Cranes with brains

But 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. It can cost in excess of Euros 500,000 to refurbish a rail mounted gantry crane. So attempting to calculate a return on investment from, say, energy saving using variable speed drives is not practical.

Health and safety and improved productivity offer far more tangible payback. Especially when you consider that today many crane systems are out of date and use old and inefficient motor control technology. This leads to poor crane safety, inaccuracy and lower reliability, as well as high maintenance costs.

The need to ensure the safety of crane drivers and factory personnel has never been greater and has led to companies like ABB investing heavily in developing variable speed drive hardware and software that meet these challenges head on.

Within a crane’s control system there are three areas to consider:

– the electric motor

– the variable speed drive

– its associated control programme or application

All need to work seamlessly together to bring an accurate hoist movement control without endangering crane operators and factory personnel. Whether the crane hook is empty or fully loaded, the crane control system must be able to regulate the speed and to respond to the maximum speed limits.

The motor

Many cranes use slip-ring machines but compared to today’s squirrel cage induction motors, it can be difficult to achieve a steady speed with these machines; they have a high starting torque which can make the crane hoist jump erratically; and they have a higher starting current which can stress the motor’s windings.

Furthermore, slip-ring motors have high rotor resistance relative to induction motors, and that resistance changes to control the speed of the hoist, and also changes with temperature.

While these machines can be retrofitted to a variable speed drive, it is not the best technical or commercial solution. When considering a refurbishment it is well worth considering replacing the old motor technology with squirrel cage induction motors. Induction motors bring accurate and soft control and feature a good torque response which is vital to control the positioning of the load.

The drive

In 1996, ABB launched a revolutionary motor control platform, called Direct Torque Control (DTC) that, at the time, was reported to “change the law of physics,” ABB says, and is widely regarded as the most dynamic and accurate motor control technique available, bringing total control of an AC motor. It plays a critical role in bringing the performance levels, inherent safety and protection functions needed for a crane application.

DTC gives accurate speed and torque control and achieves full torque at zero speed without any feedback device. The torque is controlled with an accuracy of 1% from zero speed through base speed.

So what does this mean in practice? DTC is designed for constant torque applications such as that of overhead cranes, where a load is suspended and the torque must always remain the same, from zero speed to maximum. The drives have an excellent torque response which is vital to the control of the load.

With a suspended load, if the torque is not controlled, the load will swing, with the risk of damage through uncontrolled impacts. During lowering of the load, the drive must constantly dissipate energy. By using an ABB regenerative drive, the energy can be fed back into the electricity grid rather than dissipated to the atmosphere. Alternatively, the same result can be achieved with brake choppers that are built into the drive.

So DTC enables the motor to take full control of a hanging load whether zero load, full load or maximum or minimum. Behind all the wizardry is the fact that it can process data 1,000 times faster than anything previous. With a fast control loop approaching 25 microseconds, there is little risk of overshoot by the crane hoist.

It is the speed of the control loop that lies at the heart of DTC’s success. Traditional variable speed drives using either pulse width modulation or vector control offer control loop speeds substantially slower at between 1 to 3 microseconds. The software within DTC updates the speed and torque signals extremely fast while at the same time rapidly checking the status of the load and carrying out immediate 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 adaptive programming which has been likened to having a small PLC inside the drive. Programmable logical blocks that make up the adaptive programming routine are easy to programme using the drive’s control panel or another PC tool. Adaptive programming enables the user to integrate external control logic or create new functions, so the software can be customised quickly and easily.

For both new build crane manufacturers and the retrofit market this is good news. With the trend towards distributed control, whereby the processing is as close as possible to the motors, the wiring is greatly simplified. In addition, the use of fieldbus simplifies communication between drives, reduces wiring costs and remote I/O.

In crane applications braking is a must, not an option. A mechanical brake is used to hold the load firmly in place. When the drive starts the motor, there is a need to carefully order the release of the brake. If the brake is released too early, the load will fall uncontrollably. It is therefore essential to achieve maximum torque at zero rpm. DTC achieves this objective.

Similar challenges arise when stopping the crane. If the brake is closed too soon, the load jumps. It is essential to gradually and softly reduce speed, while maintaining full torque and then closing the brake. Once the brake is applied, the flux can be removed from the machine.

