Torque ripple reduction in three-stage brushless synchronous machines

Considering the large torque ripple and poor start performance with load which are caused by the excitation current pulsation of the main generator in the electric start process of three-stage brushless synchronous machines, the impact of the excitation current pulsation was analyzed on operating characteristics of the main generator and distribution characteristics of output voltage vector error caused by harmonic currents when vector control strategy was adopted. In order to improve the start performance of the three-stage brushless synchronous machine with load, a start control method was proposed using the polar coordinate to determine the output voltage vector. Compared with the traditional vector control strategy, this method does not produce amplitude error, but only small angle error, of the vector voltage. The experimental results show that, at a speed of 100 r/min, the electric start system adopting the new method can operate steadily with a load of 40 N·m, while for the traditional vector control method, it is only 5 N·m. Therefore, the new method is more suitable for the low speed and heavy load operating conditions at the start mode of aircraft engines.