Vijay Jayabalan - Captronic Systems
Anu Kalidas - Captronic Systems, PVT. LTD.
Essentially, a starter generator rotates the turbine engine fast enough to ignite the fuel, so that the engine takes over as a generator providing DC power to the electrical system after the start of an aircraft.
The starter generator is tested in two different modes, generator and starter modes, to check its performance at various load conditions. Both modes have a common control desk and a data acquisition system (DAS) to control, monitor, and store the required parameters.
In this mode, the equipment used includes an inverter duty AC motor, an AC drive, a step-up gearbox, a torque sensor, a spline shaft, and cooling blower. The AC motor and step-up gearbox is mounted on a mild steel-base plate. This setup is grouted onto a foundation/stress-relieved mild steel structure. The gearbox output spline shaft is coupled to the generator.
During operation, the generator is driven by an AC motor connected to a 1:10 step-up gearbox. The generator was flange mounted onto a hollow shaft torque sensor, and the torque sensor was then mounted on the step-up gearbox. The AC motor speed is varied through a closed-loop flux vector drive. The generator was loaded at various speeds in steps using the resistive load bank in steps and a momentary load for 0.3 s at the beginning of each step of loading.
In this mode, the equipment used includes a powder dynamometer, an adapter plate, a mounting pad, and a spline shaft. The powder dynamometer is mounted on a mild steel-base plate. The starter motor is mounted on the reaction-type, hallow-shaft torque sensor. The power supply to the starter motor is fed from a DC power supply of a maximum of 4000 A and 24 V. There is a separate field supply of 30 V, 20 A for the field coil excitation.
The starter generator can be tested in both auto mode and manual mode. In auto mode, the CompactRIO data acquisition and control system takes control of the entire test. In manual mode, the user can control the test by performing appropriate operations in sequence through the inputs available in the control desk. The control desk is used for getting all the operational instructions. Test inputs such as generator speed, load, and starter torque are fed through the PC. The control disk consists of potentiometers, selector switches, illuminated push buttons, and emergency stop and reset buttons for getting the operator instructions.
Auto Mode of Operation in Generator Mode
In generator mode, auto mode has user-programmable entries available for functions such as the speed of the generator, load-and-load duration, and generated voltage. The system establishes the test conditions including switching on the drive, blower motor, and lube oil pump. Once the initial condition is established, generator speed is set through the flux vector drive by generating appropriate analog output voltage. The system regulates the generator voltage through the field excitation and uses the digital PI controller for regulating the generator voltage.
The proportional integral controller is implemented in the FPGA. Nonlinear behavior of load versus generated voltage enforces the need for piece-wise linearization of the system around the operating points. The system uses the gain scheduling at appropriate loads for generator voltage control. Once the generator attains the rated voltage, the generator is loaded through auxiliary contactors. The generator is loaded dynamically for a duration of 60 s with six dynamic loads; each consists of overload applications at the beginning of each step for the duration of 300 ms. With the CompactRIO real-time controller, we can provide the time-deterministic loading duration.
LabVIEW and CompactRIO helped us solve our problem in a timely and cost-effective manner. The generator mode of testing uses a 250 kW regenerative vector drive to power the AC motor, which generates EMC noise for a few meters. Even properly shielded and grounded signal cables do not prevent the noise from getting coupled with signal cables. Electromagnetic shielding of the entire drive is quite expensive, and it takes time to purchase and implement.
Thanks to LabVIEW, the EMC noise problem was solved with the median filter in the software. The median filter provides the great advantage that it does not change the end-to-end transfer function of the system, which is very important for any dynamic testing.
Auto Mode of Operation in Starter Mode
In starter mode, auto mode of testing performs the five starting operations. The starter is tested for a series of motor-start operations. The system sets the required torque by communicating to the power-type dynamometer, and it sets the appropriate voltage to the remote power supply using the CompactRIO analog output module. The system establishes the required initial conditions such as water supply to the dynamometer and cooling air to the starter before starting the actual test.
The user has the provision of aborting the test at any point by pushing the emergency stop button. The user can bring the systems to the initial condition by pushing the reset button. The entire system handles three digital PID controllers, one for controlling the generator voltage, another for motor speed in the generator rig, and the last for controlling the torque in the starter rig. Controllers are tuned for slightly lower than the critical damped response.
Manual Mode of Operation
In manual mode, the operator performs all the operations through the selector switches, push buttons, and potentiometers. Users can load the generator continuously in manual mode. The software reads all the analog and digital inputs, and based on the user inputs (push button and selector switch actions) and interlocks, the software updates the digital and analog outputs. The sequence, as well as the operational interlocks, are implemented in the software. This implementation eliminates the need for external relay logics.
We developed a flexible, durable, cost-effective automated starter-generator test system using CompactRIO and LabVIEW. The system based on LabVIEW provides advantages over the PLC-based system in terms of analog signal processing and the graphical user interface.
No 3, Victorian Meadows
Tel: 080-4037 3900
Fax: 080-4037 3839