150 THE MAGNETIC CIRCUIT [ART. 48 



Prob. 8. The mac'hine specified above is to be used as a synchronous 

 motor. Determine graphically the required field excitation when the 

 useful output on the shaft is to be 700 kw., and in addition the machine 

 must draw from the line 600 leading reactive kva. The efficiency of the 

 machine at the above-mentioned load is estimated to be about 91 per 

 cent. Ans. 12.5 kiloampere-turns. 



Prob. 9. Draw to the same scale as the diagram shown in Fig. 37, 

 another similar diagram, for the same value of the current and of the 

 phase angle 0, except that the current is to be leading. Assume a 

 reasonable shape of the saturation curve in determining the new value of 

 M n . Show that a much smaller exciting current is required with the 

 same kva. output, than in the case of a lagging current. 



Prob. 10. Solve problem 9 for the motor diagram shown in Fig. 38, 

 assuming the current to be lagging with respect to the line voltage. 



Prob. 11. For a given alternator, show how to determine the voltage 

 e (Fig. 37), analytically or graphically, when Mf, i, and </> are given; 

 explain when such a case arises in practice. 



Prob. 12. For a given synchronous motor, show how to determine 

 the reactive component i 2 of the current (Fig. 38), analytically or graphic- 

 ally, when Af /, e and i\ are given ; explain when such a case arises in 

 practice. 



Prob. 13. Work out the details of the above-mentioned method 

 for the determination of the overload capacity of a synchronous motor 

 by trials. Hint: Introduce the components of e and i, in phase and in 

 quadrature with E', rewrite eqs. (71) and (72) by projecting the figure 

 OABD on the direction of E and on that perpendicular to E. Use no 

 angles in the formulaB, and neglect the small terms containing r, where 

 they lead to complicated equations of higher degrees. 



48. The Direct and Transverse Armature Reaction in a Synchro- 

 nous Machine with Salient Poles. In a machine with non-salient 

 poles the armature reaction shifts the field flux but hardly distorts 

 its shape. In a machine with projecting poles the flux, generally 

 speaking, is both altered in value and crowded toward one pole- 

 tip (Fig. 36). It is convenient, therefore, to resolve the traveling 

 wave of the armature m.m.f . into two waves, one whose crests coin- 

 cide with the center lines of the poles, the other displaced by 90 

 electrical degrees with respect to it. The first component of the 

 armature m.m.f. produces only a " direct " effect upon the field 

 flux, that is, it either strengthens or weakens the flux, without dis- 

 torting it. The second component produces a " transverse " 

 action only, viz., it shifts the flux toward one or the other pole-tip, 

 without altering its value (that is, neglecting the saturation) . 



We have seen before that an armature current, which reaches 

 its maximum when the conductor is opposite the center of the 



