EXAMPLE OF ALTERNATOR DESIGN 



353 



129 and 130 can be applied for obtaining curves giving the ap- 

 proximate connection between terminal volts and exciting current 

 for any other power factor. We shall confine ourselves here to 

 calculating the inherent regulation by the more correct method as 

 outlined in Art. 110, and since much of the work has already 

 been done in connection with full-load current on 80 per cent, 

 power factor, this is the condition which we shall choose for the 

 purpose of illustration. 



We know that although 27,000 ampere-turns per pole will 

 develop the specified terminal voltage when no current is taken 

 from the machine, this excitation must be increased to 37,000 

 ampere-turns to give the same terminal voltage under full-load 

 conditions (80 per cent, power factor). If then, we can calculate 

 the voltage, with this greater field excitation, when the load is 

 thrown off, the inherent regulation can be predetermined, and, 

 incidentally, we shall obtain a point on the open-circuit charac- 

 teristic corresponding to a fairly high value of the excitation. 



The required flux curve, marked A , has been plotted in Fig. 

 141. It is derived, like any other flux curve, from the m.m.f. 

 curve Mo of Fig. 142, by using the saturation curves of Fig. 139 

 which must be extended beyond the limits of the diagram in 

 order to read the flux values for the higher degrees of excitation. 

 Careful measurements of the flux curve A give an area over the 

 pole pitch of 129 sq. cm., and if we assume the form factor of 

 the resulting e.m.f. wave (not plotted) to be the same as for the 

 open-circuit curve at normal voltage, i.e.j 1.11, the voltage cor- 

 responding to the flux Ao will be 



3,810 X 129 

 106.3 



= 4,625 



where 106.3 is the previously measured area of flux curve A. 

 The inherent regulation at 80 per cent, power factor is therefore 



4,625 - 3,810 



- 5-^777 = 21.4 per cent. 



o,olU 



This is well within the specified limit of 25 per cent, which 

 again points to the fact that a somewhat smaller air gap, or a 

 lower flux density in the rotor teeth would have been permissible. 



It should be pointed out here that the external power factor 

 corresponding to the flux distribution curve C of Fig. 141 is not 

 necessarily exactly 0.8; because the method of determining the 



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