Using iron coil cores will certainly increase the strength of the magnetic flux flowing through the coils as the air gap becomes much smaller. Therefore the voltage generated at a certain rotational speed will be much higher. However, iron cores may cause strong fluctuation of the clogging torque depending on the number of armature poles and stator coils. This fluctuation is maximal if the number of armature poles is equal to the number of stator coils. This fluctuation can be minimised if the number of armature poles is two more or two less than the number of stator coils but this requires a special winding (see public reports KD 560, KD 580 or KD 632).