There are two ways to explain how a voltage is generated in a coil. Assume we have a constant magnetic field in which a rectangular coil is rotating. The coil has two in parallel long sides and two in parallel short sides. The axis of rotation is lying half way the short sides and is in parallel to the long sides. The axis of rotation is perpendicular to the direction of the magnetic field. Assume that the position of the coil for which the coil area is perpendicular to the magnetic field, is called the zero position.
First way of explanation
The maximum number of field lines are flowing through the coil for the zero position. If the coil has rotated 90°, the coil is in parallel to the magnetic field and no magnetic field lines are flowing through the coil. The generated voltage is proportional to the speed for which the magnetic flux is changing. The change is zero for the zero position and maximal if the coil has rotated 90°. This is because the direction of the magnetic field with respect to the coil, changes at this point. It can easily be proven that the change of the number of magnetic field lines flowing through the coil is sinusoidal and that it has a maximum for a rotational angle of 90° and 270°. So the generated AC voltage is maximal for a rotational angle of 90° and 270°
Second way of explanation
A voltage is generated in a straight wire which is moving perpendicular to a magnetic field. The voltage is proportional to the strength of the magnetic field, to the length of the wire and to the component of the speed perpendicular to the magnetic field. A short side of the rectangular coil can be divided into a part left from the turning point and a part right from the turning point. The voltages generated in these parts are just opposite each other and so the total voltage is zero. The voltage generated in a long part of the rectangular coil is proportional to the component of the speed perpendicular to the magnetic field. This component is zero for the zero position and at a rotational angle of 180°. This component is maximal at a rotational angle of 90° and 270°. The generated voltages in both long parts of the coil are strengthening each other. The component of the speed perpendicular to the magnetic field also changes sinusoidal so the generated voltage varies sinusoidal.
So both ways of explanation give the same result. If a PM-generator has coils with an iron core, like for most radial flux generators, the magnetic flux is concentrated in the iron and almost no field lines are flowing through the air in the grooves which are used for the coils. For this kind of generator, it is therefore easiest to to use the first way of explanation for the generation of the voltage.
For an axial flux generator with two steel sheets with magnets at the inside and a disk with coils without iron in between, the second way of explanation can better be used. As there is no iron which concentrates the magnetic field, there is about a constant magnetic field in between two opposite magnets. A coil has two legs and an inner and an outer coil head. The voltage is only generated in the legs as only the legs are moving through the magnetic field. The coil heads are required to connect the legs but the length of the coil heads should be made a small as possible to reduce the Ohmic resistance of a coil. The average pitch in between the left and the right leg should be the same as the pitch in between two adjacent magnets because then the generated voltages in both legs are in phase. If the generator has eight poles, the pitch angle in between two adjacent magnets is 45°. This means that the average pitch angle in between two legs of a coil should also be 45°. But as a coil has a certain thickness, the pitch angle of the inner turns will be somewhat smaller than 45° and of the outer turns will be somewhat larger than 45°.
For a 1-layer, 3-phase winding, the number of armature poles must be dividable by four. The number of stator coils is 3/4 of the number of armature poles. So for an 8-pole armature we get six coils. The coils are laid in the sequence U1, V1, W1, U2, V2 and W2. The optimum positions of the heart of the coil legs are then respectively 0° and 45°, 60° and 105°, 120° and 165°, 180° and 225°, 240° and 285°, 300° and 345°.