by the lines 38a 38c respectively to junctions 92a 92c between pairs of series connected switch circuits s1 s6 a neutral line 94 to the main generator 12 is connected at a junction between filter capacitors c1 and c2 across the dc link filter 44 although not shown the excitation inverter and control 48 may be of generally similar construction to the main inverter 46 illustrated in fig4 alternatively other circuits may be utilized for either or both of the main inverter 46 and the excitation inverter and control 48 as is well known although no implementation for the control of the excitation inverter and control 48 is shown herein reference may be had to the rozman et al co pending application incorporated by reference herein for illustrative embodiments thereof with reference to fig5 a block diagram illustrates the implementation of the gccu 22 see fig3 according to the invention including the main inverter control 74 the main inverter control 74 includes a pwm generator 100 the pwm generator 100 receives the position signal on the line 70 a voltage command on a line 102 and a commutation angle command on a line 104 the pwm generator 100 derives the base drive commands which are transferred on the line 88 to the base drive circuits b1 b6 of the main inverter 46 see fig4 the pwm generator 100 may be of any.