New developments in differential-type hydraulic transmissions and controls

The differential-type hydraulic transmission has been used as a constant speed drive for aircraft a-c generators for approximately 8 years. The experience gained in this time on both the B36 and P5M airplanes has been a major factor in establishing future aircraft electrical system and component requirements. However, the final decision to establish closer electrical system tolerances for voltage and frequency, real and reactive load balance, and necessary fault protection should be determined on the basis of component and system reliability, simplicity, and performance. The following discussion considers these important characteristics for the components controlling system frequency, real-load division between paralleled a-c generators, and automatic paralleling of a-c generators. The improvements in new system performance were gained by utilizing components with faster response times, and also by the introduction of nonlinear circuit elements. Design and performance details are cited to substantiate steady-state frequency control to within 0.25 per cent of 400 cycles per second, real-load division to within 2 kw, and automatic paralleling of generators to maintain current transients within generator full-load ratings.