Machine/drive circuit interactions in small variable-reluctancestepping and brushless DC motor systems

The authors investigate the torque, losses, and efficiency of small brushless DC and stepping-motor systems, and explain how performance is influenced by excitation mode, drive circuit, and machine parameters. Quantitative deductions are made for a three-phase reluctance-type machine based on computed steady-state results. For a given machine and drive circuit, maximum torque is obtained with an excitation mode that allows each phase to be turned on for half of the complete excitation cycle. However, if maximum efficiency is the aim, phase excitation must occur for a shorter period, e.g. by exciting only one phase at a time. When making comparisons between drive circuits, the most important measure of drive-circuit capability is the circuit power available at low speeds. Drive circuits producing rapid current decay at phase turn-off benefit torque production in the two-phase-on excitation mode, but are detrimental with the one-phase-on mode. At high speed the pull-out torque depends on the unsaturated inductance parameters of the machine. For maximum torque these parameters must be correctly proportioned, their values being dependent on the excitation mode