Konecranes World №7

@ KONECRANES KNOW-HOW
Figure 3. Torque controlled inverter
configuration
loading device can be jammed into
external structures while hoisting. Jamming
causes an immediate stop of the
loading device. Depending on the elasticity
of the ropes and supporting structure,
the loading of the hoisting machinery
is increased by rotating inertia
according to the stopping time of the
machinery. Generally the overload limiter
senses the jamming situation and stopping
is initiated by a hard-wired circuit
which engages the brake and interrupts
the power supply to motor. However, this
is not the fastest way to stop the machinery.
In Konecranes drives, snag protection
is initiated when the measured load is
exceeded by a predetermined factor
during the hoisting movement. The initiation
activates a fast stop mode, which
immediately switches the hoisting mode
with speed reference to maximum negative
braking torque reference mode and
engages the brake. The torque direction
change is much faster than brake closing
time. Combined with the initiation of the
fast stop, the occasional loading of the
machinery is reduced significantly.
Combined Multi-Drives
Master-Slave hoisting
Although torque control provides
notable benefits for single drive applications,
even more advantages are realised
with multi-drive systems. Mechanicallyconnected
hoisting systems, like twin
drum or twin motor systems, can easily
begin to vibrate or even tackle against
each other if driven by two separate drives.
This problem can be avoided with
the Master–Slave drive connection.
The master drive has speed and direction
reference as normal and gives as output
the run and actual torque information.
The master drive also controls the
brake and its “slow down” and “stop” limit
switch functions. The slave operates in
18 KONECRANES’ WORLD No 7
torque reference mode, with run and
direction references given by the master.
Special care is taken in abnormal or
tripping situations. If one drive stops, both
drives are stopped.
Master–Slave traversing
An unbalanced situation occurs with
travelling motion when both sides of a
moving crane are unequally loaded. In a
case where there are two synchronised
but separate speed reference- controlled
inverters driving both sides of the crane,
the inverters may start to tackle against
each other while trying to follow the
speed reference path.
Unbalanced loading conditions typically
occur in connection with:
■ the travelling trolley of an overhead
crane with a long span reaches the
end of the span
■ a gantry crane with cantilever travelling
The advanced solution is to drive the
other side of the crane as the master unit
with speed reference. The master outputs
the actual torque and run references.
Brake control, “slow down” and “stop”
limit switch functions are initiated through
the master. The slave unit is controlled
with torque reference according to outputs
from master. If tripping occurs, both
drives are stopped and prevented from
re-start until the obstruction has been
removed.
AC drive response time
The change from DC to AC drives has
been rapid over the past few years. A
common discussion topic among drive
suppliers is whether AC drives can meet
the requirements for response time. From
the operator’s viewpoint, any response
time (or the delay from actuating a controller
to motor motion) exceeding 250 ms
is characterised as a delay.
Analysing the basic technology of
both DC and AC motors shows why there
is a measurable difference. When motor