Determining pulsed alternator thyristor converter firing angles to produce a desired launcher current

Proposed electromagnetic launch systems using pulsed alternators usually employ a phase-angle-controlled thyristor converter to supply DC to the railgun launcher. By carefully choosing the firing angles of the individual thyristors, the system designers can control shape of the current pulse delivered to the launcher, thereby controlling projectile acceleration and magnetic energy recovery from the launcher after muzzle exit. This paper describes a simple method for determining the required phase angles for a specified launch current waveform, using a simplified equivalent controlled DC source model for the pulsed alternator and converter system. Once a desired current profile is chosen to deliver the required action to the projectile, the railgun equations are used to determine the required breech voltage profile. The thyristor firing angle vs. time to produce this breech voltage is then calculated from the equivalent DC source model. Improvements to the basic model incorporating field current decay and rotor speed reduction with time are discussed. The method has been embedded in a Mathcad™ worksheet. Results from the simplified model show good agreement when compared to a detailed Saber™ system simulation.