Theory and Simulation of the Brushless DC 120° Inverter System

The brushless dc motor- inverter system is becoming increasingly popular in servo and variable speed applications. One type of inverter, the 120° inverter, does not require rotor position sensing hardware. In this thesis, a theory of the brushless dc motor with a 120° inverter is set forth and used to create a fast simulation procedure for steady state operation. This simulation is well suited for evaluation of torque speed curves since the effects of changes of system parameters can be rapidly assessed. Furthermore, it was found that the system operation may be classified into a finite number of distinct operating modes. The properties of these modes and physical reason for their existence is discussed. Classifying the operation into modes provides a powerful tool for understanding system behavior. The theory presented and classification scheme are then used to formulate an approximate analytical method for the steady-state torque, which is quite accurate for normal operating conditions. The approximations presented are suitable for control system design