High Speed Induction Generator for Applications in Aircraft Power Systems

Electric generators have higher power density when the operating speed is increased. High speed electric generators direct coupled to gas turbines provide an ideal source of electric power for airborne applications because of reliable operation and high power density. To function reliably in the speed range of 60000 RPM to 120000 RPM, the rotor of the electric generator must be robust. Examples of the robust rotor technologies for the generator include: permanent magnet (PM), induction and switched reluctance. The objective of the present paper is to describe the current activities in the field of high speed induction generators and associated controllers. A range of induction generators and controllers rated from 5 kW to 200 kW operating at speeds to 62000 RPM is currently under development. The rotor is designed using high strength magnetic materials for the magnetic paths, and high strength alloys for the conductors that form the squirrel cage. Tests have been successfully conducted to demonstrate the high yield strength of the rotor construction materials. High power density of the induction generator designs is demonstrated by the electromagnetic weights of the induction generators: 5 kW weighs 1.7 Lbs., 30 kW weighs 5 Lbs., and 200 kW weighs 37 Lbs. in electromagnetics. The benefits of induction generator for a variety of airborne applications are: Simple, low cost, robust rotor construction; Electrical excitation allows de-excitation instantly in case of internal failure to prevent further damage; Output voltage can be well regulated dc or ac, under steady state and transient conditions; For very high voltage or for very low voltage applications, transformers can be used get the desired voltage output, which helps improve the controller power density; For generator control, the rotor speed information and not the rotor position information is needed, which simplifies the design of the sensor itself; Because no permanent magnets are required, the generator can function in hostile temperature environment. The 5 kW and 30 kW generators are designed for operation at 500°F for location inside the turbine. Three generators rated at 5 kW, 30 kW, and 200 kW are in various stages of fabrication and testing. Closed loop operation using DSP based controllers is implemented. Test data relating to the performance of the induction generators and controllers will be presented. The test conditions will include the steady state voltage regulation, transient voltage regulation, and generator efficiency analysis.