Case Study: The Drive to Win

New Berlin, WI, Summer, 2010 . . . As the saying goes, “Everything is bigger in Texas.” The new home of the Dallas Cowboys is certainly no exception. At a cost of almost $1.3 billion (USD), this enormous structure encompasses over three million square feet, and seats up to a hundred thousand fans. Even though the first football playing season is barely over, the stadium already holds some significant records -- such as the world’s largest enclosed NFL stadium, world’s longest single-span roof structure, and world’s largest high-definition video display.

While football might be what this icon is best known for, the engineering that went into designing the structure is no less impressive. From the mechanization of the retractable roof, to the 600-ton video board hoist system, ABB drives have played a major role. With industry leading features such as Direct Torque Control and easily configurable master/follower operation built-in, the choice for drives was clear. This stadium, in short, has raised the bar on “moving architecture” for facilities that are used by millions of people.

Moving Architecture for “America’s Team”

When Dallas Cowboys’ owner, Jerry Jones, set out to build a new home for “America’s Team,” he did it in true Texas style: Bigger is Better. “As a team of firsts, this new stadium represents the Dallas Cowboys’ innovation and progressiveness for the future, while upholding the traditions of Texas Stadium. The iconic design is architecturally significant and reflects the Dallas Cowboys worldwide brand,” Jones noted.

Building on a legacy that started 50 years ago, the Dallas Cowboys have gained worldwide recognition via winning five Super Bowls, eight NFC crowns and 19 division titles. Along the way, their home always has been as impressive as their record. The original home of the Cowboys, Texas Stadium, was widely known for the hole in its roof – a curious feature at the time. Former Dallas linebacker D.D. Lewis once said that Texas Stadium has a hole in its roof, “So that God could watch his favorite team play.”

With all kidding aside, the open roof is one feature that was carried over into the new stadium for good reason: The retractable roof and moveable end-zone glass doors allow ample sunlight and natural ventilation to flow through the stadium. This type of “open design” gives fans the sense of being outside, in the elements, where the game of football began. If mother nature should become too much to bear, as it frequently does during the hot Texas summer, these openings can be closed in a matter of minutes. It’s this kind of flexibility that makes the new stadium a truly multi-purpose venue. And with the football season lasting only six months each year, Jones is counting on a host of other events to make the venue profitable in the off-season.

Retractable Roof

The retractable roof is comprised of two moveable panels – each weighing 1.68 million pounds. Supporting that enormous load, a full 3.5% of the entire roof weight, are two box trusses that span the length of the stadium – 1225 feet to be exact. Sitting on top of that truss is a steel rail similar to that used by train cars. The panels roll freely along this rail, but are anchored in place by a gear rail, or rack and pinion system. This is a critical component that allows a team of 128 7.5HP motors with planetary gear reducers to pull the panels up the inclined roof.

The slope of the incline varies, up to 24 degrees when the panels are fully open. Multiple gear motors were chosen for the design in order to provide redundancy and manage the safety risk created by the steep travel path. . The multiple motor brakes and gear teeth engaged with the gear rack prevents the failure of any single component from allowing the roof panels to quite literally roll off the roof and fall into the parking lots. This redundant design also allows the retractable roof to be operated with up to 5 of the 32 motors in each quadrant off line.

Optimal Torque at Zero Speed – DTC Ideal in Application

The steep incline creates one additional problem, too. When the panels are fully open and a command is given to close, the motors must start under an enormous load. This high starting torque demand normally requires the high-performance motor control provided by a closed- loop vector drive. This approach, however, was not advisable, due to the high cost and complexity associated with so many motor encoders. Instead, the engineers at Uni-Systems.1 took advantage of ABB’s Direct Torque Control (DTC), which allows an almost identical level of performance without the headache of encoders. Using a 100MHz digital signal processor, the DTC algorithm calculates the current state of the motor 40,000 times per second and determines the best IGBT switching pattern to produce the needed torque. This feature is unique to ABB and one of the main reasons these drives were chosen over those from competing vendors. In a sophisticated application like this, the competition simply wasn’t up to the task.

The ACS800 drives are also line- regenerative drives. This feature allows the drive to decelerate the motors without the use of a brake resistor. As the panels move from an almost level and fully closed position to a fully open position on a downward slope, they cross a point where the motors transition from motoring to braking. It is during this braking phase that the drives are called upon to slow the motors and keep the opening speed under control.

This braking technique is accomplished by converting the kinetic energy of motion to electrical energy inside the drive -- in a process called dynamic braking. Normally the drive dissipates this excess energy as heat, using a brake resistor in much the same way a car braking down a hill causes the brakes to get hot. This heat, or thermal energy, is essentially wasted. A line regenerative drive is an alternative solution that sends this energy back to the utility, instead. While the amount of energy recovered is small, about $14 Dollars worth per opening cycle2, the benefits of not having to install brake resistors were significant, and justified the additional cost of line-regenerative drives.

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