High-Efficiency Current Excitation Strategy for Variable-Speed Nonsinusoidal Back-EMF PMSM Machines

Abstract

Opportunities for energy efficiency improvements in motor drives remain a forefront topic of interest in recent times due to their extensive use and applications. Then, it is the right juncture to revisit various stator current excitation strategies and their impact on the efficiency of brushless dc (BLDC) machines favored in high volume variable speed applications. BLDC machines with nonsinusoidal back electromotive force (EMF) are considered in this paper, and the influence of sine, square, and nonsinusoidal harmonic current on their performance is investigated. To facilitate such a study, analytical expressions for the generalized back EMF of all the three excitation schemes have been derived to highlight their influence on torque ripple and motor harmonic losses, thus leading to efficiency evaluation. An analytical approach on the estimation of iron losses under loaded operation has been elaborated to highlight the impact of stator current harmonics on the efficiency of the motor. Analytical as well as experimental results show that it is advantageous to use the nonsinusoidal harmonic injection (NSHI) scheme in the speed range until the sum of the drive conduction and stator resistive losses exceeds the iron losses of the machine. When iron losses become dominant, the algorithm is modified to sinusoidal current control, thus maintaining higher efficiency operation. To achieve such an operation, a hybrid control scheme encompassing sinusoidal to NSHI field-oriented sensorless controller has been implemented.