In order to improve the performance of motor in traction application

we propose a method using synchronous SHEPWM modulation. The switching angles is stored in the chips. When the modulation index changes

the switching angles

which are got by micro-distance linear interpolation way

will change too. Therefore

the switching angles are more precise. Meanwhile

when the carrier frequency changes

the fundamental frequency will also change so as to ensure that the carrier frequency is 6 times the fundamental frequency. The carrier frequency got from the way of 15-division can guarantee the symmetry between positive half-wave

negative half-wave

and 1/4 cycle. The proposed changing way from SHEPWM（7） to SHEPWM（5）

SHEPWM（3）

SHEPWM（1） to a six-step mode can improve the performance of changing. The experimental results demonstrate the feasibility of this method

the entire dynamic performance is obvious

smooth switching between modulation schemes can transit to the six-step mode automatically. The method can be used as traction modulation strategy.