Crane control programme

Another benefit to braking can be found in the latest crane control programme developed by ABB. Safety and reliability of the mechanical brake control using the torque memory feature ensures that the hook does not drop at the start-up sequence. The integrated brake control logic utilises torque memory and pre-magnetising to open and close the mechanical brake safely and reliably. So the motor torque is stored in the memory when stopping the motor and closing the brake. In the next new start, the same torque level is fed out to the motor to avoid load drop.

These features are part of ABB’s recently launched crane control programme which is promoted as a new, flexible version with open parameters and functions to meet customer’s personalised requirements.

The programme can accommodate different start torque methods and it also includes a brake monitoring function for start, stop or running sequences.

New features of the crane control programme include the ability to check the position of a mechanical brake – every time a brake is applied with no voltage, it must operate effectively. For example, when the line supply falls away the brake must close.

However, it could be that the mechanical opening or closing of the brake is problematic. In this case a small proximity switch indicates if the brake is open or closed.

A warning to the operator indicates that there is a problem with the mechanical brake and helps place the load electrically, thereby using the torque to lower the load.

The drive features good security measures to warn the crane driver. For example, the driver is allowed one minute to take action and lower the load to the ground without any catastrophic collapse.

Another function is speed monitor and speed matching. Here the speed monitor function ensures that the crane motor speed remains within safe limits to prevent overspeed. Speed matching function continuously compares the speed reference and the actual motor shaft speed to detect any difference. One of these functions will stop the motor immediately if a fault occurs in operation of motor.

Other features include master-follower and synchronisation control for common or separate drums/motors, for load sharing or synchronisation of ‘x’ numbers of motions.

Crane control programme is an optional, easy-to-use add-on for ABB industrial drives. It eliminates the need for an external PLC and incorporates all the functions commonly required for hoist, trolley and long travel motions in industrial cranes, harbour cranes or tower cranes.

A PLC, for example, would normally be contained within the overriding control system and is used to check the brake setting and the speed of the motor. However, the ABB industrial drive has a PLC function built-in, thereby eliminating delays or risk in wasted time or communication errors.

Summary

Today’s drives technology has, like all technology, progressed significantly in the last few years to bring huge cost savings and productivity improvements. AC drives provide a cost-effective way to update existing crane systems, thereby enhancing their safety at all levels from start-up through to handling and then braking.

Productivity is improved through the accurate handling of the load – there are no unforeseen movements and the crane is always “listening” for orders.

Maintenance costs are lower particularly as the soft start feature of the drive means there is less stress or heavy wear and tear on ropes, wheels, gearing, bearings, switchgear and motors.

Other noticeable benefits, as a result of the applications being built within the drive, include less space needed for the drive system; greater protection functions; no need for an external PLC within the overriding control system, thereby eliminating time delays or communication errors; lower noise levels; and increased operator satisfaction with the level of control accuracy.

DTC gives accurate speed and torque control and achieves full torque at zero speed without any feedback device. The torque is controlled with an accuracy of 1% from zero speed through base speed ABB 11 Today’s drives technology has, like all technology, progressed significantly in the last few years to bring huge cost savings and productivity improvements ABB 1 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 ABB 2 It can cost in excess of Euros 500,000 to refurbish a rail mounted gantry crane ABB 3 Health and safety and improved productivity offer far more tangible payback. Especially when you consider that today many crane systems are out-of-date and use old and inefficient motor control technology ABB 4 The need to ensure the safety of crane drivers and factory personnel has never been greater and has led to companies like ABB investing heavily in developing variable speed drive hardware and software that meet these challenges head on ABB 5 Many cranes use slip-ring machines but compared to today’s squirrel cage induction motors, it can be difficult to achieve a steady speed with these machines ABB 6 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 ABB 7 AC drives provide a cost-effective way to update existing crane systems, thereby enhancing their safety at all levels from start-up through to handling and then braking ABB 8 When considering a refurbishment it is well worth considering replacing the old motor technology with squirrel cage induction motors ABB 9 Throughout its decades the drive has performed well in four key areas: safety, productivity, efficiency, environment ABB 10