9905-877

About the 9905-877

This overspeed protection device continually monitors for overspeed conditions by using three magnetic pickups (MPUs) interfaced with three individual speed-sensing units, which each perform its own speed calculation. When overspeed conditions are detected after a two-out-of-three vote, a trip relay is activated. Model 9905-877 is an energize-to-trip system.

Features of this model include remote reset, alarm output, built-in self-test, and a locked enclosure.

The locked enclosure of model 9905-877 ensures no unauthorized changes are made to the systems programming and no unauthorized testing is performed, as key access is required to switch between MONITOR and PROGRAM modes. The 9905-877 enclosure meets NEMA 4 / IP 65 standards.

Speed-sensing units may be tested or replaced one at a time while the prime mover remains online. The 9905-877 system is designed in a way that an accurate output can be maintained with only two of the three units. If one of the units does trip, it needs to be reset or have the power cycled before it can be brought back online.

A few things that might cause a trip are MPU failure or loss of signal, interlock failure, an overspeed test, or the unit simply was not reset before the turbine was re-started. If a unit trips, the trip will be indicated by an alarm output and an LED on that unit’s operator interface.

Information on installation, troubleshooting, and additional 9905-877 specs are in Woodward’s manual 85191 (version B).