Description
Key Technical Specifications
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Model Number: 9905-131
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Manufacturer: Woodward Inc.
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Product Family: 2301A Load Sharing and Speed Control (LSSC)
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Power Supply: 10-40 V DC (Low Voltage Model)
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Power Consumption: ≤15 W
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Actuator Output: 0-200 mA current signal
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Actuation Type: Forward-acting, single actuator
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Speed Sensing: Magnetic Pickup Unit (MPU), 1.0 Vac minimum to 30 Vac maximum
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Speed Sensor Input Impedance: 100-300 Ω
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Speed Ranges (Switch-Selectable): 500-1500 Hz, 1000-3000 Hz, 2000-6000 Hz (standard), 4000-12000 Hz
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Control Modes: Isochronous or Droop (selectable via external switch/relay)
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Load Sharing: Self-contained load sensor with 0-6 Vdc analog load-sharing lines
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Operating Temperature: -40°C to +85°C (-40°F to +185°F)
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Vibration Rating: 4 Gs, 5 to 500 Hz
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Shock Rating: 60 Gs
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Dimensions: 219.075 mm W × 184.15 mm H × 63.5 mm D
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Weight: < 4 lbs (1.8 kg)
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Enclosure: Sheet-metal chassis with single PCB architecture
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Start Fuel Limiting: Automatic, adjustable to reduce emissions and engine wear
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Compatibility: Woodward SPM-A Synchronizers, AGLC, APTL, Import/Export Controls
Woodward 9905-090
Field Application & Problem Solved
In the field, the biggest challenge with multi-generator installations is maintaining stable frequency while ensuring equal load distribution across units. Without proper load sharing, one genset ends up carrying the bulk while others “motor” or fight each other, creating thermal stress, fuel inefficiency, and potential reverse power trips. The 9905-131 solves this by providing true isochronous load sharing through analog load-sharing lines that balance kW output proportionally across all units in the system.
You will typically find this controller in power generation applications where reliability trumps complexity—think offshore platform auxiliary generators, drilling rig power plants, hospital backup systems, and industrial cogeneration setups. It’s particularly common on Caterpillar, Waukesha, and Cummins engine packages from the 1980s through early 2000s that shipped with Woodward governing packages. The 9905-131 is the low-voltage, forward-acting, single-actuator variant, meaning it drives standard fuel racks (increasing current increases fuel) and runs on 24V DC battery systems rather than 125V DC station power.
Its core value is maintaining bus frequency within ±0.25% of rated speed while seamlessly transitioning between isochronous mode (isolated bus operation with load sharing) and droop mode (utility parallel or infinite bus operation). The integrated load sensor eliminates the need for external load sensors—this control measures generator current and voltage internally, calculates real power, and generates the load-sharing signal. The automatic start fuel limiting is another field-saver; it caps maximum fuel during startup to prevent wet stacking in diesels or over-firing in gas engines. For technicians, the all-front-access potentiometer design means you can tune gain, stability, and droop without pulling the unit from the panel—a significant advantage when you’re working in cramped switchgear rooms.
Installation & Maintenance Pitfalls (Expert Tips)
Verify Your Actuator Direction Before Power-Up
A common rookie mistake is assuming all actuators are forward-acting. The 9905-131 is specifically forward-acting (0-200 mA output where increasing current increases fuel). If you’re retrofitting this onto a reverse-acting actuator setup (like some EGB governor configurations), the engine will slam to minimum fuel when it should be starting. Check your actuator part number and compare against Woodward’s compatibility matrix before applying power. If you smell smoke or hear the actuator rattle against the fuel stop immediately after start, kill power and verify direction. The reverse-acting equivalent is the 9905-133.
Load-Sharing Line Terminations Are Critical
The analog load-sharing lines (terminals 10 positive, 11 negative) must be wired in parallel across all units with proper shielding. I’ve seen installations where the shield was grounded at both ends, creating ground loops that introduced 60 Hz hum into the load-sharing signal. This manifests as erratic kW hunting between units—one genset surging while another sags. Ground the shield at terminal 12 (control end only), and keep the load-sharing wiring twisted-pair, separate from high-current power cables. When mixing 2301A controls with other Woodward systems, do not connect shields at the load-sharing bus junction.
