Control Techniques’ Drive Centre in Holland has recently completed the retrofitting of four floating grab cranes. Two of the cranes are rated at 16-tonnes, retrofitted with AC Unidrive variable speed drives, and two larger 25-tonne cranes have gone for a DC solution, with Mentor II DC drives.
All four cranes, situated at Amsterdam, have the four-rope grab system and are mainly used for ship to quay bulk handling. Each crane is powered by an on-board diesel generator set that supplies power to the crane.
A typical project was the 16 tonne floating grab crane, ‘ONE,’ built by Figee at Amsterdam and owned by IGMA. The floating grab crane, mounted on a barge, is a Lemniscate type. For the different movements standard AC-motors were used.
The AC-motors for the hoist and close movements of the grab, as well as the motors for the luffing and slewing movement, are all equipped with Control Techniques Unidrive AC drives.
The data communication between the Unidrives in the control room is based on Control Techniques’ high speed network,CTNet.
Control Techniques Holland provided a turn-key service, including design, engineering, software and programming, the building of the panels and the final installation and on-site commissioning at IGMA Amsterdam.
The scheme comprised Unidrive AC drives on a DC-Bus system, with CTNet-communication between the grab hoist and the grab close drives.
Squirrel cage AC motors control the following movements:
• Hoist/grab close 2x160kW; driven by 2x double size5 Unidrives
• Luffing 1x40kW; driven by 55 kW Unidrive
•Slewing 2x39kW; driven by 55 kW Unidrive
The configuration is a standard drive system with a single quadrant rectifier and brake choppers. Since the crane is powered by a diesel engine/generator set, energy stored in the system cannot be regenerated as is the case with many other dockside cranes. A diode bridge rectifier supplies the inverters for hoist, slewing and luffing via a common DC-bus giving high reliability.
Large brake choppers are required to convert potential energy stored in the hoisting system, or kinetic energy stored in the moving masses, into heat, since no regeneration into the grid can take place. The brake resistors are mounted outside the control panel. The crane control system requirements include:
• Slewing control
• Grab hoisting and grab closing
• Load dependent speed control on the hoist movement
All this software functionality was achieved without a PLC, using the integrated software solution inside a plug-in programmable application module built-in to the drive.
On many conventional cranes, the slewing movement is undertaken with slip-ring motors. The slip-ring motor, in combination with rotor resistors, meets the crane driver’s needs in most instances.
There is good motor torque control for acceleration and deceleration and it is possible to coast when the controller is moved to zero. However, this method of control is very poor at low speeds, with sudden steps in torque between resistor steps wasting a lot of energy, and the system requires regular and intensive maintenance.
When replacing the slip-ring motors with a modern drive system, the results can be very disappointing. Figure 1 shows the behaviour of a conventional speed controlled drive system which makes it almost impossible to control the swaying of the load.
To counter this effect, Control Techniques has developed a programme that gives the crane driver optimal control over the swaying load, without the need for a PLC.
The Control Techniques slewing control system provides the driver with control over both the speed and the motor torque. Speed control is important for accurate positioning at low speed. It also provides compensation for the wind forces on the load.
Torque control is crucial for controlling the sway. In this way, the driver is always able to anticipate the movement of the load and compensate for it. By bringing the controller back to zero, the movement is effectively coasting, which gives a major dampening effect on the sway of the load (Figure 2).
Using Control Techniques drives, there is the dual option of using existing software for a refit, where it exists, and, where this does not apply, there is the further option of using software that is integrated into the drive itself. In either case, it is possible to finetune the system to meet the crane driver’s needs, for an effective solution at a very cost-effective price.