DESIGN OF ALTERNATORS 



241 



A will also be induced in B, but after an interval of time repre- 

 senting a quarter of a period. This diagram shows the positions 

 of the poles at the instant when the e.m.f . in A is at its maximum, 

 while in B it is passing through zero value. From these two 

 windings we can, therefore, obtain two-phase currents with a 

 phase displacement of 90 electrical degrees. 



In the bottom diagram the arrangement of three windings is 

 shown, from which three-phase currents can be obtained, with a 

 phase angle between them of 120 degrees, or one-third of a cycle. 

 It will be seen that, at the instant corresponding to the relative 

 positions of coils and poles as indicated on the diagram, the e.m.f. 

 in A is at its maximum, -while in B and C it is of a smaller value 

 and in the opposite direction. 



68. Number of Poles. Frequency. For a given frequency 

 the number of poles will necessarily depend upon the speed. 



Thus p = 





where N stands for the speed in revolutions per 



minute. Since / is usually either 25 or 60, it follows that N must 

 be some definite multiple of the number of poles p. 



69. Usual Speeds of A.C. Generators. The speed at which a 

 machine of a given kilowatt output should be driven will depend 

 upon the prime mover. The speed may be very low, as when 

 the generator is direct-coupled to a slow-speed steam engine or 

 low-head waterwheel. Higher speeds are obtained when the 

 generator is belt-driven or direct-coupled to high-speed steam or 

 oil engines. Very high speeds are necessary when the generator 

 is direct-connected to a steam turbine. 



For usual speeds the reader is referred to the table on page 

 81, the values there given being applicable to both D.C. and 

 A.C. machines. In hydro-electric work the generator is usually 

 direct-coupled to the waterwheel, the speed of which will be 

 high in the case of impulse wheels working under a high head. 

 As an example, a PELTON waterwheel to develop 1,500 hp. under 

 a head of 1,000 ft. would have a wheel about 5 ft. in diameter, 

 running at 500 revolutions per minute. This would be suitable 

 for direct coupling to a six-pole 25-cycle generator. 



In the case of steam turbines with a blade velocity of about 

 5 miles per minute the speeds are always very high. The actual 

 speed best suited to a given size of unit will depend upon the 

 make of the turbine, but the following table gives the approxi- 

 mate range of speeds covered by modern steam turbines. 



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