This article discusses the benefits of variable speed drives for cranes. It explains that drives have been used for over 30 years to control crane motors and provide more precise control than traditional techniques. Safety and productivity are the main benefits, as drives allow for accurate speed and torque control to safely move loads without swinging. The article focuses on ABB drives and their direct torque control technology, which provides ultra-fast and precise control of cranes without needing feedback devices. This improves both safety and productivity in crane operation.

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The international factory crane magazine October 2007 Issue 65
HOIST
www.hoistmagazine.com
ON SITE
Cranes with brains Exploring the benefits of drives
Exploring the benefits of drives
Historic hoists Slings LiftEx preview

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DRIVES
Cranes with brains
For over 30 years variable speed drives have been at the heart of many crane control
operations. In its most basic form, a drive controls the speed and torque of electric
motors, with a precision and accuracy often not achievable with more traditional
techniques, says Mikael Holmberg, application manager, drives, ABB Oy
T
hroughout its decades the hardware and software that meet these
drive has performed well challenges head on.
in four key areas: Within a crane's control system
• safety there are three areas to consider:
• productivity • the electric motor
• efficiency • the variable speed drive
• environment • its associated control programme or
But unlike other industries where application
the main reason for using a drive is All need to work seamlessly
energy saving, in cranes the number together to bring an accurate hoist
one reason is often health and safety movement control without
related. It can cost in excess of Euros endangering crane operators and
500,000 to refurbish a rail mounted factory personnel. Whether the crane
gantry crane. So attempting to hook is empty or fully loaded, the
calculate a return on investment from, crane control system must be able to
say, energy saving using variable speed regulate the speed and to respond to
drives is not practical. the maximum speed limits.
Health and safety and improved
productivity offer far more tangible The motor changes to control the speed of the DTC gives accurate
payback. Especially when you consider Many cranes use slip-ring machines but hoist, and also changes with speed and torque
control and achieves
that today many crane systems are out compared to today's squirrel cage temperature. full torque at zero
of date and use old and inefficient induction motors, it can be difficult to While these machines can be speed without any
motor control technology. This leads to achieve a steady speed with these retrofitted to a variable speed drive, it feedback device. The
torque is controlled
poor crane safety, inaccuracy and machines; they have a high starting is not the best technical or with an accuracy of
lower reliability, as well as high torque which can make the crane hoist commercial solution. When 1% from zero speed
maintenance costs. jump erratically; and they have a higher considering a refurbishment it is well through base speed,
according to ABB
The need to ensure the safety of starting current which can stress the worth considering replacing the old
crane drivers and factory personnel has motor's windings. motor technology with squirrel cage
never been greater and has led to Furthermore, slip-ring motors have induction motors. Induction motors
companies like ABB investing heavily in high rotor resistance relative to bring accurate and soft control and
developing variable speed drive induction motors, and that resistance feature a good torque response which
Unlike other
industries where the
main reason for
using a drive is
energy saving, in
cranes the number
one reason is often
health and safety
related

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DRIVES
regulation. It is an ultra fast loop,
continuously checking and controlling
the information. The benefit to
operational safety is immense.
If you really want a slow speed that
is what you get. For example, if the
crane is carrying a heavy load, it can
take it to the end position and stop
without suffering any over-swing of the
load. Loads can be hoisted and moved
quicker and more safely, thereby
improving the productivity of the
industry in which the crane is operating.
DTC is at the heart of the ABB
industrial drives portfolio. This is the
largest of all of ABB's low voltage AC
drives with the power range from
0.55kW to 5600kW and is the
preferred drive for crane applications
due to its robust design and control
method. The drive also features
ABB says drives is vital to control the positioning of torque is not controlled, the load will adaptive programming which has been
provide a cost
the load. swing, with the risk of damage through likened to having a small PLC inside
effective way to
update existing uncontrolled impacts. During lowering the drive. Programmable logical blocks
crane systems, The drive of the load, the drive must constantly that make up the adaptive
enhancing their In 1996, ABB launched a revolutionary dissipate energy. By using an ABB programming routine are easy to
safety from start-up
to handling motor control platform, called Direct regenerative drive, the energy can be programme using the drive's control
and then braking Torque Control (DTC) that, at the time, fed back into the electricity grid rather panel or another PC tool. Adaptive
was reported to "change the law of than dissipated to the atmosphere. programming enables the user to
physics," ABB says, and is widely Alternatively, the same result can be integrate external control logic or
regarded as the most dynamic and achieved with brake choppers that are create new functions, so the software
accurate motor control technique built into the drive. can be customised quickly and easily.
available, bringing total control of an So DTC enables the motor to take For both new build crane
AC motor. It plays a critical role in full control of a hanging load whether manufacturers and the retrofit market
bringing the performance levels, zero load, full load or maximum or this is good news. With the trend
inherent safety and protection functions minimum. Behind all the wizardry is the towards distributed control, whereby
needed for a crane application. fact that it can process data 1,000 the processing is as close as possible
DTC gives accurate speed and times faster than anything previous. to the motors, the wiring is greatly
torque control and achieves full torque With a fast control loop approaching simplified. In addition, the use of
at zero speed without any feedback 25 microseconds, there is little risk of fieldbus simplifies communication
device. The torque is controlled with overshoot by the crane hoist. between drives, reduces wiring costs
an accuracy of 1% from zero speed It is the speed of the control loop and remote I/O.
through base speed. that lies at the heart of DTC's In crane applications braking is a
So what does this mean in success. Traditional variable speed must, not an option. A mechanical
practice? DTC is designed for drives using either pulse width brake is used to hold the load firmly in
constant torque applications such as modulation or vector control offer place. When the drive starts the motor,
that of overhead cranes, where a load control loop speeds substantially there is a need to carefully order the
is suspended and the torque must slower at between 1 to 3 release of the brake. If the brake is
always remain the same, from zero microseconds. The software within released too early, the load will fall
speed to maximum. The drives have an DTC updates the speed and torque uncontrollably. It is therefore essential
excellent torque response which is vital signals extremely fast while at the to achieve maximum torque at zero
to the control of the load. same time rapidly checking the status rpm. DTC achieves this objective.
With a suspended load, if the of the load and carrying out immediate Similar challenges arise when

