>>12375897AC case:
suppose the loop starts off having zero flux going though it (loop plane orthogonal to field lines) and define an orientation to the loop (i'm going to say it's pointing up, so we go CCW around the loop in the picture). as the loop rotates, the magnitude of the magnetic flux changes in time; specifically it goes from 0, to negative, to zero, to positive, to zero over the course of one turn of the loop. lenz's law indicates this will cause a current in the loop that tends to cause a flux that opposes the change. thus, we expect the current to go 0, +, 0, -, 0 over the course of one turn. hence the term AC
DC case:
same analysis, except now there's a split ring communtator. flux goes from 0 to -, causing a current that goes CCW in the picture. when the flux is most negative, the split ring commutator then breaks the circuit and effectively switches which terminals the ends of the loop are connected to. now the flux goes from - to 0 to +. this increase in flux causes a current that goes CW, but since the contacts have been reversed, the external circuit sees current going in the same direction as the first part of the cycle. finally, when the flux is most positive, the connects are switched again. flux decreases, inducing a CCW current, and we get back to the starting configuration for another cycle.
btw, i've never studied motors. they are clever little shits
