Harmonic elimination in polyphase machines by graded windings

The usual method of controlling the harmonic components of the magnetomotive-force and electromotive-force waveforms of a polyphase machine is by distributing and chording the winding. Although this method attenuates most of the harmonics (apart from slot harmonics), it can only eliminate a limited proportion. The method described makes use of a double-layer graded distributed winding, i.e. one in which the turns per coil are not constant but are selected in such a way as to provide an m.m.f. waveform from which all harmonics are eliminated except the slot harmonics (whose relative magnitude no distribution or chording can affect). Correct selection of the turns per coil results, in general, in a slight variation of the total number of conductors in a slot, but approximations are possible which lead to this number remaining constant at the expense of less perfect harmonic elimination. Finally it is shown that these same windings eliminate the same harmonics in the line e.m.f. of 3-terminal 2-phase and star-connected 3-phase machines.