This article deals with new recommended control structures for adjustable-speed direct current (DC) drives with a flexible shaft. Two cases will be analyzed: a two-inertia system (i.e., single motor driving a load through an elastic coupling; e.g., suction fan drive) and a three-inertia system (i.e., two motors driving a common load through two elastic couplings; e.g., dual-kiln drives). The first recommended control configuration is an extension of the rigid system controller with an added control term that is proportional to the differential speed. The second recommended configuration is the variable-structure sliding-mode control (VSC) that is known to be robust, insensitive to parameter variations, and easy to implement. Simulation results for both structures are presented to illustrate the effectiveness of the proposed controllers.
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