Retractable roofs have become a popular feature in stadiums because they provide the ability to control the stadium's interior environment more effectively. Only a select number of companies have the knowledge and expertise to mechanize and control these immense structures, which, themselves, become architectural feats.
Uni-Systems, based in Minneapolis, MN, has experience doing this type of engineering work on Minute Maid Park and Reliant Stadium, Houston, TX; Miller Park, Milwaukee, WI; RFK Stadium, Washington, DC; and most recently, the Arizona Cardinals' Stadium, Glendale, AZ.
With its curved roof track, the Arizona Cardinals' project presented interesting challenges in designing the retractable roof mechanism. Uni-Systems partnered with ABB (New Berlin, WI) and selected the ABB ACS800 drive to provide optimum control of the roof's torque-distribution system.
When the National Football League's (NFL) Arizona Cardinals made the decision to construct a new arena several years ago, the desert climate was a major consideration in the design plan. The heat can take its toll on fans and players, alike, and can be detrimental to the playing surface, as well -- especially if it's natural grass. On the other hand, in cooler months, the world-famous climate is perfect for hosting outdoor activities.
The exterior design of the stadium resembles the basic form of the barrel cactus. The retractable roof can be closed so the facility can be air conditioned in the hot months, and then opened in the cooler months. The roof's panels consist of a PTFE (polytetrafluoroethylene) coated woven fiberglass fabric, and are much lighter than a traditional, clad roof. The stadium design includes not only a retractable roof, but also a retractable playing surface.
The innovative, roll-out field will save an estimated $50 million in operating costs, as it is more economical to move the field outside, rather than having the entire roof retract to allow the necessary sunshine to reach the grass. The retractable, natural-grass playing surface is contained in a 16.9-million pound tray that is 234 ft wide by 400 ft long -- the first of its kind in North America. All in all, the design of the stadium is so unique that, to date, it's been featured in a multiple series on The Discovery Channel.
This roof was different than any that Uni-Systems had previously constructed. According to Lennart Nielsen, Danish master electrician and senior electrical designer with Uni-Systems, one of the most important decisions was selecting motor drives to control the roof's movement.
"An important factor in choosing the ABB drives was the inherent risks associated with running a roof on a sloped track," says Nielsen. "This caused us to look for a Variable Frequency Drive (VFD) that would allow us to test the drive torque before each roof motion, to ensure that each drive was operational and capable of a 100% torque output. The ACS800 Drive was capable of this, so before each motion, the Programmable Logic Controller (PLC) checks the torque output from the VFDs at 0 Hz, before committing to opening the motor brakes." Nielsen also says the ABB drives could be installed and operated without the need for closed-loop encoders -- a cost-effective option that helped the company meet budget requirements.
Nielsen explains that for other roofs that Uni-Systems designed, if the brakes were to release and the motors didn't start up for some reason, the roof would simply remain in place. However, at the Cardinals' new stadium -- with its sloped roof track -- if the brakes were released and the motors didn't start up, the roof sections would fall into the parking lot.
"The ABB set-up, with their direct torque control, can measure the feedback from the motor. . . to read the rest of the story, click here.
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