A Switched-Reluctance Motor Drive with Zero Torque Ripple and a Constant Inverter Bus Current

Switched-reluctance motors appear to be ideal industrial prime movers capable of precision speed and position control. The efficiency can be higher than for a similar-sized induction motor and the electronics less complicated for precise speed control. While the switched-reluctance drive is common in some applications, it has not been widely accepted because of the large amount of torque ripple produced. The torque ripple from the widely used induction motor is quite low and it causes less vibration in the mechanical drive train following the motor. A four-phase switched-reluctance motor can he operated in such a way as to produce a constant zero-ripple torque output. The currents in at least two of the four phases are set so that the total torque produced is constant. By precisely setting the currents in three of the four phases, a constant torque output can be obtained at a constant d.c. supply current, and the switched-reluctance motor then has similar characteristics to a d.c. series motor. A mathematical description of these non-linear currents is derived along with the individual and mutual torque contributions to the total constant torque. The equations are also shown in graphical form.