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DRIVES
recently launched crane control Crane control programme is an
programme which is promoted as a optional, easy-to-use add-on for ABB
new, flexible version with open industrial drives. It eliminates the need
parameters and functions to meet for an external PLC and incorporates all
customer's personalised requirements. the functions commonly required for
The programme can accommodate hoist, trolley and long travel motions in
different start torque methods and it also industrial cranes, harbour cranes or
includes a brake monitoring function for tower cranes.
start, stop or running sequences. A PLC, for example, would
New features of the crane control normally be contained within the
programme include the ability to check overriding control system and is used
the position of a mechanical brake - to check the brake setting and the
every time a brake is applied with no speed of the motor. However, the
voltage, it must operate effectively. For ABB industrial drive has a PLC
example, when the line supply falls function built-in, thereby eliminating
away the brake must close. delays or risk in wasted time or
However, it could be that the communication errors.
mechanical opening or closing of the
brake is problematic. In this case a Summary
small proximity switch indicates if the Today's drives technology has, like all
brake is open or closed. technology, progressed significantly in
A warning to the operator indicates the last few years to bring huge cost
that there is a problem with the savings and productivity improvements.
When considering stopping the crane. If the brake is mechanical brake and helps place the AC drives provide a cost-effective way
a refurbishment closed too soon, the load jumps. It is load electrically, thereby using the to update existing crane systems,
it is well worth
considering replacing essential to gradually and softly reduce torque to lower the load. thereby enhancing their safety at all
the old motor speed, while maintaining full torque and The drive features good security levels from start-up through to handling
technology with
then closing the brake. Once the measures to warn the crane driver. For and then braking.
squirrel cage
induction motors brake is applied, the flux can be example, the driver is allowed one Productivity is improved through the
removed from the machine. minute to take action and lower the accurate handling of the load - there are
In its most basic
form, a drive controls
load to the ground without any no unforeseen movements and the
the speed and torque Crane control programme catastrophic collapse. crane is always "listening" for orders.
of electric motors, Another benefit to braking can be Another function is speed monitor Maintenance costs are lower
with a precision and
accuracy often not found in the latest crane control and speed matching. Here the speed particularly as the soft start feature of
achievable with programme developed by ABB. Safety monitor function ensures that the the drive means there is less stress or
more traditional and reliability of the mechanical brake crane motor speed remains within heavy wear and tear on ropes,
techniques
control using the torque memory safe limits to prevent overspeed. wheels, gearing, bearings, switchgear
feature ensures that the hook does not Speed matching function continuously and motors.
drop at the start-up sequence. The compares the speed reference and Other noticeable benefits, as a
integrated brake control logic utilises the actual motor shaft speed to detect result of the applications being built
torque memory and pre-magnetising to any difference. One of these within the drive, include less space
open and close the mechanical brake functions will stop the motor needed for the drive system; greater
safely and reliably. So the motor immediately if a fault occurs in protection functions; no need for an
torque is stored in the memory when operation of motor. external PLC within the overriding
stopping the motor and closing the Other features include master- control system, thereby eliminating
brake. In the next new start, the same follower and synchronisation control for time delays or communication errors;
torque level is fed out to the motor to common or separate drums/motors, for lower noise levels; and increased
avoid load drop. load sharing or synchronisation of 'x' operator satisfaction with the level of
These features are part of ABB's numbers of motions. control accuracy.