DESIGN OF ALTERNATORS 



251 



there is, however, a tendency to use high values of q in order to 

 limit the length of the armature (and increase the critical speed) 

 and also to increase the armature m.m.f. with a view to lowering 

 the short-circuit current. 



(6) With low frequency it is easy to keep the iron loss small, 

 and either more copper, or a greater current density in the con- 

 ductors, is permissible. 



(c) If the e.m.f. is low, the insulation occupies less space, and 

 there is more room for copper without unduly reducing the cross- 

 section of the armature teeth. 



The approximate figures given in the above table may be 

 increased or reduced about 20 per cent., the highest values being 

 used only when there is a combination of low voltage, low fre- 

 quency, and large number of poles. In single-phase machines, 

 the value of q may be about 20 per cent, greater than in poly- 

 phase machines. 



76. Flux Density in Air Gap. Since the pole shoe is shaped 

 to give as nearly as possible a sinusoidal distribution of flux 

 density over the pole pitch, it is convenient to think of the 

 maximum value of the air-gap density, because this will determine 

 the maximum density in the iron of the teeth. The frequency 

 being usually higher than in D.C. machines, lower tooth densities 

 must be used in order to avoid excessive loss. The allowable 

 flux density in the air gap will depend upon the proportions of 

 tooth and slot; but the following values may be used for pre- 

 liminary calculations. 



For a frequency of 25, B g 4,000 to 5,800 gausses. 

 For a frequency of 60, B g = 3,500 to 5,000 gausses. 



These values of B g stand for the average density over the pole 

 pitch. If 3> = the total number of maxwells per pole; and the 

 shape of the flux distribution over the pole pitch is assumed to 

 be a sine wave, we have: 



< 

 Area of pole pitch = -~-' 



&0 



which determines the axial length of the armature core. The 

 maximum air-gap density, on the above assumption, is ~ B 0) and 







after deciding upon the tooth and slot proportions, it is advisable 

 to see that this density will not lead to an unreasonable value 

 for the apparent tooth density. As a check, it may be stated 

 that the tooth density in alternators is rarely higher than 18,000 

 gausses in 25-cycle machines, and 16,000 gausses in 60-cycle 



