Abstract

Energy efficiency is on the forefront once again after the 1970s and greater regulation on that score is on the rise with stimulus funds from the federal government in the USA. Then this is the right juncture to revisit various control strategies and their impact on motor efficiency of brushless dc (BLDC) machines that are finding favor in some high volume applications. BLDC machines with non-sinusoidal back EMF are considered in this study and in them influence of sine, square and non-sinusoidal harmonic currents on motor performance are investigated. To facilitate such a study, analytical expressions for generalized back EMF of all the three excitation schemes have been derived to highlight their influence on torque ripple and stator copper losses thus leading to efficiency evaluation. To achieve non-sinusoidal current injection, a scalable sensorless field oriented vector control scheme was implemented using non-sinusoidal coordinate transformation while back EMF harmonics in the machine were identified online using an adaptive filtering technique. The experimental results show that it is advantageous to use non-sinusoidal current injection only in lower frequency range until drive conduction and stator resistive losses are higher than iron losses of the machine after which the algorithm can be modified to sinusoidal current injection thus maintaining higher efficiency operation.

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