A 27-year-old tower crane used on the NZ$102m ($57m) Westpac Trust Stadium construction site in Wellington, New Zealand has capabilities that much newer cranes on the site cannot match, thanks to the replacement of old control systems with new electronic AC motor controllers.
The stadium’s opening day is set for Wellington Anniversary, 24 January 2000. The role of the tower cranes on the site is particularly important because about 90% of the structure is made up of precast concrete components, most of which have to be lifted into their final position as soon as they arrive by truck from the supplier. Dan Dunne managed the AC motor controller conversion for project contractor Fletcher Construction. He points out: “In this kind of job the only acceptable kind of crane is one that is on the job giving 100% productivity 100% of the available time.” The size of the stadium meant that four of Fletcher’s heavier tower cranes had to be pressed into service to provide adequate site coverage – including a 27 year-old German-built crane. The 20t maximum capacity of the crane, one of the largest in New Zealand, was also needed, but Fletcher Construction had to be sure that the crane was reliable before committing it to the stadium job.
The most troublesome component was the slewing mechanism. There are three slew drives for jib rotation. These are designed to give equal torque on pinions spaced 120O apart around the slewing ring. Each drive unit consisted of a constant speed motor driving a reduction gearbox via an eddy current coupling. Slewing effort was adjusted by varying the DC excitement of the coupling.
The eddy current couplings were prone to failure, both mechanically and electrically. When a coupling failed, the remaining two or one could over-energise, sometimes to a point where complete lock-up was possible. If not identified early enough, mechanical failure of some part of the slewing mechanism followed. A worst case scenario is to strip a tooth from the slewing ring gear at NZ$100,000 ($56,000) for the new ring plus the cost of dismantling and reassembling the crane. A replacement ring is not available ex-stock, so it would have to be manufactured and shipped from Germany.
A mechanical solution, using fluid (hydraulic) couplings was considered but rejected. Instead, Fletcher enlisted the help of Arthur Ede Ltd to design an innovative solution using PDL Microdrive Elite motor controllers. The original drive motors are now direct coupled to the gearboxes and all direction, speed, acceleration, torque and braking is controlled via a bank of Elite controllers.
“We have achieved three very important outcomes”, says Dunne. “First, we have improved the reliability which is crucial to the job. Secondly, the cost was about a third of the hydraulic option we had investigated. Thirdly, we avoided any problems from the limited support for old cranes by the manufacturer. In fact the technology we have put in is similar to the crane manufacturer’s latest technology.” He continues: “The smoothness of control is a bonus, particularly important when placing precast concrete. Operator skill requirement has not been reduced, but with the AC motor controller crane a skilled operator should get more done in a day.” Dunne also found that an electronic system had advantages when fine-tuning the crane on site. “We had done our best in the yard during the R&D phase without actually being able to load the thing. Once working on site we found a problem with the trolley control when the crane was loaded. This hadn’t been anticipated and could have resulted in a major rework at a time when the crane was needed for production. However, Arthur and PDL were able to rectify the problem by reprogramming the Elite drives. Even now we could quickly change its characteristics. No mechanical or structural changes were needed. That is a major asset.” Arthur Ede, managing director of Arthur Ede Ltd, has used PDL Microdrives in many industrial applications of AC motor controllers. “These drives offer us a lot of advantages,” says Ede. “For example there is no inrush current. An ordinary motor might take 100 amps when you first switch it on, but with the Elite if the motor is rated for 20 amps that’s all it will use. This is important on construction sites where the power supply can be a bit weak to start with. We also found that the stress transmitted to the tower structure by the original slew drive settings could exceed the certified limits. With the new drives it was an easy matter to adjust the torque curve to fit within those limits.” The Microdrive Elite controllers are synchronised in a “master and two slaves” configuration, with control data between them transmitted by fibre optic cable. The slave motors’ torque is set by, and equal to, the master. The fibre optic link is immune to the electrical noise common in industrial environments and can handle a very high digital information flow, compared with conventional wiring.
The movement of the hoist trolley along the boom is also controlled by a Microdrive Elite, using PDL Vista software. The trolley control is speed based and incorporates a “slow down” signal and automatic deceleration to a “stop” position close to each end of its track, thus providing a safety margin against operator error. Trolley braking is also provided through the motor, avoiding mechanical wear and providing more positive control.
The use of AC motor controllers on tower cranes is relatively new internationally and already the team have thought of new ideas for using the application of Microdrive Elite and Vista on tower cranes. “We could link the trolley and slew functions together via a PLC to create a collision prevention device”, says Dunne. “For instance we could ensure that the crane would not swing over a road with a load on. We can’t do it with our other electric cranes because the fluid couplings prevent us from controlling slew deceleration and braking without operator intervention. With the refitted old crane, automatic controlled deceleration is available right down to zero.”
