Abstract
If a d.c. armature is placed in a rotating magnetic field, the emf induced in the individual armature conductors is proportional to the relative speed of field and armature, both in magnitude and frequency. Due to the action of the commutator the voltage appearing between brushes placed on the commutator has the same frequency as the rotating field irrespective of the speed of the armature, but its magnitude depends on the relative speed and the spacing of the brush arms around the commutator. These characteristics have been used to design a number of polyphase commutator machines, but currently only two types are in general use and are made in relatively large numbers : stator-fed and rotor-fed motors with shunt characteristics. Motors with series characteristics are useful for some applications, and are also made, but in relatively small quantities.
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