Description
Key Technical Specifications
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Model Number: 9905-031
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Manufacturer: Woodward
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Sensing Voltage: 90-150 VAC or 180-300 VAC (jumper-selectable)
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Frequency Range: 45-65 Hz
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Phase Window: ±5° electrical (fixed)
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Slip Frequency: 0.05-0.5 Hz (field-selectable via DIP)
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Speed Bias Output: 0-7 VDC into 5 kΩ, drives 2301A speed reference
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Breaker Control: Form-C relay, 5 A @ 28 VDC / 120 VAC
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Power: 10 W max, 24 VDC or 125 VDC supply options
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Operating Temperature: –40 °C to +71 °C
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Mounting: 8.5″ × 2″ Euro-card, slides in 2301A chassis
Woodward 9905-031
Field Application & Problem Solved
Paralleling a genset by hand is a good way to blow up a breaker or worse. The 9905-031 sits between the generator PTs and the 2301A speed control, watching voltage, frequency, and phase angle. When slip drops below 0.1 Hz and phase lines up within ±5°, it outputs a speed bias to hold the engine there and closes the “DEAD BUS” relay to fire the breaker. I use it on 2 MW landfill-gas engines and small steam turbines where operators used to stare at a synchroscope and guess when to hit the button. One card replaces the light-bulb scope, the human reaction time, and the inevitable late closures that weld contacts. Real value: it keeps closing even if the operator walks away—no more broken switches or 120° out-of-phase slams that trip the unit offline.
Paralleling a genset by hand is a good way to blow up a breaker or worse. The 9905-031 sits between the generator PTs and the 2301A speed control, watching voltage, frequency, and phase angle. When slip drops below 0.1 Hz and phase lines up within ±5°, it outputs a speed bias to hold the engine there and closes the “DEAD BUS” relay to fire the breaker. I use it on 2 MW landfill-gas engines and small steam turbines where operators used to stare at a synchroscope and guess when to hit the button. One card replaces the light-bulb scope, the human reaction time, and the inevitable late closures that weld contacts. Real value: it keeps closing even if the operator walks away—no more broken switches or 120° out-of-phase slams that trip the unit offline.
Installation & Maintenance Pitfalls (Expert Tips)
CT Polarity Reverses the Phase LED
If the “GEN LEADS BUS” light is on when the engine is actually slow, swap the CT polarity at the terminal strip—not at the breaker—so you don’t disturb the metering circuits.
If the “GEN LEADS BUS” light is on when the engine is actually slow, swap the CT polarity at the terminal strip—not at the breaker—so you don’t disturb the metering circuits.
Set Slip Before You Close the Breaker
Default 0.2 Hz works for 4-pole gensets. On a 2-pole unit dial it down to 0.05 Hz or the breaker slams at 120° out and trips on instantaneous overcurrent.
Default 0.2 Hz works for 4-pole gensets. On a 2-pole unit dial it down to 0.05 Hz or the breaker slams at 120° out and trips on instantaneous overcurrent.
Keep Sensing Leads Under 10 Feet
Long unshielded runs pick up VFD noise and the card thinks phase is jumping. I run #18 shielded twisted pair, ground one end only, and route away from 480 V buckets.
Long unshielded runs pick up VFD noise and the card thinks phase is jumping. I run #18 shielded twisted pair, ground one end only, and route away from 480 V buckets.
Interpose the Dead-Bus Relay
The Form-C contact is rated 5 A but inductive breaker coils will weld it shut. I always drive a 24 VDC ice-cube relay first; costs $8 and saves a $300 card replacement.
The Form-C contact is rated 5 A but inductive breaker coils will weld it shut. I always drive a 24 VDC ice-cube relay first; costs $8 and saves a $300 card replacement.
Technical Deep Dive & Overview
The 9905-031 is an analog phase-locked loop on a single Euro-card. Input transformers drop bus and gen voltage to 12 VAC; zero-cross detectors generate square waves feeding a 4046 phase comparator. The resulting error voltage is filtered to 0-7 VDC speed bias and sent to the 2301A auxiliary input. A separate window comparator watches slip frequency; when both slip and phase are inside limits it energizes the dead-bus relay. No microprocessor—op-amps and comparators only—so it survives RF storms and never needs firmware updates. The card boots in 100 ms and will re-sync after a utility disturbance without operator intervention.
The 9905-031 is an analog phase-locked loop on a single Euro-card. Input transformers drop bus and gen voltage to 12 VAC; zero-cross detectors generate square waves feeding a 4046 phase comparator. The resulting error voltage is filtered to 0-7 VDC speed bias and sent to the 2301A auxiliary input. A separate window comparator watches slip frequency; when both slip and phase are inside limits it energizes the dead-bus relay. No microprocessor—op-amps and comparators only—so it survives RF storms and never needs firmware updates. The card boots in 100 ms and will re-sync after a utility disturbance without operator intervention.
