Most servomechanism drives optimized for speed of response are incapable of returning any of the energy stored in a reactive load to the source. Even transmissions incorporating dynamic braking are often not fully optimized with respect to energy recovery. In this paper, drives based on modulated convertors such as separately excited d.c. motors and variable-displacement hydraulic motors are studied to discover optimum control strategies for operation in all four quadrants of the torque-speed plane, i.e., for power supply and recovery. Quite different results are found for the two nominally analogous types of converters. Although several types of energy loss are not considered in the simplified mathematical models used, the effects of electrical armature resistance and hydraulic line pressure loss together with constant voltage and constant pressure accumulators indicate basically different optimum control strategies. For the types of systems studied, it is found that gyrotors such as electric motors have good efficiency for sustained high speeds while transformer systems such as hydraulic motors are relatively more efficient for transient operation near zero speed. Naturally, the absolute efficiency of a practical drive system depends on a number of additional factors not considered here.

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