishes a loop with
the armature, field, and resistance connected in parallel. The terminal velocity
is proportional to the ratio of the field voltage to the armature voltage, and
these voltages are normally selected to limit the terminal velocity to a maximum
of 40 percent of the rated hoisting speed with rated load on the hook.
4.5.3.3
Speed Limiting. Since the speed of series-wound motors is inversely
proportional to the torque, the motor speed will increase to a dangerous level if
its load is removed. The loss of motor load may be caused by a broken wire rope
or a sheared shaft. Series-wound drive motors that are unattended or are not
under the control of a qualified operator require a speed-limiting device (such as
a centrifugal switch) in the control circuit.
In the case of shunt-wound and compound-wound motors, their shunt fields
inherently limit their speeds under unloaded conditions.
4.5.4
AC Drive Motors. Two categories of AC motors are used widely on cranes
"squirrel cage" and wound-rotor. Being induction motors, they are simpler in
design and require less maintenance than the DC motors. Although their
operational characteristics differ from those of DC motors, their controls and
additional equipment can be arranged to accommodate the requirements of virtually
all crane drives.
4.5.4.1
Squirrel Cage Motor Drives. Squirrel cage motors have a primary winding
(stator), which is connected to the power source, and a rotor, which carries the
induced secondary current. They are inherently constant-speed motors; the speed
being determined by the power supply frequency and the number of pole pairs. The
maximum speed of an unloaded motor is the synchronous speed, and as load is
applied, the speed is reduced only slightly. The precise relationship between the
speed and load (torque) varies with the design details of the motor design. The
speed of these motors can be changed significantly only by means of complex
electronic controls. The motors are available in single speed or two-speed
construction. Two-speed motors have two separate stator windings, one for each
operating speed. The most common speed ratio is 3:1, but 2:1 and 4:1 are also
available.
88
Previous Page
