Analytical study of Torque vs. speed characteristics of PM brushless DC drives

PM brushless DC (BLDC) motors should be fed by square-wave currents synchronized with the back emfs to develop a constant torque with the required value. In practice, phase inductances and limited source voltage produce non ideal square-wave currents and unbalanced phase currents during commutations. Such phenomena affect the torque developed by the PM BLDC drives by deviating its value from the requested one and by giving rise to torque ripple. In this paper, an analytical study of the torque characteristics, namely motor torque and torque ripple, of PM BLDC drives is carried out as a function of the motor speed and is supported by experimental data. Torque characteristics are expressed in terms of a quantity distinctive of the motor, thus giving a generalized formulation to the results.