SYSTEM RENOVATION

In 1999, the Washington State Department of Transportation (WSDOT) decided it was time to replace the bridge's original control system. The old system was becoming obsolete and required increasing amounts of maintenance work and repairs. It was decided that the rehabilitation project would include installing a new bridge control system and a new DC motor drive system.

The WSDOT wanted a non-proprietary system that could be operated from a control desk located in the bridge tower. This demanded the ability to control traffic gates and lights, channel lights, load limits, trunnion limits, and the drive motors operating both the North and South bridge leafs. Furthermore, the WSDOT required that the original 75 year old series wound DC motors be reused in the new system.

DRIVEN BY TECHNICAL CHALLENGE

The WSDOT needed a capable and experienced supplier to meet the technical equirements of the project. One challenge to the successful operation of the new system included the re-use of the originally installed, refurbished Westinghouse series wound 65 HP DC motors. In order to obtain acceptable motor torque and speed characteristics within the constraints of the existing mechanical system, a linear proportional relationship needed to be established between the applied motor armature current and the resulting motor torque. Systems Interface accomplished this by operating the existing series wound motors in a separately excited shunt configuration using a current controlled high ampacity DC supply to separately excite the series fields. This conversion enabled the use of industry standard armature DC/SCR drives and allows the WSDOT to have the required control over the speed and movement of the bridge when raising and lowering each leaf.

The drive system also had to be able to respond to special bridge considerations in order to control the leafs and avoid any oscillations while operating the system. This meant that the drive system must have the ability to compensate for load to no-load transitions caused by the bridge leaf center of gravity passing over the trunnion. This balance condition combined with the machinery windup causes the load to reverse from being driven--to no load--to overhauling. In addition, the balance of the bridge varies with ice and snow loading. The dynamic load of the machinery also varies with wind conditions.

MODERN DC DRIVE AND CONTROL SYSTEM

The new bridge control system consists of one operator control desk and one main control panel. The desk is used in conjunction with the panel to issue commands in order to change traffic lights, operate traffic gates, engage the centerlock, and command North and South leaf motion. Systems Interface not only manufactured the new DC motor drive system, but also designed it.

The DC motor drives consist of two leaf drive/control systems. One system controls the South bridge leaf and the other system controls the North bridge leaf. The North and South leaf drive systems are virtually identical. Each leaf drive cabinet houses two Siemens 75 HP DC four quadrant (reversing/regenerative) digitally controlled SCR motor drives, isolation transformers, tachometers, overspeed switches, limits interface, brake interface and logic/sequence controllers to interface between the motor drives and the bridge control system. The leaf drive systems are connected to the existing 65 HP Westinghouse series wound DC motors (two motors per leaf).

In addition, two separate Siemens 75 HP single quadrant (nonregenerative) SCR drives have been integrated into each leaf drive system. These drives are used to separately excite the DC motor series fields.

The public will now benefit from the completion of this fully functional operation system. Systems Interface successfully integrated the project pieces together in the best way to serve the WSDOT and the public who own the bridge. March 31, 2000 marked its first successful test run. WSDOT personnel were on hand to witness the feat of engineering that brought this historic bridge into the 21st century.