238 PRINCIPLES OF ELECTRICAL DESIGN 



are of a type not commonly met with, they will not again be 

 referred to. 



The remainder of this book will be devoted to a study of the 

 synchronous alternating-current generator, and since multipolar 

 polyphase generators with stationary armatures are more com- 

 mon than any other type, they will receive more attention than 

 the less frequently seen designs; but the case of the high-speed, 

 steam-turbine-driven units, with a small number of poles and 

 distributed field windings, will also be considered. 



Apart from the absence of commutator, the chief point of dif- 

 ference between an A.C. and D.C. generator is that the frequency 

 of the former is specified, whereas, in the latter, this is a matter 

 which concerns the manufacturer only. It follows that, for a 

 given speed, the number of poles is determined by the frequency 

 requirements, and this fact necessarily influences the design. In 

 Europe a frequency of 50 cycles per second is common, the idea 

 being that this is high enough for lighting purposes while being 

 sufficiently low to allow of the same circuits being used occasion- 

 ally for power purposes also. A lower frequency is usually to be 

 preferred for power schemes, and the standards in America are 

 25 cycles for power purposes and 60 cycles for lighting. 



66. Classification of Synchronous Generators. It is well to 

 distinguish between two classes of alternators: 



1. Machines with salient poles, driven at moderate speeds by 

 belt, or direct-connected to reciprocating steam, gas, or oil 

 engines, or to water turbines. The peripheral speed of the ro- 

 tating part (usually the field magnets) will generally lie between 

 the limits of 3,000 and 8,000 ft. per minute. 



2. Machines direct-coupled to high-speed steam turbines, in 

 which the peripheral velocity usually exceeds 12,000 ft. per min- 

 ute, is commonly about 18,000, and may attain 24,000 ft. per min- 

 ute. In these machines the field system is always the part that 

 rotates; the number of poles is small, and although salient poles 

 are sometimes used on the lower speeds, the cylindrical field 

 magnet with distributed windings is more common. The me- 

 chanical problems encountered in the design of these high-speed 

 machines are relatively of greater importance than the electrical 

 problems; but since these are beyond the scope of this book, 

 they will not be considered in detail. Such differences as occur 

 in the electrical calculations will be pointed out as the work 

 proceeds. 



