From what I understand, the net emf of a DC motor is made of its supply emf and back emf. The supply emf is constant, whilst the back emf depends on how fast the coil is spinning.
Initially, there is no rotation and hence the coil doesn't experience any change in flux so back emf is zero. As the coil starts rotating, the the coil experiences a faster rate of change of flux and hence induces a larger back emf, thereby resulting in then net emf to decrease. This continues until the net emf approaches zero (in an ideal motor), thereby producing a graph like this:
However, this implies that as the coil reaches a max speed, the back emf is a constant and hence the net emf is zero, but that doesn't make sense to me.
We know from AC generators that when they are rotating at a constant speed, the output emf is sinusoidal, so shouldn't the graph for this DC motor also be sinusoidal at maximum speed? It still experiences a rate of change of flux.