Evaluation of DC-Link Capacitor RMS Current in Switched Reluctance Motor Drive

In this article, a method to evaluate the dc-link capacitor rms current of a switched reluctance motor (SRM) drive under different operating conditions is presented. Square current reference in both hard and soft chopping modes of pulsewidth-modulated current control is considered. By approximating the current controller response and using a recently proposed flux linkage model, the switching-cycle-average phase voltage and current are evaluated over an electrical cycle. The phase voltage and current are further used to obtain the rms values of dc-link current and capacitor current. The proposed method is also used to determine the capacitor current for low-torque-ripple current reference and single-pulse operation. The analytical predictions pertaining to different control methods under different operating conditions are verified by simulation and experimental results obtained from a 4 kW, 1500 r/min, four-phase SRM drive. While the capacitor current is found to be maximum at the lowest operating speed with hard chopping, the maximum capacitor current is observed at an intermediate speed with soft chopping, both for a given magnitude of square wave current reference. The proposed method is useful to size the dc-link capacitor and evaluate its loss in an SRM drive. Furthermore, closed-form expressions for capacitor rms current are also derived, assuming perfect tracking of square wave current reference at low speeds. These expressions bring out the dependence of capacitor rms current on the machine characteristics.