Description
Key Technical Specifications
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Model Number: 9905-355
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Manufacturer: Woodward
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Supply Voltage: 20-40 VDC (low-volt); 88-131 VAC or 90-150 VDC (high-volt) selectable
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Actuator Output: 0-200 mA into 40 Ω max, reverse-acting (decreasing current = increasing speed)
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Speed Reference Input: 1-5 VDC (or 4-20 mA via 250 Ω jumper)
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MPU Input: 1-30 V p-p, 1 kHz–25 kHz, 0.010–0.040 in. air-gap
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Load Sharing Input: 0-6 VDC from 5 A CT secondary, gain trim on board
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Droop Range: 0-10 % via 20-turn pot; isochronous when pot fully CCW
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Operating Temperature: –40 °C to +85 °C
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Power Consumption: 8 W typical, 15 W max
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Isolation: 500 V input-to-output, opto-coupled speed share line
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Mounting: 8.5″ × 2″ Euro-card, slides in 2301A chassis or DIN clips
Woodward 9905-090
Field Application & Problem Solved
Steam-turbine throttle valves are fail-closed—lose signal and the valve slams shut to prevent overspeed. The 9905-355 is the reverse-acting version of the 148: 200 mA holds the valve wide open, 0 mA lets it spring closed. I use it on 10 MW extraction units where the throttle is air-to-open, spring-to-close. The card lives in the 2301A chassis, takes MPU pulses from the gearbox, and drives a Woodward TG-17 actuator. When the unit trips or the cable breaks, current collapses to zero and the valve closes under spring force—no overspeed, no runaway. Commissioning is backwards from a forward-acting card: at 0 % speed demand you want 200 mA (valve closed), at 100 % you want 0 mA (valve open). Get it wrong and the turbine screams to 120 % speed on start-up.
Steam-turbine throttle valves are fail-closed—lose signal and the valve slams shut to prevent overspeed. The 9905-355 is the reverse-acting version of the 148: 200 mA holds the valve wide open, 0 mA lets it spring closed. I use it on 10 MW extraction units where the throttle is air-to-open, spring-to-close. The card lives in the 2301A chassis, takes MPU pulses from the gearbox, and drives a Woodward TG-17 actuator. When the unit trips or the cable breaks, current collapses to zero and the valve closes under spring force—no overspeed, no runaway. Commissioning is backwards from a forward-acting card: at 0 % speed demand you want 200 mA (valve closed), at 100 % you want 0 mA (valve open). Get it wrong and the turbine screams to 120 % speed on start-up.
Installation & Maintenance Pitfalls (Expert Tips)
Verify Acting Direction Before You Start
Bump the speed reference to 10 %—actuator current should drop from 200 mA toward 180 mA. If it rises, you have a forward-acting card (9905-148) in the slot. Swap it before you spin the shaft.
Bump the speed reference to 10 %—actuator current should drop from 200 mA toward 180 mA. If it rises, you have a forward-acting card (9905-148) in the slot. Swap it before you spin the shaft.
Set the Null with the Valve Closed
At 0 % demand, dial the null pot until you read exactly 200 mA. Any less and the valve leaks steam; any more and you don’t have headroom to close fully. I use a 0.1 Ω shunt and a DVM—clamp meters lie at 200 mA.
At 0 % demand, dial the null pot until you read exactly 200 mA. Any less and the valve leaks steam; any more and you don’t have headroom to close fully. I use a 0.1 Ω shunt and a DVM—clamp meters lie at 200 mA.
MPU Polarity Matters on Reverse-Acting Cards
Reverse the MPU wires and the card thinks the shaft is spinning backwards—valve drives full open when you ask for stop. Check rotation with a scope: Channel A should lead B by 90° when viewed from the governor end.
Reverse the MPU wires and the card thinks the shaft is spinning backwards—valve drives full open when you ask for stop. Check rotation with a scope: Channel A should lead B by 90° when viewed from the governor end.
Woodward 9905-090
Don’t Trust the Droop Pot Lock-Washer
Vibration walks the 20-turn pot. After setting droop to 4 %, hit the shaft with a dab of thread-lock or nail polish. I’ve seen pots drift to 8 % droop and the plant shed 400 kW before anyone noticed.
Vibration walks the 20-turn pot. After setting droop to 4 %, hit the shaft with a dab of thread-lock or nail polish. I’ve seen pots drift to 8 % droop and the plant shed 400 kW before anyone noticed.
Technical Deep Dive & Overview
The 9905-355 is an analog reverse-acting speed control on a single Euro-card. A magnetic-pickup zero-cross detector feeds a phase-locked loop; the loop compares shaft frequency to an internal 60 Hz crystal and generates a 200-0 mA error signal (inverted from the 148). Load-sharing current from the generator CT is converted to 0-6 V, summed at the PID junction, and drooped by the front-panel pot. An on-board 5 V reference feeds the speed-set pot; an external 1-5 V or 4-20 mA command can override it via jumper. The output stage is a PNP current source that fails to zero on loss of supply—intentional, for fail-closed valve safety. No microprocessor—everything is op-amps and comparators—so it boots in 50 ms and never needs firmware.
The 9905-355 is an analog reverse-acting speed control on a single Euro-card. A magnetic-pickup zero-cross detector feeds a phase-locked loop; the loop compares shaft frequency to an internal 60 Hz crystal and generates a 200-0 mA error signal (inverted from the 148). Load-sharing current from the generator CT is converted to 0-6 V, summed at the PID junction, and drooped by the front-panel pot. An on-board 5 V reference feeds the speed-set pot; an external 1-5 V or 4-20 mA command can override it via jumper. The output stage is a PNP current source that fails to zero on loss of supply—intentional, for fail-closed valve safety. No microprocessor—everything is op-amps and comparators—so it boots in 50 ms and never needs firmware.
