Question

why is the emf zero when the armature is perpendicular to the magnetic field​

Answer 1

When the coil is upright there is no change in magnetic flux (i.e emf=0) because the coil isn't 'cutting across' the field lines. The induced emf is zero when the coils are perpendicular to the field lines and maximum when they're parallel. Remember, the induced emf is the rate of change in magnetic flux linkage.