The Challenge:

Developing a flexible, rugged and cost effective automated test system for testing the performance of the Starter-Generator at various load conditions.

The Solution:

Developing a flexible, rugged, cost effective automated “Starter Generator Test System” using NI cRIO and LabVIEW.

Introduction:

Briefly, Starter generator rotates the turbine engine fast enough, so the fuel can be ignited and takes over as a generator providing DC power to the electrical system after start in the aircraft.

There are two types; shunt start and series start. The shunt type can usually be identified by having two large terminals and two small terminals located on the terminal block marked A, B, D and E. For starting, terminals B and E supply current to the armature and stator windings. At the same time, voltage is applied to the shunt field terminals A and D from the Generator Control Unit.

Three large terminals on the terminal block marked as C+, B and E- can identify series start type units. Shunt field terminals A and D are located on either side of the larger terminals. Terminals C+ and E- supply current to the armature and stator windings during the start cycle and the starter generator works the same as a DC electric motor. The shunt windings are not used in this mode. Following the starter operation, the Generator Control Unit regulates shunt field current with voltage supplied from generator residual voltage and maintains a nominal system voltage of 28 volts DC.

The Starter Generator is tested in two different modes namely Generator mode and Starter mode to check its performance at various load conditions. Both the modes have a common Control desk & Data Acquisition System (DAS) to control, monitor and store the required parameters.

Generator Mode:

In this mode the equipment used is “Inverter duty AC motor, AC drive, Step up Gear Box, Torque Sensor, Spline Shaft &Cooling blower”. The AC motor and Step up Gear Box were mounted on a Mild Steel base plate. This entire setup was grouted on to a foundation / stress relieved Mild Steel structure. The Gearbox output spline shaft is coupled to the Generator.

In generator mode of operation, the generator is driven by an AC motor connected to a 1:10 Step up gear box. The generator was flange mounted on to a Hollow shaft Torque sensor. The torque sensor was 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 seconds at the beginning of each step of loading. The following Figure 1, Figure 2 shows the generator mode test rig and generator mode schematic diagram of starter generator test system.

Starter Mode:

In this mode the equipment used is “Powder Dynamo meter, adaptor plate, mounting pad, Spline Shaft”. Powder Dynamo meter 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 DC power supply of 4000Amps max and 24 Volts maximum. There is a separate field supply of 30V, 20Amps for the field coil excitation. The following Figure 3, Figure 4 shows the starter mode test rig and starter mode schematic diagram.

Starter Generator can be tested in both Auto mode and Manual mode. In Auto Mode, cRIO based data acquisition and control system takes over the control of entire test. In Manual mode, user can perform the entire test by performing appropriate operation in sequence through the inputs available in the control desk. Control desk is used for getting all the operational instructions. Test inputs like generator speed, load, starter torque etc… are fed through the PC. Control disk consists of potentiometers, selector switches and illuminated push buttons, emergency stop and reset buttons for getting the operator instructions. Figure 5 shows the picture of control disk.

Auto Mode of Operation

Generator Mode

Auto mode of generator testing has following user programmable entries like Speed of the generator, Load and Load duration and generated voltage. System establishes the test conditions like switching on the drive, blower motor and Lube oil Pump etc… Once the initial condition is established, Generator Speed is set through flux vector drive by generating appropriate AO voltage. System regulates the generator voltage through the field excitation. System uses the digital PI controller for regulating the generator voltage. Proportional Integral controller is implemented in the FPGA. Nonlinear behavior of Load Vs Generated voltage enforces the need of piece wise linearization of system around the operating points. System uses the Gain Scheduling at appropriate load for the generator voltage control. Once the generator attains the rated voltage, generator is loaded through Auxiliary contactors. Generator is loaded dynamically for the duration of sixty seconds. Generator is loaded with six dynamic loads; each consists of overload application at the beginning of the each step for the duration of 300ms. Real time cRIO controller allows us to provide the time deterministic loading duration. Figure 6 Shows the Generator Mode - Auto Mode testing panel

It is worth sharing, how LabVIEW and cRIO helped us to solve our problem in timely and cost effective manner. Generator Mode of testing uses 250kW regenerative vector drive for driving the AC motor, which generates EMC noise to few meters. Even, properly shielded and grounded signal cables do not prevent the noise from getting coupled with signal cables. Electro magnetic Shielding of the entire drive is quite expensive more over it takes its time for purchase and implementation. Thanks to LabVIEW, EMC noise problem has been solved by using the “median filter” in the software. 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. Figure 6 shows the Generator Mode – Auto mode user interface.

Starter Mode

Starter Mode – Auto Mode of testing performs the five starting operation. Starter is tested for series motor start operation. System sets the required torque through series communication to the Power type dynamometer and sets the appropriate voltage to the remote power supply through cRIO AO module. System establishes the required initial conditions like water supply to dynamometer and cooling air to starter etc… before starting the actual test. Figure 7 shows the Starter Mode – Auto Mode testing panel.

User has the provision of aborting the test at any point of time by pushing the Emergency stop button. User can bring the systems to initial condition by pushing the Reset button. Entire system handles three digital PID controllers one for controlling the generator voltage, another for motor speed in generator rig another for controlling the torque in starter rig. Controllers are tuned for slightly lower than the critical damped response.

Manual Mode of Operation

Generator Mode - Manual Mode, operator performs all the operation through the selector switches and push buttons and through the potentiometers. User can load the generator continuously in manual mode. Software reads all the analog and digital inputs and based on the user inputs (Push button and selector switch actions) and interlocks software updates the digital and analog outputs. Sequence as well as operational interlocks is implemented in the software. This implementation eliminates the need of external relay logics. Starter Mode – Manual Mode is same as that of generator mode - manual mode.

LabVIEW based system provides advantage over the PLC based system in terms of analog signal processing as well as the graphical user interface.

Conclusion

We have developed flexible, rugged, cost effective automated “Starter Generator Test System” using NI cRIO and LabVIEW.