Radford, VA--As with many industries these days, improved efficiency and fewer emissions are critical to the gas turbine business. That's why Cooper-Rolls Coberra, a joint venture between Cooper Cameron Corp. and Rolls Royce plc., now offers electromechanical fuel control on their Coberra 2000 and 6000 turbines.
Built to power electric generators worldwide, as well as centrifugal compressors used for pipeline transmission, these turbines have traditionally relied on hydraulic valves to regulate the supply of fuel to the combustor.
Switching to electromechanical fuel control, says Cooper development engineer Greg Crudele, allows faster positioning with better accuracy. This development, in turn, lowers emissions and boosts efficiency. Crudele adds that the absence of high-pressure oil reduces fire hazards, offers immunity from fluid contamination, and simplifies system installation.
Two components contribute to the new valve's successful implementation: A GOLDline brushless dc servomotor, and FAST Drive servoamplifier. Modified by Kollmorgen Motion Technologies Group for valve manufacturer Whittaker Controls, the motor/controller combination will move the fuel control valve one inch in 100 milliseconds with an accuracy of better than 0.001 inch.
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"The GOLDline motor is a good choice for this application," observes Kollmorgen design engineer Reinhard Beatty, "because of its very high torque-to-inertia ratio." This property, he says, gives the valve its fast acceleration and high step response--important characteristics when the application calls for the delivery of so many pounds of fuel per minute to the combustor. "Any more fuel would be wasteful," Beatty explains, "while any less means the engine would run too hot."
Motor modifications include a precision-ground, stainless-steel ball screw with special bearings, and an explosion-proof housing. The valve stem connects to a nut which rides up and down the motor's lead screw. Angular contact bearings handle the valve's high thrust loads.
Other motor features include: an embedded thermostat for over-temperature detection; 180C winding insulation; a banded rotor for high-speed operation; and a double-duty resolver for both motor commutation and position feedback.
Because the valve uses a brushless motor, it needs a controller. Kollmorgen's FASTDrive servoamplifier, placed in an explosion-proof enclosure, meets the Cooper-Rolls specs. The fully-digital motor drive system lets users set the drive architecture in software and tune the control loops via the serial port. Located remotely from the valve and motor, the controller is programmable for a variety of dynamic tuning variations such as load, speed, acceleration, dynamic response, current limits, and position control. Command and feedback cycle time over the serial port is less than five milliseconds for 16-bit position control.
Presently undergoing system qualification and field evaluation, the Whittaker gas turbine fuel control valve is also under consideration at the General Electric Company.
Additonal details, valve…Contact Whittaker Controls, Inc., 12838 Saticoy St., North Hollywood, CA 91605, (818) 765-8160.
Additional details, motor/controller…Contact Kollmorgen Motion Technologies Group, 501 First St., Radford, VA 24141, (540) 639-9045.