After reading this:
I want to know, does it improve the motor characteristics if you supply the motor flat DC instead of pulsed (fully rectified) DC?
And what happens when you want to speed-control (e.g. PWM) these two methods, is there a significant advantage in those cases?
To conclude: what's the shape of voltage and thus switching-type that you can best use to speed-control a universal motor to maximize motor characteristics (less heat/noise/friction/wear, more efficiency)?