TransactionsPublished under the care of the General Secretary and the Treasurer . nd winding is wound as usual around thewhole magnet core; an air-space, however, is left throughthe middle of the core and gives the pole-shoe such aform that, even with no load, one-half cd of the magnetcore on the trailing side, Pig. 43, is completely saturated,(5 = 17,000, say), while the induction in the second half ab ismuch smaller (B = 10,000, say). If the machine is heavily loaded,the armature current endeavors to distort the field, which is pos- ARNOLD AND LA COUR: COMMUTATION. 847 sible only on one-half
TransactionsPublished under the care of the General Secretary and the Treasurer . nd winding is wound as usual around thewhole magnet core; an air-space, however, is left throughthe middle of the core and gives the pole-shoe such aform that, even with no load, one-half cd of the magnetcore on the trailing side, Pig. 43, is completely saturated,(5 = 17,000, say), while the induction in the second half ab ismuch smaller (B = 10,000, say). If the machine is heavily loaded,the armature current endeavors to distort the field, which is pos- ARNOLD AND LA COUR: COMMUTATION. 847 sible only on one-half the field on the leading side, because thepart cd of the magnet core is strongly saturated. The compoundwinding is chosen of such power that it magnetizes the halfab of the magnet core and one gets almost the same field curvewith light load as with full load; so that the armature field and, therefore, ^q becomes infinitely small. By a well-chosen form of pole comer on the leading side, the form of the field curve in thecommutating zone can be made very flat. Although the com-. FiG. 43. Pole Constritction and Compound Winding foe OBTAiNiwaA Stable Commutation Eield. pound winding must be chosen of great strength, the ratio field ampere turns — can be brought down close to 1. The de-armature ampere turns gree of efficiency of the max^hine becomes rather high, therefore,at the lesser loads, because the excitation needs little ). In the machine of D^ri, as it is now installed, not only thenecessary commutating field is provided, but even the entire arma-ture field is done away with. For this purpose D^ri arranges,Ixiside the shunt winding N, around the pole, a compensating wind-ing K, Fig. 44, on the main jx>le P^ which is provided (as wasproposed and carried out by Eyan) with auxiliary poles excitedby the armature current and situated in the neutral zone. Thismachine is expensive and demands a very exact setting of thebrushes. 84S ARNOLD AND LA COUR: COMMUTATION. f).
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