General Online Current-Harmonic Generation for Increased Torque Capability With Minimum Stator Copper Loss in Fault-Tolerant Multiphase Induction Motor Drives

This paper proposes a current-reference generation method including current harmonic injection (CHI) for enhancing the torque capability of multiphase induction machines (IMs) with negligible space-harmonic effects, which is useful, e.g., during transient overload in electric vehicles. The admissible torque is increased because the harmonics reduce the phase-current peaks, so that the instantaneous peak-current constraint of the drive, usually associated with the converter ratings, is respected. The harmonics are injected in the so-called x-y subspaces of the IM, which do not produce torque, so that no torque ripple is introduced. The optimum harmonics are found online for each load, making it possible to minimize the stator copper loss (SCL) per torque in the entire torque range, ensuring full-range minimum loss (FRML). The method is suitable for healthy operation or open-phase faults, and for multiphase machines of any phase number and with either symmetrical or asymmetrical windings. Compared with FRML methods without CHI, higher torque is achieved. Although some techniques were available for increasing the torque capability by CHI, FRML was not attained, laborious offline optimization was needed, or they were only suitable for specific drives or for healthy conditions, unlike the proposal. Experimental results with a symmetrical six-phase IM are provided.