Driven by Technical Challenge

The WSDOT needed a capable and experienced supplier to meet the technical requirements of the project. One challenge to the successful operation of the new system included the reuse 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.