The Bently Nevada 3500/50 is a dual-channel tachometer monitoring module designed for integration into the 3500 Monitoring System rack. Its core mission is to provide machinery protection and essential operational information. The module accepts pulse signals from proximity transducers or magnetic pickups, calculates the shaft speed of rotating machinery in real-time, and drives alarm outputs based on user-configured setpoints. This enables immediate annunciation of abnormal conditions such as overspeed, underspeed, zero speed, and reverse rotation.
The module's design embodies high flexibility and integration:
Plug-and-Play Configuration: Shipped unconfigured, it can be programmed on-site using the 3500 Rack Configuration Software to meet specific application needs. This "one-module-fits-many" approach reduces spare parts inventory costs.
Dual Independent Channels: Two independent monitoring channels allow simultaneous speed monitoring of two measurement points on a single machine or two separate machines.
System Integration: Beyond basic speed monitoring and alarming, the 3500/50 can supply Keyphasor® signals to other monitor modules (e.g., vibration monitors) within the rack. The Keyphasor is a critical digital timing reference signal essential for measuring 1X vibration amplitude and phase, making it indispensable for rotor balancing and fault diagnosis.
2. Core Functions and Monitoring Types
The 3500/50 module can be software-configured to perform four primary monitoring functions, addressing diverse operational requirements:
Rotor Speed Monitoring: The fundamental function. It continuously measures and displays shaft speed (RPM). Configurable alarms include Overspeed, Underspeed, and Speed Band (to avoid specific undesirable speed ranges like resonance zones).
Rotor Acceleration Monitoring: Monitors the rate of change of speed (RPM/min). This is particularly useful for detecting abnormal acceleration/deceleration during startup or shutdown, helping to identify potential issues like friction or misalignment.
Zero Speed Monitoring: Specifically designed to detect when a machine has completely stopped rotating. This function requires dual-transducer input (two channels) and employs voting logic to minimize false indication from a single transducer failure. A zero-speed signal is a critical permissive for safely engaging turning gear or performing maintenance.
Reverse Rotation Monitoring: Detects and quantifies reverse rotation of machinery. This is vital for preventing damage in equipment like compressors or fans caused by reverse flow. It monitors reverse speed, peak reverse speed, number of reverse rotations, and provides immediate alarm upon detection of reverse rotation.
3. Advanced Features and Configuration Options
Triple Modular Redundant (TMR) Support: For ultra-high safety applications (e.g., nuclear power, critical compressor trains), the 3500/50 can be configured for TMR operation. Three identical modules are installed adjacently as a group. Alarm outputs use a 2-out-of-3 voting logic, and proportional values are compared between modules. A single module failure cannot cause a system false trip or failure to trip, dramatically enhancing system availability and safety. TMR configuration supports both bussed (single shared sensor) and discrete (redundant sensors) input schemes.
Comprehensive Software Switches & Bypass: Module-level and channel-level software switches allow maintenance personnel to temporarily bypass alarms, enable/disable zero speed monitoring, reset peak hold values, etc. This facilitates online testing and maintenance without physically disconnecting wiring or modifying configurations.
Flexible Status & Alarm Management: The module provides detailed module and channel status information, including "OK", "Alert/Alarm 1", "Danger/Alarm 2", "Bypass", "Off". These statuses are accessible in real-time via the Communication Gateway Module, configuration software, and operator display software. Alarm setpoints are configurable for each enabled proportional value, with options for latching/non-latching modes, adjustable alarm delays, and hysteresis.
Transducer Health Diagnostics: The module continuously monitors the DC voltage of the transducer input signal (for proximity probes) and compares it against preset "OK Limits". This allows it to distinguish between a "stopped machine" condition and a "failed transducer" condition, preventing false "zero speed" alarms due to transducer failure and increasing diagnostic intelligence.
4. System Integration and I/O Module Options
The 3500/50 module must be used with a corresponding I/O Module to adapt to different field wiring and hazardous area requirements. Primary I/O module types include:
Internal Termination I/O Module: Field sensor cables connect directly to Euro-style terminal blocks on the rear of the module. Suitable for cabinet installation in non-hazardous areas with short wiring distances.
External Termination I/O Module: Sensors connect first to separate External Termination Blocks, then to the I/O module via multi-conductor cables. This method simplifies cabinet wiring, eases maintenance and isolation, and is the more common industrial installation.
Internal Barrier I/O Module: Incorporates intrinsic safety barriers. Allows direct connection of intrinsically safe transducers certified for use in hazardous areas (flammable/explosive atmospheres), meeting stringent certifications like Class I, Division 1/Zone 0 (CSA, CENELEC [EEx ia] IIC).
TMR I/O Module: Designed specifically for TMR configurations, supporting both bussed and discrete input methods.
5. Maintenance, Diagnostics, and Lifecycle Support
The manual provides exhaustive maintenance guidance, emphasizing preventive verification over calibration:
Periodic Verification: Recommends establishing a maintenance interval (typically annually) based on machine criticality and environment. Using standard test equipment (power supply, function generator, multimeter) to simulate transducer signals, procedures verify channel threshold, alarm setpoints, speed accuracy, OK status, and recorder output. Step-by-step procedures lower the technical barrier for maintenance.
Fault Diagnostics: The module features robust self-diagnostics.
System Event List: Logs all significant module events (e.g., power faults, configuration errors, communication loss, self-test results) with severity classification (Severe/Potential Problem/Typical). This is the primary tool for troubleshooting.
LED Diagnostics: Specific flashing patterns of the three front-panel LEDs allow quick determination of module state (unconfigured, faulted, normal communication, bypassed).
Alarm Event List: Clearly records alarm entry and exit times.
Firmware Upgrade: Instructions are provided for safely replacing the firmware IC on the module for functional upgrades.
Spare Parts & Ordering: Complete ordering information for modules, I/O modules, termination blocks, cables, and tools ensures reliable lifecycle support.
6. Safety Notices and Critical Application Alerts
Bently Nevada strongly emphasizes the product's intended use and safety responsibilities within the manual:
Not for Control or Overspeed Protection: The Bently Nevada 3500/50 Tachometer is not designed, and must not be used, as a component of a speed control or overspeed trip protection system. It does not provide the protective redundancy and response speed required for such systems. Its primary purpose is condition monitoring and alarm.
Output Signal Purpose Limitation: Its analog proportional outputs are suitable for data logging or chart recording purposes only. Speed alarm setpoints are for annunciation purposes only.
Safety Responsibility: The user is responsible for the proper disposal of the product at the end of its life in compliance with applicable laws. Misuse of the product may result in property damage and/or bodily injury.
