| Abstract: | Aircraft power systems require power supplies which are capable of generating constant frequency output power from variable speed mechanical power. One type of this power supply system is known as a variable speed constant frequency system (VSCF). The VSCF system is an assemblage of an alternator and solid state frequency changers and allows the alternator shaft speed to vary directly with turbine speed. The type of VSCF system used most often is that of a synchronous machine applied as a main generator and a naturally commutated cycloconverter utilized as a frequency changer to convert the variable frequency power output to a constant frequency power output. Major disadvantages of using naturally commutated cycloconverters are their capability of reactive power generation, limited output frequency and large number of circuit component requirements. However, VSCF systems, especially in aircraft power systems, need to be compact, light-weight and efficient. To meet these requirements, a permanent magnet generator is considered here as a prime power source and force turn-off frequency changers are employed as a link between the generator and a constant 400 Hz frequency supply. For this new VSCF system, dynamic control laws, which reduce power ratings of the system and increase efficiency, are established. The dynamic control method uses input power factor control of the force turn-off frequency changer to optimize system's operation. The input power factor control of the frequency changer is performed by a feed-back controller. For the control of a force turn-off single stage frequency changer, new pulse width modulation methods were proposed. Look-up table based control algorithms have been successfully developed for the microprocessor-based control. A three stage converter structure, which allows input power factor control and offers a high utilization factor, is proposed and analyzed. Then the direct modulation method, which is ideal for the microprocessor-based realization, is developed. The direct modulation method is further investigated to improve harmonic distortion by allowing freedom in varying carrier intervals. A systematic mathematical model of the frequency changer system including input filters is developed and used for digital simulation. The generalized input/output waveform harmonic models are established to analyze the harmonic spectrum. This harmonic analysis tool is subsequently used to design an input filter for the frequency changers. Finally, a realization of the whole VSCF system is studied. |
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