Sarah Mildred Long Bridge.

Photo by JayDuck. Edited from original. Licensed under the Creative Commons CC BY-SA 3.0.

Project Description

UA was contracted on a time and material basis to redesign the control system for a vertical lift bridge between Kittery, ME and Portsmouth, NH.

The old system was comprised of two 100HP DC motors and drives as the primary movers with 35 HP AC across the line starter backups which could lift in the bridge in a reduced speed mode. The new design was to replace all four motors with identical 100 HP AC VFD duty motors run by state of the art flux vector drives. In addition, the submarine cable that provided all control signaling and motor current to the North side of the bridge was to be replaced by a new substation on the North side. This presented many technical challenges because the two sides now were run from differing power sources. There is currently no electrical connection between the North and South sides.

UA provided a communication solution via fiber optics with a wireless backup in the case of communications failure over fiber. The control system is run by redundant GE Rx3i controllers (PLC’s) with Ethernet remote I/O nodes strategically placed to reduce wiring and simplify the overall electrical design. The original south side PLC enclosure contained over three thousand terminations. When we finished, it had less than 500. Reliability of the bridge has increased dramatically. In over 500 lifts to date, the bridge has never gone out of “skew”. This is when the north and south sides are not moving in parallel. This was a common occurrence on the older system and can be a very serious problem.

Project Details

  • Client State of NH-DOT, State of ME-DOT, Interstate Bridge Authority
  • Tags GE Intelligent Platforms, PLC - PAC
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