EXAMPLE OF ALTERNATOR DESIGN 321 



some of the larger machines running at higher peripheral veloci- 

 ties, and the electrical features of the design will be considered 

 alone, reference being made to mechanical details only as occa- 

 sion may arise. 



It is proposed to work through the consecutive items of a design 

 sheet, as was done for the D.C. dynamo; but the sheets will include 

 fewer detailed items, more latitude being allowed in the exercise 

 of judgment and the application of knowledge derived from the 

 work done on previous designs. An attempt will be made to 

 render the example of use in the design of slow-speed salient-pole 

 machines, and, with this end in view, references will be made to 

 the text when taking up in detail the items of the design sheet. 

 For the same reason namely, to make the numerical example of 

 broad application the writer may take the liberty of digressing 

 sometimes from the immediate subject, if matters of interest 

 suggest themselves as the work proceeds. 



113. Single -phase Alternators. Since the selected design is 

 that of a polyphase machine, it seems advisable to state here one 

 or two matters of special interest in the design of the less common 

 single-phase generator. It is easier to design a polyphase than 

 a single-phase alternator, although this fact is not always recog- 

 nized, even by designers. Many of the single-phase machines 

 in actual service are less efficient than they might be; but the 

 problems which are peculiar to single-phase generators receive 

 comparatively little attention because these machines are rarely 

 used at the present time, the development during recent years 

 having been mainly in the direction of power transmission and 

 distribution by polyphase currents. 



It is the pulsating nature of the armature m.m.f., as explained 

 in Art. 94, Chap. XIII that leads to eddy-current losses that 

 are practically inappreciable in the case of two- or three-phase 

 machines working on a balanced load, that is to say, with the 

 same current and the same voltage in each of the phase windings, 

 and with the same angular displacement between current and 

 e.m.f. in the respective armature circuits. Sometimes the poly- 

 phase load is not balanced, and in that case pulsations of the 

 armature field occur as in the single-phase machines, the amount 

 of the pulsating field being dependent upon the degree of un- 

 balancing of the load. The effect is then as if an alternating 

 field were superposed on the steady armature m.m.f. due to 

 the balanced components of the total armature current. 



Without going into detailed calculations, it may be stated that 

 21 



