242 PRINCIPLES OF ELECTRICAL DESIGN 



Output, kilowatts Approximate range of speed, 



revolutions per minute 



2,000 ................. 3,000 to 6,000 



5,000 ................. 2,000 to 4,000 



10,000 ................. 1,200 to 2,500 



20,000 .............. ... 900 to 1,800 



70. E.m.f. Developed in Windings. 



Let $ = flux per pole (maxwells). 

 AT = revolutions per minute. 

 p = number of poles. 



The flux cut per revolution is then & X p and the flux cut' per 



N 

 second is &p ~. The average value of the e.m.f. developed in 



each armature conductor must therefore be 



E c (mean) = 6Q x 1Q8 volts. 



If the space distribution of the flux density over the pole pitch 

 follows the sine law, the virtual value of the e.m.f. is 1.11 times 

 the mean value. In other words, the form factorj or ratio 



r.m.s. value TT . 



- i } is o /-, or 1.11, in the case of a sine wave. 

 mean value 2v2 



Concentrated and Distributed Windings. If each coil-side may 

 be thought of as occupying a very small width on the armature 

 periphery, and if the coil-sides of each phase winding are spaced 

 exactly one pole pitch apart, the arrangement would constitute 

 what is usually referred to as a concentrated winding. With a 

 winding of this kind, all conductors in series in one phase would 

 always be similarly situated relatively to the center lines of the 

 poles, and the curve of induced e.m.f. would necessarily be of 

 the same shape as the curve of flux distribution over the armature 

 surface. In practice, a winding with only one slot per pole per 

 phase would be thought of as a concentrated winding. When 

 there are two or more slots per pole per phase, the winding is 

 said to be distributed; and since the conductors of any one 

 phase cover an appreciable space on the armature periphery, 

 all the wires that are connected in series will not be moving in 

 a field of the same density at the same instant of time. Except 

 in the case of a sine-wave flux distribution, the form factor may 

 depend largely upon whether the winding is concentrated or 

 distributed. The wave shape of the developed voltage can 

 always be determined when the flux distribution is known; but, 





