252 METHODS OF CALCULATION 



Case of Field Magnets with Divided Windings. In certain machines, 

 particularly turbo-alternators, circular field magnets are found in 

 which the windings are carried along the air-gap in slots like those 

 of the armature. The preceding diagrams (Figs, i to 7) apply like- 

 wise to these machines only on the condition of assuming the two 

 coefficients of reaction equal even when the field magnet is entirely 

 divided into slots. Moreover, the field-magnet winding must be 

 affected by a coefficient of ampere-turns K\, reducing them to KiNiI t 

 (with ^! = o.4 to 0.5 in completely uniform windings), and by the 

 Hopkinson coefficient Vi, which is calculated like the stray field of a 

 slot in an asynchonous motor. All the other coefficients are calculated 

 as in the ordinary case, by supposing the breadth of the reactive flux 

 equal to that of the field-magnet poles. This method has given me 

 satisfactory results in practice for this type of machines. 



PART II. CALCULATION OF CONSTANTS. 



Practical Calculation of Reactions. In order to apply the diagrams, 

 the coefficients s, I, K t and K must be determined (/ and K t are of 

 course only two expressions of one and the same coefficient). The 

 stray coefficient is determined by known methods frequently indicated 

 for asynchronous motors, and they need not, therefore, be alluded 

 to here. For K and K t , I have employed for several years the most 

 direct method, which consists in determining for the same machine 

 curves of distribution of magnetic potential, and of the flux in the 

 air-gap, assuming that the armature is traversed by a known current 

 either active or reactive. By taking into account the position of 

 the pole pieces in these two cases, and their form, as well as 

 that of the slots, the reactions may be determined with sufficient 

 precision. 



Let us consider, for example, the case of three-phase currents: 

 the three phases occupy in a double field six slots, or groups of slots, 

 and at the passage of each slot, the magnetic potential along the 

 entrefer undergoes a sudden positive or negative increase equal to 



N- 

 0.47: multiplied by the number of ampere-turns -^- contained in the 



slot. It suffices to mark off on a straight line, representing the 

 development of the circumference of the armature, lengths equal to 

 the distance between the axes of the slots, and on successive ordinates, 

 the variations of the magnetic potential thus calculated. The horizontal 



