DC Chopper Drive for Brushed DC Separately Excited Motor
if the drive is to be run at 50% speed for a long time, then this reduced power factor puts a strain on the AC Generators.


That is not exactly true. At 50% speed, the power required from the generators will be 50% while the total RMS value (including harmonics) of the current remains constant. However the effect of the harmonics may be increased somewhat.


  Will the above arrangement improve power factor?


The displacement power factor will be near 1.0, but there will still be harmonic content that will cause the total power factor to be less that 1.0, considerably less under some conditions. However the total RMS current at 50% speed is likely to be less than you are experiencing with the SCR drive. Also the SCR line notching will be eliminated.

You can reduce the harmonic content of a PWM drive in several ways. The best thing to do would to determine from the details of your power distribution system how much harmonic distortion to expect. Make a detailed study of the problems of your current installation. Then determine the cost and benefits of the various alternatives.