The Central Inverter in a Spacecraft Power System

The central static inverter used to establish a 1250 VA, 115/200 volt, 3-phase, 400 c/s ac power distribution system for the Apollo Spacecraft Command and Service Module is one of the loads operating in parallel from the dc power distribution system. Some of the other dc loads cause transient and steady-state ripple voltages on the dc system bus of 10 c/s to 1000 c/s frequency that is critical because passive filter networks of reasonable size and weight cannot provide adequate attenuation. A static inverter is susceptible to dc input voltage ripple, and a demodulator network is required when ac output voltage modulation must be less than 1/2 percent. The various static inverter circuits which can provide the demodulation function are discussed. Particular emphasis is given to the non-dissipative approach afforded by the voltage buck-boost regulator5 type of dc link static inverter in which available and reliable components have been incorporated. A circuit model is analyzed in order to substantiate the demodulation approach and the data taken from an actual hardware inverter.