The induced emf in a coil in AC generator is given as:
$$\mathbb E = NAB\omega \sin \theta $$
$\omega = d\theta/dt$
Now, when the angle between the normal of plane and magnetic field is zero degrees, the induced emf is zero i.e.
$$\Bbb E = NAB\omega \sin 0 = 0$$
But we also define emf as the time rate of change of magnetic flux so, why do we get zero emf in the above case, magnetic flux is still changing with time?