Comparison of a single pulsed alternator with two or more in parallel for driving a railgun

Pulsed alternators, particularly multipole, multiphase machines incorporating a flexible rectifier/inverter subsystem, can be used for powering large DC railguns. Such machines are able to store energy for several shots; presently, these designs generally lead to much lighter and smaller systems than those designed with other pulsed power options. For large caliber railguns that are designed to accelerate projectiles to 10 MJ or more, however, the reaction torque associated with a pulse discharge from a single alternator can be excessive. If two (or any even number) counter-rotating alternators are used, the torque on the overall structure can be virtually eliminated. Use of two identical counter-rotating machines also overcomes some of the gyroscopic effects on the vehicle. Most practical system concepts, therefore, utilize an even number of multipole machines. This raises several questions that must be answered. How does this mode of operation affect the energy density of the system? How must the machines be connected together, in series or parallel? What are the issues involved in each configuration? Must the field coils of the two or more machines be connected in series or in parallel? If the machines must operate in parallel, at what location must they be linked? This paper attempts to define the issues involved in answering these questions.