Buck converter based model for a brushless DC motor drive without a DC link capacitor

This study presents a simple model for a brushless DC (BLDC) motor drive operated directly from rectified mains without a DC link capacitor. A switching strategy with single switch control is adopted to regulate either the speed or torque. Operation of the drive during any switching state is similar to that of a buck converter and, thus, a buck converter based model is proposed to accurately predict its behaviour. Using the model, analytical expressions are derived for the motor current to investigate the impact of operation without the DC link capacitor on torque. Theoretical behaviour is compared with Matlab/Simulink based simulations as well as that observed from a 250 W prototype motor drive to demonstrate the validity of the modelling technique. Results convincingly indicate that the proposed buck converter based model is accurate and can be used as a design tool, being simple and requiring less computational time and applicable to conventional BLDC motor drives with similar switching patterns.