284 ELECTRICAL EQUIPMENT 



pitch factor, k w , its values for different per cent pitch being given 

 in Fig. 153. They are simply based on the formula: 



*= sin (l5o x90 )' 



where x is the per cent pitch. 



For single-phase generators the armature is generally wound 

 similar to a three-phase machine, one-phase being left normally 

 idle. With this arrangement the slot factors k s are the same as 

 given for three-phase windings. If the winding is furthermore 

 distributed as with purely single-phase generators, when it covers 

 considerably more than two-thirds of the armature surface, the 

 values of these slot factors should be reduced. 



The flux <f>, obtained from the previous formula is that which is 

 necessary in the armature for inducing the required e.m.f., i.e., 

 the useful flux. Due to the leakage between the poles it is, how- 

 ever, necessary to provide a greater flux in the field poles and the 

 yoke to compensate for this leakage, and this must be considered 

 when calculating the ampere turns of the field winding. This 

 increased flux is obtained by multiplying the useful flux by a leak- 

 age coefficient. The average values for this factor at no load, 

 depending on the diameter per pole, may be obtained from Table 

 XLII. 



TABLE XLII 



POLE LEAKAGE COEFFICIENTS 



Diameter per pole, inches: 2345 6 7 8 



Leakage coefficients: 1.4 1.35 1.3 1.26 1.22 1.18 1.16 



Effect of Power Factor on Operation. Assuming all conditions 

 except the load constant, the terminal voltage of an alternating- 

 current generator will fall as the load increases. This is due to 

 the resistance of the armature conductors and the synchronous 

 reactance, the latter combining the effects of the armature reac- 

 tion and the armature reactance or self-induction. For a con- 

 stant terminal voltage with increased load, the armature resistance 

 and self-induction require an increase in voltage while the demag- 

 netizing effect requires only an increase in the magnetic flux to 

 make up for the reduction in flux caused by the armature current. 

 The latter does not require any increase in the generated voltage 

 since the action is confined to the magnetic flux. 



The drop in voltage, due to the armature resistance, requires 



