Consider a DC motor driven by a DC supply. When supply voltage is suddenly reduced back emf becomes momentarily more than the supply voltage, which should drive a current in the circuit opposing the polarity of the supply. Will this damage the DC supply due to wrong polarity?
Commutation in DC generator is pretty straight forward. You reverse the coil terminal polarity at neutral plane where there is no flux, so no induced emf and current, which aids commutation without sparking. But for DC motor you are supplying pure DC all the time and so there is load current all...