An elementary book on electricity and magnetism and their applications . tter in-sulation than those with sine curves. On the other hand, ifthe wave is flat-topped, the hysteresis loss is increased, whichmay greatly increase the cost of operating transformers. Figure 248 is an oscillogram which shows the actual curveof the terminal voltage of a 6600-volt generator, and it willbe seen to follow very closely the form of a sine wave. Ex-periments show that the wave form of depends on theway in which the magnetic flux is distributed around thepoles and also on the way in which the inductors
An elementary book on electricity and magnetism and their applications . tter in-sulation than those with sine curves. On the other hand, ifthe wave is flat-topped, the hysteresis loss is increased, whichmay greatly increase the cost of operating transformers. Figure 248 is an oscillogram which shows the actual curveof the terminal voltage of a 6600-volt generator, and it willbe seen to follow very closely the form of a sine wave. Ex-periments show that the wave form of depends on theway in which the magnetic flux is distributed around thepoles and also on the way in which the inductors are distrib-uted over the surface of the armature. Besides these fac-tors, it is necessary to consider the shape and length of the airgap, the relative strength of field magnets and armature wind-ing, the power factor of the load, and the method of connectingthe phases of a three-phase machine. Usually, therefore,the first and most necessary consideration about an alternatoris that it shall generate very nearly a sine wave of ALTERNATING-CURRENT GENERATORS 365. Fig. 248. — Actual voltage curve at no load. 150 kv-a. and 60 cycles. 249. Efficiency of alternators. The efficiency of any machinemeans the ratio of the power delivered by the machine to thopower received by it. Efficiency = <^ In the case of a large alternator it is generally impracticableto measure directly the input and so we determine the outputand the losses and calculate the efficiency thus: Efficiency = ^PSL^ Output + total losses The results of tests on alternators of varying sizes from 100kv-a. to 10,000 kv-a. show that the efficiencies run from about91 per cent to 97 per cent. It is, of course, possible to getvery high efficiency by increasing the first cost of the experience has shown that sometimes it does not pay to build machines for the highest possible efficiency. Hencewe must distinguish between efficiency and economy. 366 ELECTRICITY AND MAGNETISM If we know the effic
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