Average-value modeling of brushless DC motors With trapezoidal back-EMF

Average-value modeling is indispensable for large and small-signal analysis of electromechanical systems with power electronic drives. Development of an accurate average-value model for brushless dc motors with 120° inverter systems is particularly challenging due to the complexity of voltage and torque equations. This paper presents an improved average-value model for the 120° BLDC motor-inverter system with trapezoidal back-EMF. In the proposed model, a proper qd model of the permanent magnet synchronous machine is utilized, wherein the multiple reference frame theory is used to obtain appropriate average-value relationships for currents and torque. The presented studies are based on a typical industrial BLDC and include measurements and simulations using the detailed and the average-value models. The proposed model is shown to be more accurate in comparison to the conventional models that assume sinusoidal back-EMF.