GB190825465A - Improvements in and relating to Alternating Current Dynamo Electric Machines. - Google Patents

Abstract

25,465. British Thomson - Houston Co., [General Electric Co.]. Nov. 25. Dynamos, alternating-current armatures.-The electromotive-force wave of an alternator is deprived of any selected harmonics by using a fractional-pitch armature winding, the coils of which are connected in series and displaced from each other by distances not equal to the exact pole-pitch. Thus, a three-phase alternator having its armature star-connected in the usual way to eliminate the third harmonic and its overtones, may have the fifth harmonic removed by winding the armature with a fractional pitch equal to four-fifths of the pole pitch, and the seventh harmonic may be removed by displacing half the coils of each phase by one-seventh of the polepitch. Should it be required to eliminate higher harmonics, as for telephone work, further displacements would be employed. Thus, by connecting a coil of fractional pitch equal to 1 Π1/n in series with a similar coil displaced by 1 Π1/m and two similar coils displaced by 1 Π1/p, the nth, mth, and pthharmonics are removed. In the Y-connected eight-pole armature shown, there are nine slots per pole and an average of three coils per phase per pole. Each coil has a pitch of seven-ninths, which is sufficiently near to four-fifths to eliminate the fifth harmonic. Considering, say, the phase shown in heavy lines, the groups b<1>, b<3>, b<5>, b<7> have the normal number of coils, namely three, while the groups b<6>, b<8> and b<2>, b<4> have respectively two and four coils. As a result, the coils are divided into two groups, for example b<1> and b<5> or b<2> and b<6>, which are displaced by nineteen slots instead of the normal number eighteen, or the two groups are asymmetric by one slot or one ninth of a pole pitch. This is close enough to one-seventh and one-eleventh practically to eliminate the seventh and eleventh harmonics.