Comparison between self-excitation and pulse-excitation in air-core pulsed alternator system

Abstract

Air-core pulsed alternators greatly enhance the power density capabilities of pulsed power supply for electromagnetic launchers. Compared with iron-core electric machines, they require very high excitation currents. Usually, the self-excitation mode is adopted in these systems. However, the self-excitation mode is a positive feedback process which should be precisely controlled to avoid the system being destroyed. The excitation losses are very large during the process of establishing the large field current. To solve these problems, a new excitation mode, pulse-excitation mode, is presented in this paper. The main idea is that a pulsed current charges the field coil so that the field coil and the armature coil conduct only for a short time around the main discharge pulse. This will avoid the machine being overheated and the risk of positive feedback process. Compared with self-excitation mode, the pulsed alternator can be operated in a higher discharge frequency and the circuit schematic of the system is simpler. A larger field capacitor is used which charges the field coil to the desired current just by a pulse. As the energy density of pulse capacitor is enhanced rapidly, the volume and mass of the field capacitor are acceptable. Generally speaking, the pulse-excitation mode is a combination of pulsed alternator and pulsed capacitor. In this paper, the characteristics of self-excitation and pulseexcitation are analyzed respectively. The performances of both systems are simulated, and the main waveforms are presented. Finally, there is a practical comparison between them.