Droop vs. Isochronous Switch Wiring
The mode selection depends on terminal 14 being connected to terminal 16 (common) for isochronous, or opened for droop. Many field issues stem from improper auxiliary contact wiring from the circuit breaker. If you’re paralleling with the utility, you need droop mode; if you’re on an isolated bus with other Woodward controls, you need isochronous. I’ve seen plants where the breaker auxiliary contact was wired backwards, causing the unit to run isochronous into the grid—resulting in immediate reverse power trips or instability. Verify your breaker contact logic with a multimeter before commissioning. Remember: both droop contact AND breaker contact must be closed for isochronous load sharing.
Speed Sensor Phasing Matters
The MPU input is sensitive to phasing. While the control will typically “run” with reversed leads, you’ll see erratic speed readings and poor stability. If your speed indication jumps around or the control hunts excessively, swap the MPU leads at the terminals. Also, ensure your MPU gap is set correctly—typically 0.020 to 0.030 inches for standard magnetic pickups. Too wide a gap gives weak signal; too close risks contact damage from shaft runout. The 9905-131 accepts 1.0 to 30 Vac input, so you have margin, but a weak signal degrades stability.
CT Polarity Determines Load Sharing Direction
The current transformer connections must be phased correctly for the load sensor to work. If your CTs are reversed, the control will see negative load and try to “correct” by adding fuel, causing runaway or reverse power conditions. During commissioning, verify that increasing generator load shows increasing voltage on the load-sharing lines (terminals 10-11). If load increases but the voltage decreases, swap your CT leads. The standard connection is Wye, though open-delta is acceptable for some applications.
Gain and Stability Settings Interact
The front-panel GAIN and STABILITY pots are interactive. High gain gives fast response but can cause oscillation; high stability damping prevents oscillation but slows response. The rule of thumb is: set GAIN for 5-10% overshoot on a step load change, then increase STABILITY until the oscillation just disappears. In multi-unit plants, all units should have identical settings to prevent hunting. I’ve seen installations where one unit had high gain and another had low gain—the system never stabilized because the controls fought each other.
Woodward 9905-090
Technical Deep Dive & Overview
The Woodward 9905-131 is an analog load sharing and speed control module from the 2301A series, representing a mature, field-proven architecture that predates fully digital governor systems. It consists of a single printed circuit board housed in a rugged sheet-metal chassis, with all tuning potentiometers accessible from the front panel—no software interface, no programming cables, no firmware revisions to track.
The control operates as a closed-loop speed regulator with an inner current loop driving a proportional actuator. Speed feedback comes from a magnetic pickup unit sensing gear teeth on the engine flywheel or accessory drive, converted internally from frequency to voltage via a discriminator circuit. This speed signal is compared against a reference (set by front-panel SPEED ADJUST potentiometer or external trim), with the error amplified and conditioned through GAIN and STABILITY (reset) networks before output to the actuator driver stage. The 0-200 mA output is a current source, not a voltage source—this provides better noise immunity and consistent torque from the actuator regardless of wiring length.
The load sharing function operates through an analog current loop between paralleled units. The 9905-131 contains an internal load sensor that measures generator current (via CTs) and voltage (via PTs), calculates real power using analog multipliers, and generates a 0-6 Vdc signal representing load magnitude. This voltage is shared across the parallel load-sharing lines (terminals 10-11). The control adjusts its speed reference slightly to equalize these voltages, resulting in proportional kW sharing without master/slave hierarchy. The load-sharing-line relay (internal to the control) energizes when both the droop contact and breaker auxiliary contact are closed, connecting the load-matching circuit to the system.
From a system architecture perspective, the 9905-131 functions as the primary fuel control authority, receiving permissive and mode commands from external switches (idle/rated, raise/lower, droop/isochronous select) and interfacing with accessories like Woodward SPM-A synchronizers for automatic paralleling, AGLC modules for soft loading, or APTL controls for utility paralleling. It does not provide serial communication or remote diagnostics—tuning requires physical access to the front panel, which is both a limitation and a reliability advantage in harsh environments where network-based controls might fail. The 10-40V DC power input makes this unit suitable for 24V nominal battery systems common in mobile and marine applications, with internal DC-DC isolation protecting against ground loops and allowing operation during battery transients up to 77V for five minutes without damage.
