Abstract :
Designers of alternating-current machinery have long been able to make rapid estimates, by means of published curves, of the heating effect due to current displacement in slot-wound conductors. The wave-shape of the current in the windings of direct-current armatures is complex, since it is rich in harmonics. The estimation of the heating effect due to current displacement in direct-current armatures has hitherto necessitated laborious calculation, for each case on its own merits. The busy designer rarely has the time to carry out such a long process, and it is probable that in most cases guesswork is resorted to. In this paper an attempt is made to provide curves, by the use of which the heating of direct-current armatures may be estimated rapidly. It is believed that results are obtainable with fair accuracy, and with almost the same rapidity as is associated with the estimation of the loss in an alternating-current machine. It is not possible to construct one curve for use in all cases; a ?standard? curve is given, which refers to a single-turn full-pitch winding, having 15 slots per pole and 45 commutator segments per pole. Two additional curves give coefficients which enable other full-pitch single-turn windings to be dealt with. The heating ratios so obtained do not differ from those obtained from individual calculations by more than 2 per cent in any practical case. Curves are also given for windings which differ from the standard winding in having (a) two turns per coil, (b) three turns per coil, (c) chording. The question of the critical height of conductor, and the question of the use of unequal heights for the two layers in a slot, are alsotouched upon. It is, perhaps, not generally realized that current displacement is present at all times in a direct-current armature winding; it is not confined to the period of commutation. A set of current-distribution curves has been calculated for one case, to show this. The author hopes to deal with the problem of t- he rotary convertor, and the problem of unusual winding arrangements designed to reduce heating losses, in a subsequent paper.
