OPERATING MODES OF THE BRUSHLESS DC MOTOR WITH A 120-DEGREES-C INVERTER

The brushless dc motor is often supplied from either a 180° or 120° voltage—source inverter. The analysis of the 180° inverter — permanent-magnet synchronous machine combination (brushless dc motor) is quite straightforward since the applied voltages are known. The analysis of the brushless dc motor with a 120° inverter is more difficult since the current in the phase to be switched off may take on various waveforms. In particular, during the 60° switching interval when the transistors associated with a phase current are ungated, this current may be negative (N), positive (P), or zero (Z) depending upon the rotor speed, dc source voltage and the advance in firing angle. The status of the current being switched off is used to classify the modes of operation possible during motor operation of a brushless dc motor with a 120° inverter supply. It is shown that six modes of operation are possible for the system considered. This paper should prove useful in calculating and predicting the various modes of operation and thereby provide a clearer understanding of the operation of this type of system. © 1990 IEEE