Principles of Electrical Design; D. C. and A. C. Generators

by Alfred Still

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1916 ...if necessary a diverter would be provided to adjust the current in accordance with results obtained on test. 52. Prevention of Sparking--Practical Considerations.--It is not suggested that the method of considering commutation phenomena as outlined above covers the subject completely. The designer aims at obtaining "ideal" commutation under certain load conditions, knowing well that, even when series-wound commutating poles are used, the required conditions cannot be exactly fulfilled at other loads. He relies on the resistance of the Fig. 67.--Armature coil near end of commutation period. carbon brush to give sparkless commutation even when the conditions depart appreciably from those of "ideal" commutation. The extent to which the ideal condition can be departed from without producing destructive sparking is not easily determined except by experimental means. In Art. 45, page 146 the effect of the brush-contact resistance was considered, and it was seen that the value of this resistance has no effect on the problem of commutation provided the change of current in the short-circuited coil takes place in accordance with the "ideal" or straight-line law. The reason is that, when "straight-line" commutation is obtained, the distribution of the current over the contact surface of a brush of rectangular section is necessarily uniform. If, now, we wish to examine the conditions of commutation when the changes of current do not follow the ideal straight-line law, it is necessary to consider the effect of the brush-contact resistance when the current density is no longer uniform over the entire surface. The diagram, Fig. 67, is generally similar to Fig. 56, except that the coil connecting segments A and B has moved nearer to...

108 pages, Paperback

First published July 18, 2009