REGULATION OF ALTERNATORS 319 



Item (2) may lead to very great additional copper losses if 

 solid conductors of large cross-section are used in narrow slots 

 of considerable depth. The calculation of the losses due to the 

 reversals of the slot leakage flux could be made without difficulty 

 if it were not for the fact that the dampening effect of the unequal 

 distribution of the current density through the section of the 

 solid conductor actually decreases the amount of the slot flux 

 and so reduces the loss. The best and most thorough treatment 

 of this subject known to the writer is that of PROF. A. B. FIELD 

 in the Trans., A. I. E. E., vol. 24, p. 761 (1905). The remedy in 

 the case of heavy losses due to slot leakage flux through the cop- 

 per is to laminate the conductors in a direction parallel to the 

 flux; thus, if copper strip is used, it must not be placed on edge 

 in the slot, but should be laid flat with the thin edge presented 

 to the flux lines crossing the slot, exactly as in the case of the 

 armature stampings, which are so placed relatively to the flux 

 from the poles. Owing to the fact that the leakage flux is con- 

 siderably greater in amount near the top than the bottom of the 

 slot, the losses due to both causes of flux reversal in the space 

 occupied by the "active" copper are of more importance in the 

 upper layers of conductors than in those near the bottom of the 

 slot. For this reason, the upper conductors will sometimes be 

 laminated, while the lower conductors are left solid. When the 

 method of lamination has been decided upon, the probable in- 

 crease in loss can be obtained from figures and curves published 

 by PROF. FIELD in the paper previously referred to; but it is 

 suggested that, for the purpose of estimating the probable 

 efficiency, the calculated PR loss in the armature windings 

 be increased 15 per cent, in the case of slow-speed machines of 

 medium size, and 30 per cent, in the case of steam-turbine- 

 driven units of large output. This addition is intended to 

 cover not only the losses due to eddy currents in the armature 

 windings, but all indeterminate losses in end plates, supporting 

 rings, etc., which increase with the load. 



