Impact of Digital Signal Processing on FOC Current Feedback in High-Speed PMSM Drive

In applications where size and weight of the electric motor are among major design concerns, Permanent Magnet Synchronous Motors (PMSMs) with wide operational speed-range are commonly preferred. Due to limited inverter switching frequency, high-speed operation of a drive results in a low ratio between the switching frequency and the fundamental frequency of motor voltage. Such operating conditions have been recently identified as a source of errors in determining motor current feedback using the typical technique dedicated for microprocessor-based controllers. First, sampling the phase currents synchronously with the peaks of modulation carrier provides results of mean currents under an assumption of linear current changes in steady voltage intervals. This assumption is over-simplistic in high-speed drives. Second, transforming the phase currents into a rotating reference frame, required in Field Oriented Control (FOC), is carried out using discretized rotor angle. If rotor covers a substantial angular distance in the control cycle, such simplification distorts transformation results considered as mean values. This paper analyses the joint impact of both these oversimplifications on the errors of field-oriented current components derived in a microprocessor as a feedback to the control algorithm. The analysis was carried out for a highspeed PMSM drive operating at switching frequency of 5 kHz and maximal fundamental frequency of 350 Hz. Simulation results show that the considered oversimplifications lead to notable errors in digital processing of current feedback. The direct-axis current is affected by a notable offset, while the quadrature-axis current is supplemented with a high-frequency error component.