190 



PRINCIPLES OF ELECTRICAL DESIGN 



load terminal voltage. Then, since the ampere-turns on the 

 shunt at no load are OA, they will obviously have increased to 

 OB at full load on account of the higher terminal voltage (the 

 "long shunt" connection is here assumed). The ampere-turns 

 necessary to produce the required full-load flux will be OC; 

 but the field excitation must be greater than this in order to 

 balance the distortional and demagnetizing effects of the arma- 

 ture current. It was found that the ampere-turns necessary 

 to counteract the effects of the armature current were repre- 

 sented by the distance P b P a in Fig. 49 (Art. 42, page 133). These 



A B C 



Ampere "Earns per Pole 

 Fia. 74. Open-circuit saturation curve of dynamo. 



SI to Compensate 

 7 for Demagnetising 

 andDrstortional 

 Effect of 

 Armature 



ampere-turns had to be put on the field poles, not to increase 

 the air-gap flux and thus develop a higher voltage, but merely 

 to counteract the effects of the armature current and restore 

 the air-gap flux to its original value on open circuit. It is 

 therefore correct to say that additional ampere-turns approxi- 

 mately equal to this amount must be added to the field wind- 

 ings in order that the necessary flux shall be cut by the armature 

 conductors. This addition is shown in Fig. 74, where the 

 distance CD has the same value as PbP a in Fig. 49. It follows 

 that OD represents the total ampere-turns required per pole 

 at full load. Of this total amount, OB will be due to the shunt 

 winding, and the balance, BD, must be provided by the series 

 winding. 



58. Arrangement and Calculation of Field Windings. Since 

 the ampere-turns required in the shunt winding have now been 



