Excitation Control of a Brushless Synchronous Motor

A simple system is presented for controlling the excitation of a brushless synchronous motor independently of the motor speed or load angle. The system comprises a pulsewidth modulated (PWM) inverter provided with controllable pulsenumber. Experimental measurements of the induced rotor excitation shows that a wide range of excitation control is obtainable by simply varying the pulsenumber of the output waveform of the inverter while keeping the fundamental frequency of the inverter unaltered. An approximate formula for the rotor current is also given which confirms the dependence of the rotor current upon the pulsenumber.