CHAPTER II 



PREDETERMINATION OF THE FIELD-EXCITATION OF ROTARY 



CONVERTERS 



IN the preceding chapter we discussed the conditions attending 

 the supply of electric current to rotary converters, and, in particular, 

 the effects of reactance in the supply-circuit, and the various ways 

 of using it to obtain a given range of impressed voltage for a given 

 range of load without exceeding a certain maximum reactive current, 

 and even while making this current approach a minimum value. There 

 still remains the second portion of the problem to be solved, namely, 

 the determination, of the field-exciting ampere-turns, due to either the 

 series or the shunt-winding coils, which enable the rotary converter to 

 follow approximately this law of variation, in consequence of the effects 

 of the reactive currents. 



In what follows it will be assumed that we are dealing with the 

 ordinary case of a converter which is operating without lag, at its average 

 load, and which consequently, has, at lighter loads, a positive reactive 



current ( lagging behind the E.M.F. ) whereby there is produced a 



magnetizing effect on the field. 1 This is the case which was discussed 

 in the preceding chapter. 



Characteristic Features of the Rotary Converter. We will begin 

 by considering the magnetic features of the rotary converter, which 

 involve two new important elements: 



i. The excitation-curve, or the curve showing the variation of in- 

 duced E.M.F. as a function of the field-exciting ampere-turns due to 

 the field-windings. This curve is supposed to be known from the 

 shop-tests made of the machine at the time it was finished. 



In the excitation-curve (Fig. 15) the ordinates represent the E.M.F.'s 



1 It is easy to see that any reactive current which lags behind the impressed 

 E.M.F. EI is leading with respect to the internal E M.F. e, since E and e are 

 practically in opposition. 



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